Vaillant ecoTEC exclusive Fault codes & diagnostics

57 fault codes with plain-English explanations, severity ratings, DIY guidance, and repair cost estimates.

CombiSystem Natural GasLPG 27-43 kW 2007-2016 Discontinued
GC Numbers 47-044-3747-044-6347-044-64

The ecoTEC exclusive range comes in 2 types

Pick the one you have. The fault codes, GC numbers and manual differ by type.

Download the Vaillant ecoTEC exclusive manual The official installation & service manual (PDF) — the exact document these fault codes were verified against. PDF

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Severity at a glance

Each cell = one fault code. Hover to identify.
4 Emergency 44 High 9 Medium

Which version do you have?

The GC number on your boiler's data badge identifies the exact appliance and production years, as recorded in the UK Product Characteristics Database.

GC number Appliance (register name) Produced
47-044-37 ecoTEC exclusive 832 2007–2016

All 57 documented codes

Tap any card for details

10

Your boiler has detected an issue with the internal ventilation system that helps it exhaust gases safely, so it has turned itself off to prevent any risk.

High Engineer only £250-450

Parts commonly replaced

  • Fan assembly
  • Wiring harness
  • Printed Circuit Board (PCB)

Related codes

12

Your boiler is not detecting the connection to your wall-mounted thermostat control, meaning it doesn't know when to turn the heating on or off.

Medium DIY-safe £90-180

What to check first

  1. Check that your wall thermostat has fresh batteries installed.
  2. Ensure the thermostat display is active and not showing a connection error.
  3. Check the wiring connection at the boiler terminal block for any loose or disconnected wires.

Parts commonly replaced

  • eBUS communication interface
  • PCB
  • Room thermostat unit

Related codes

13

Your boiler has lost communication with its wall-mounted thermostat, meaning it doesn't know when to switch your heating on.

Medium DIY-safe £90-180

What to check first

  1. Turn off the electrical power to your boiler for 60 seconds
  2. Turn the power back on and wait for the system to reboot
  3. Check all cables between the boiler and your wall thermostat for loose connections or damage

Parts commonly replaced

  • eBUS communication module
  • Room thermostat interface
  • Wiring loom
14

Your boiler is struggling to switch between heating your radiators and heating your hot water because an internal valve is sticking.

Medium Engineer only £150-250

Parts commonly replaced

  • Diverter valve motor
  • Diverter valve cartridge

Related codes

21

Your boiler's internal control system is having trouble communicating with an optional external accessory, such as an extra pump or a remote timer device.

Medium Engineer only £120-250

Parts commonly replaced

  • External control relay
  • Wiring harness
  • Printed Circuit Board (PCB)

Related codes

23

Your boiler is struggling to push water around your heating system because the internal pump is not working correctly.

High Engineer only £250-450

Parts commonly replaced

  • High-efficiency pump
  • Pump wiring harness
  • PCB

Related codes

24

Your boiler is failing to light the flame properly, which means it will stop working to keep your home warm and provide hot water until it is repaired.

High Engineer only £150-280

Parts commonly replaced

  • Ignition electrode
  • Ignition lead
  • PCB

Related codes

con

Your boiler's internal computer brain is unable to talk to the front display panel, meaning the system cannot receive instructions to provide heating or hot water.

High Engineer only £180-450

Parts commonly replaced

  • Main Printed Circuit Board (PCB)
  • Display Interface
  • Internal Wiring Loom

Related codes

D.118

Your boiler has detected an electrical fault within its internal heating control system that is preventing it from operating safely.

High Engineer only £150-300

Parts commonly replaced

  • Main PCB
  • Wiring harness
  • Sensor wiring

Related codes

F.00

Your boiler has lost the connection to the temperature sensor that tracks the flow of water, causing it to stop heating as a safety precaution.

High Engineer only £120-220

Parts commonly replaced

  • Flow NTC sensor
  • Wiring harness

Related codes

Showing 11–20 of 57
F.01

Your boiler has stopped working because it has lost its connection to a sensor that measures the temperature of the water returning to the system.

High Engineer only £120-220

Parts commonly replaced

  • Return NTC sensor
  • Wiring harness

Related codes

F.02

Your boiler has lost the ability to read the temperature of your hot water supply, which means it has stopped providing hot water to your taps.

