Baxi Promax Ultra Fault codes & diagnostics

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

Combi Natural Gas 28-33 kW 2008-2015 Discontinued

Replaced by: Baxi Platinum Combi HE

This boiler uses indicator light combinations

Note which lights are on, off, or flashing and at what speed. Match the combination to the cards below. Do not reset until you have noted the pattern. How to read fault codes →

Severity at a glance

Each cell = one fault code. Hover to identify.
14 High 2 Medium
Just want the most common Baxi Promax Ultra faults?
The top 6 problems in plain English — with causes, DIY checks and repair costs.
Fault codes

All 16 documented codes

Tap any card for details

LED / flash-pattern codes
Indicator Neons

Your boiler uses a system of flashing or steady lights to tell you what part of the heating cycle it is in or if it has encountered an error that needs attention.

Medium DIY-safe £80-250

What to check first

  1. Check if the Power on neon is illuminated green
  2. Identify if any lights are flashing red, indicating a lockout
  3. Turn the selector switch to the 'Reset' position for five seconds and release

Parts commonly replaced

  • Printed Circuit Board (PCB)
  • Selector Switch
  • Overheat Thermostat
Full explanation for Indicator Neons
Overheat indicator

Your boiler has switched itself off because it has become too hot internally, often due to a temporary blockage or a circulation issue.

High DIY-safe £100-£250

What to check first

  1. Check that your radiator valves are open
  2. Ensure there is enough water pressure in the system (usually 1.0 to 1.5 bar)
  3. Press the white Reset button on the front control panel

Parts commonly replaced

  • Overheat Thermostat
  • Circulation Pump
  • NTC Sensor
Full explanation for Overheat indicator
Digital codes
E01

Your boiler has failed to light, which means it cannot heat your water or your home, usually due to a lack of gas or a problem with the ignition spark.

High DIY-safe £100-£350

What to check first

  1. Check if other gas appliances in your home, such as a hob, are working to confirm your gas supply is active.
  2. If you