High Engineer only £120-220

Parts commonly replaced

  • Hot water NTC sensor
  • Wiring harness
F.03

Your boiler has lost communication with the temperature sensor in your hot water tank, meaning it cannot accurately detect when your water is hot enough.

Medium Engineer only £120-220

Parts commonly replaced

  • Cylinder NTC sensor
  • Wiring harness
F.10

Your boiler has stopped working because it thinks the main water temperature sensor is broken or there is a problem with its internal wiring.

High Engineer only £90-180

Parts commonly replaced

  • NTC flow temperature sensor
  • Wiring harness

Related codes

F.11

Your boiler has stopped working because it cannot correctly read the temperature of the water returning to the unit due to an electrical connection issue.

High Engineer only £90-180

Parts commonly replaced

  • Return temperature sensor (NTC)
  • Wiring harness

Related codes

F.12

Your boiler has detected a faulty electrical signal related to the sensor that monitors your hot water temperature.

High Engineer only £150-250

Parts commonly replaced

  • Domestic Hot Water NTC sensor
  • Wiring harness
F.13

Your boiler has detected an electrical fault with the internal sensor that monitors the temperature of its hot water storage tank.

High Engineer only £120-£200

Parts commonly replaced

  • NTC Temperature Sensor
  • Wiring loom
  • Sensor connection plug

Related codes

F.20

Your boiler has shut down because it has detected an unsafe increase in internal temperature, likely caused by a sensor fault or an issue with water flow.

High Engineer only £120-£280

Parts commonly replaced

  • NTC Temperature Sensor
  • Overheat Thermostat
  • Circulation Pump

Related codes

F.22

Your boiler has stopped working because the water pressure is too low for it to operate safely.

High DIY-safe £0-120

What to check first

  1. Locate the filling loop (usually a silver flexible hose) underneath or near the boiler.
  2. Slowly open the valve(s) until you hear water entering the system.
  3. Watch the pressure gauge and close the valves once the needle reaches between 1.0 and 1.5 bar.

Parts commonly replaced

  • Filling loop valves
  • Expansion vessel
  • Pressure sensor

Related codes

F.23

Your boiler has detected that water isn't moving through the system fast enough, causing a large temperature difference that has triggered a safety shutdown.

High DIY-safe £180-450

What to check first

  1. Check that all radiator valves are fully open
  2. Ensure the boiler pressure is between 1.0 and 1.5 bar
  3. Restart the boiler using the reset button

Parts commonly replaced

  • Circulation pump
  • NTC Temperature Sensors
  • Main Heat Exchanger (Flushing)

Related codes

Showing 21–30 of 57
F.24

Your boiler is overheating too quickly because water isn't flowing through the system correctly, often due to low pressure or trapped air.

High DIY-safe £100-£350

What to check first

  1. Check the pressure gauge and top up the water to 1.5 bar if it is too low
  2. Bleed any air from your radiators using a radiator key
  3. Ensure all radiator valves and isolation valves are fully open

Parts commonly replaced

  • Circulation pump
  • Pump capacitor
  • Pressure sensor

Related codes

F.25

Your boiler has detected an internal electrical connection issue that is preventing it from running safely.

High Engineer only £120-£350

Parts commonly replaced

  • Wiring harness
  • Main PCB
  • Flue gas thermostat

Related codes

F.26

Your boiler has detected an issue with the valve that controls the gas supply, meaning it cannot safely start up to provide heating or hot water.

High Engineer only £250-450

Parts commonly replaced

  • Gas valve stepper motor
  • Main PCB
  • Wiring harness

Related codes

F.27

Your boiler has detected a flame when there shouldn't be one, which has caused the system to shut down for safety.

High Engineer only £150-350

Parts commonly replaced

  • Flame sensing electrode
  • Gas valve
  • Printed Circuit Board (PCB)

Related codes

F.28

Your boiler is failing to light the gas flame, meaning it cannot provide heat or hot water.

High DIY-safe £90-350

What to check first

  1. Check if your gas emergency control valve (usually by the meter) is in the 'on' position.
  2. If using a prepayment meter, ensure you have sufficient credit and the valve hasn't locked.
  3. Try resetting the boiler by pressing the flame symbol with a cross through it for five seconds.

Parts commonly replaced

  • Ignition lead and electrodes
  • Gas valve
  • Spark generator
  • Printed Circuit Board (PCB)

Related codes

F.29

Your boiler has lost its flame while running and cannot manage to restart itself, likely due to a temporary interruption in the gas supply or a component failure.

High DIY-safe £100-350

What to check first

  1. Check if other gas appliances, like a hob, are working to ensure gas is reaching the property.
  2. If you have a pre-payment meter, check that you have sufficient credit.
  3. Press the reset button (indicated by a flame symbol with a cross) for one second to see if the boiler restarts.

Parts commonly replaced

  • Ignition lead
  • Ignition electrodes
  • Gas valve
  • Printed Circuit Board (PCB)

Related codes

F.32

Your boiler has detected a problem with the fan that safely pushes waste gases out of your home, causing the system to shut down for safety.

High Engineer only £250-£480

Parts commonly replaced

  • Fan assembly
  • Wiring harness
  • Main PCB (Printed Circuit Board)
F.33

Your boiler has detected an issue with the airflow system, which means it cannot safely vent exhaust gases, so it has shut down to prevent any risk.

High Engineer only £150-300

Parts commonly replaced

  • Air pressure sensor
  • Fan unit
  • Wiring harness
  • Flue seals

Related codes

F.35

Your boiler is having trouble breathing because there may be a blockage, damage, or an obstruction in the pipework that vents exhaust gases outside.

Emergency Engineer only £150-350

Parts commonly replaced

  • Flue duct seals
  • Air pressure switch
  • Fan assembly
  • Flue terminal

Related codes

F.42

Your boiler has lost the digital instructions telling it which type of gas it should be using, causing it to stop working for your safety.

High Engineer only £150-300

Parts commonly replaced

  • Coding resistor
  • Main Printed Circuit Board (PCB)
Showing 31–40 of 57
F.49

There is a communication error between your boiler and its controls, often caused by an electrical wiring fault or a heavy load on the internal data connection.

High Engineer only £90-180

Parts commonly replaced

  • eBUS wiring
  • External controller
  • Printed Circuit Board (PCB)

Related codes

F.55

Your boiler has detected an internal issue with its combustion monitoring system and has shut itself down to keep you safe.

Emergency Engineer only £180-350

Parts commonly replaced

  • All-gas sensor
  • Gas valve
  • Electronic ignition board

Related codes

F.56

Your boiler has detected an issue with the fuel-to-air balance that could affect safe operation, so it has safely shut itself down to prevent any risk.

Emergency Engineer only £250-450

Parts commonly replaced

  • Gas valve assembly
  • Wiring harness
  • Air/Gas mixture sensor

Related codes

F.57

Your boiler has detected that a key ignition component is wearing out, meaning it is struggling to reliably start the flame needed to heat your home.

High Engineer only £150-250

Parts commonly replaced

  • Ignition electrode
  • Ignition lead

Related codes

F.61

Your boiler has detected a technical communication error between the main control board and the gas valve, preventing it from lighting safely.

High Engineer only £180-450

Parts commonly replaced

  • Gas valve
  • Printed Circuit Board (PCB)
  • Wiring harness

Related codes

F.62

Your boiler has detected a potential internal gas leak or an electrical failure that stops the gas supply from closing correctly, so it has shut down for safety.

Emergency Engineer only £250-£550

Parts commonly replaced

  • Gas valve
  • Printed Circuit Board (PCB)

Related codes

F.64

Your boiler has detected an electrical fault within its internal sensors or control board, which has caused the system to shut down for safety.

High Engineer only £120-£450

Parts commonly replaced

  • Flow NTC sensor
  • Return NTC sensor
  • Main PCB (Printed Circuit Board)

Related codes

F.65

Your boiler’s internal computer brain is overheating, which has caused the system to shut down for safety.

High Engineer only £250-£550

Parts commonly replaced

  • Main PCB (Printed Circuit Board)
  • Display Interface

Related codes

F.67

Your boiler's main control board is receiving an incorrect electrical signal from the flame sensor, meaning the system cannot safely confirm if the burner is lit.

High Engineer only £250-£450

Parts commonly replaced

  • Printed Circuit Board (PCB)
  • Wiring loom
  • Ignition electrode

Related codes

Showing 41–50 of 57
F.68

Your boiler is struggling to keep its internal flame lit steadily, which has caused it to shut down for safety reasons.

High Engineer only £150-350

Parts commonly replaced

  • Ignition electrode
  • Gas valve
  • Condensate trap
  • Wiring harness

Related codes

F.70

Your boiler’s internal computer boards are having trouble communicating because they haven't been properly synced or configured to recognize each other.

High Engineer only £90-180

Parts commonly replaced

  • Main PCB (Printed Circuit Board)
  • User Interface/Display PCB

Related codes

F.71

Your boiler has detected that a temperature sensor is sending a stuck or frozen reading, preventing it from heating your home or water safely.

High Engineer only £95-170

Parts commonly replaced

  • Flow Temperature Sensor (NTC)
  • Sensor Wiring Harness

Related codes

F.72

Your boiler has detected that its temperature sensors are giving conflicting or incorrect readings, causing it to shut down for safety.

High Engineer only £120-£220

Parts commonly replaced

  • Flow NTC sensor
  • Return NTC sensor
  • Wiring harness

Related codes

F.73

Your boiler's internal sensor that monitors water pressure has lost electrical contact or has failed, causing the system to stop working for safety.

High Engineer only £140-£250

Parts commonly replaced

  • Water pressure sensor
  • Wiring harness
  • Main PCB

Related codes

F.74

Your boiler has detected an electrical issue with its internal water pressure sensor, meaning it cannot accurately tell if there is enough water in the system to run safely.

High Engineer only £140-£260

Parts commonly replaced

  • Water pressure sensor
  • Wiring harness
  • Main PCB

Related codes

F.75

Your boiler has stopped working because it cannot detect the necessary change in pressure when the pump starts up.

High DIY-safe £180-350

What to check first

  1. Check the pressure gauge and top up the water to 1.5 bar if it is low
  2. Reset the boiler by holding the flame/cross button for five seconds
  3. Bleed your radiators to ensure there is no trapped air in the system

Parts commonly replaced

  • Water pressure sensor
  • Circulation pump
  • Expansion vessel

Related codes

F.77

Your boiler is struggling to remove excess water or is experiencing a communication error with an external safety part, causing it to shut down to prevent damage.

High Engineer only £120-280

Parts commonly replaced

  • Condensate pump
  • External accessory module
  • Exhaust gas flap

Related codes

F.78

Your boiler is struggling because it has detected an external control unit connected for hot water, but the required safety link or sensor connection is missing.

Medium Engineer only £120-250

Parts commonly replaced

  • Link wire bridge
  • External hot water sensor
  • PCB

Related codes

F.80

Your boiler is having trouble communicating with the temperature sensor in your hot water storage unit, meaning your water might not heat up correctly.

Medium Engineer only £120-250

Parts commonly replaced

  • actoSTOR NTC sensor
  • Wiring harness
  • Electronic interface module
Showing 51–57 of 57
F.81

Your boiler is struggling to heat your hot water tank effectively, causing it to time out before the water reaches the requested temperature.

Medium Engineer only £150-350

Parts commonly replaced

  • Diverter valve
  • Cylinder charging pump
  • Secondary heat exchanger
F.82

Your boiler has detected a communication issue with the safety component that prevents internal corrosion, meaning it cannot confirm the tank is protected.

Medium Engineer only £150-250

Parts commonly replaced

  • External current anode
  • PCB wiring harness

Related codes

F.83

Your boiler is failing to detect a change in temperature when it tries to fire up, which usually means there is either not enough water circulating inside the system or a sensor isn't reading correctly.

High Engineer only £120-250

Parts commonly replaced

  • NTC Temperature sensor
  • Central heating pump
  • System bypass valve

Related codes

F.84

Your boiler is confused because its temperature sensors are detecting heat patterns that do not match the expected flow of water, often caused by the sensors being swapped or losing contact with the pipes.

High Engineer only £120-250

Parts commonly replaced

  • Flow temperature sensor
  • Return temperature sensor
  • Wiring harness

Related codes

F.85

Your boiler is confused because its temperature sensors have likely been swapped or placed on the wrong pipes during installation or a recent repair.

High Engineer only £120-220

Parts commonly replaced

  • NTC Temperature sensors
  • Wiring harness

Related codes

F.90

Your boiler has lost the digital connection to its hot water storage tank, preventing the two units from communicating properly.

High Engineer only £120-250

Parts commonly replaced

  • Communication wiring harness
  • actoSTOR module PCB
F.94

Your boiler's internal water flow sensor is struggling to detect movement, which prevents the unit from starting safely.

High Engineer only £180-350

Parts commonly replaced

  • Flow sensor
  • Pump
  • Wiring harness

Related codes

72 operating states & engineer diagnostics

Not faults — these are normal operating states. Tap to expand.

P.00 Diagnostic Your boiler is currently running an automated self-cleaning cycle to remove air bubbles from the pipes and is not experiencing a fault. P.01 Diagnostic Your boiler is currently running a professional testing cycle to check it is functioning at full power after ignition, so no immediate action is required. P.02 Diagnostic Your boiler is currently performing a brief automatic test to ensure it can run safely at its lowest power setting. P.04 Diagnostic Your boiler has entered a configuration mode that resets its internal central heating settings back to their original factory defaults. P.06 Diagnostic Your boiler is currently in a manual maintenance mode used for filling or draining the system, so it is temporarily turned off to allow this process. P.14 Diagnostic Your boiler has initiated a diagnostic reset of its hot water settings, which is likely a temporary automatic process rather than a system failure. S.00 Status Your boiler is currently in standby mode because it has not received an instruction from your thermostat to provide heating or hot water. S.01 Status Your boiler is currently just in the process of starting up its internal air fan before it begins heating your home. S.2 Status Your boiler is currently warming itself up and checking that water is flowing correctly before it starts heating your home. S.02 Status Your boiler is simply preparing its internal pump before starting the process of heating your radiators; this is a normal operational status, not a fault. S.3 Status Your boiler is currently going through its starting sequence and is trying to ignite the flame to provide heating or hot water. S.03 Status Your boiler is currently trying to light the flame to start heating your home or water. S.04 Status Your boiler is currently performing a routine self-test or measurement, which is a normal part of its operational cycle. S.05 Status Your boiler is simply finishing its normal cooling process after the heating has turned off, so there is no need to worry. S.6 Status Your boiler has paused its operation because it is not receiving the correct feedback signal from the fan that pushes exhaust gases out of the system. S.06 Status Your boiler's fan is currently running to clear out any leftover gases after a heating cycle, which is a normal part of the cooling-down process. S.07 Status Your boiler is perfectly healthy and is simply circulating water around your radiators after the heating has switched off to ensure the system cools down safely. S.08 Status Your boiler is perfectly fine; it is currently in a brief, mandatory rest period to prevent the system from turning on and off too frequently. S.09 Status Your boiler is currently performing a routine self-check to ensure it is burning fuel efficiently, and this usually only lasts for a few moments. S.10 Status Your boiler has detected that you have turned on a hot water tap and is currently working to provide hot water to your home. S.11 Status Your boiler is currently preparing to heat your hot water by starting up its internal air extraction system. S.13 Status Your boiler is currently in the process of lighting the flame to heat your water, which is a normal part of its operation. S.14 Status Your boiler is currently heating water for your taps or shower, and this code simply indicates it is operating normally. S.15 Status Your boiler is simply finishing a safety cooling cycle after you have finished using hot water, and there is no cause for concern. S.16 Status Your boiler has detected an issue with the air fan system and has temporarily stopped heating to prevent further damage. S.17 Status Your boiler is simply performing a routine cooling-down process after you have finished using hot water, so there is no need for concern. S.19 Status Your boiler is currently running a brief internal testing sequence to check hot water settings and will return to normal operation by itself. S.20 Status Your boiler is not broken; it is simply letting you know it is currently busy heating up your hot water cylinder or keeping its internal hot water supply ready for use. S.21 Status Your boiler is currently going through its normal starting sequence to begin heating your water. S.22 Status Your boiler is preparing to heat your tap water by moving the pump into position; this is a normal operational state and not an actual fault. S.23 Status Your boiler is currently struggling to light the flame it needs to provide heating and hot water. S.24 Status Your boiler is currently operating normally and is in the process of heating your radiators. S.25 Status Your boiler is currently just finishing its standard cooling-down process after heating your water and is not experiencing a fault. S.26 Status Your boiler's fan is attempting to spin, but the electronic control board isn't receiving the correct signal back to confirm it is working properly. S.27 Status Your boiler is simply finishing a hot water cycle and is running the pump briefly to move the remaining heat into your radiators. S.28 Status Your boiler is temporarily pausing its heating cycle to prevent it from switching on and off too frequently after a short period of use. S.29 Status Your boiler is currently running an internal testing procedure for your hot water system and is not actually broken. S.30 Status Your thermostat has told the boiler to stay switched off because your home has reached the requested temperature or a timer has kicked in. S.31 Status Your boiler is currently set to its summer setting or is being told by your thermostat not to turn on the heating because it thinks the house is warm enough. S.32 Status Your boiler has temporarily paused operation because it detected unusual airflow that could lead to freezing or a fan issue. S.33 Status Your boiler is temporarily pausing because its internal safety sensors detect that the airflow needed to burn fuel safely is not moving correctly. S.34 Status Your boiler has noticed the temperature is very cold and has automatically turned itself on to prevent the internal pipes from freezing. S.36 Status Your temperature settings are too low for the boiler to start heating, so the system is currently on standby. S.37 Status Your boiler has temporarily stopped because it cannot get the internal fan spinning at the correct speed to safely clear exhaust gases. S.39 Status Your boiler has temporarily shut down because an external safety device, such as a condensate pump or an overheat sensor, has told it to stop running to prevent damage. S.40 Status Your boiler has detected a minor hiccup and is running in a limited mode to keep you warm while it monitors itself. S.41 Status Your boiler has too much water in the system, which is causing high pressure and has triggered a safety shutdown to protect the components. S.42 Status Your boiler has paused operation because it cannot safely clear waste gases or discharge wastewater produced during the heating process. S.46 Status Your boiler is struggling to keep its flame steady while running at a very low power setting and has temporarily shut down to protect itself. S.53 Status Your boiler has temporarily paused because the water is not flowing through the system fast enough, causing it to heat up too quickly. S.54 Status Your boiler has temporarily paused itself because it detected that the water temperature is rising too quickly, suggesting there isn't enough water circulating through the system. S.55 Status Your boiler is undergoing a mandatory safety check of its internal air quality sensors and is briefly pausing to ensure it is operating safely. S.56 Status Your boiler has detected high levels of carbon monoxide and has automatically paused operation to keep you safe while it waits for the fumes to clear. S.57 Status Your boiler is currently taking a short break to automatically re-calibrate its internal settings, during which it will temporarily pause heating and hot water production. S.58 Status Your boiler has detected unusual airflow patterns, likely caused by strong winds outside, and has temporarily limited its heat output to keep the system stable and quiet. S.59 Status Your boiler is temporarily pausing because it cannot detect enough water flowing through the system to heat it safely. S.76 Status Your boiler has detected that the water level inside the heating system has dropped too low and needs to be topped up to function correctly. S.86 Status Your boiler is reminding you that it is due for a routine professional service to check the water flow sensor. S.88 Status Your boiler is currently running an automated air-removal cycle to clear trapped air from the system, which is a normal process after maintenance or a power restart. S.92 Status Your boiler is currently running an automatic internal test to check that water is flowing correctly through the system. S.93 Status Your boiler is currently running an internal calibration check and is simply waiting for a few more minutes to complete its automatic system health test. S.96 Status Your boiler is performing a self-diagnostic check on its internal temperature sensors and has temporarily paused heating until it confirms everything is working correctly. S.97 Status Your boiler is currently performing a self-test of its internal pressure sensor and has temporarily paused heating until it finishes. S.98 Status Your boiler is temporarily pausing to double-check that its internal temperature sensors are reading correctly before it starts heating your home. S.99 Status Your boiler is currently performing an automatic internal check and is not experiencing a fault. T.01 Diagnostic Your boiler is struggling to circulate water properly because it cannot communicate correctly with its internal water pump. T.02 Diagnostic Your boiler is struggling to switch between heating your radiators and providing hot water to your taps, which may result in a loss of heating or hot water. T.03 Diagnostic Your boiler has identified a communication issue with its internal airflow system and has gone into a protective mode, which will prevent your heating and hot water from working. T.04 Diagnostic Your boiler is struggling to pump hot water to your separate hot water tank, which means your taps and shower may not get warm enough. T.05 Diagnostic Your boiler is having trouble moving water around the system, which means it cannot deliver heating or hot water effectively. T.06 Diagnostic Your boiler is having difficulty communicating with an external water pump, which may prevent your heating or hot water from circulating properly. T.08 Diagnostic Your boiler is currently performing a routine self-check as it starts up, and you do not need to take any action.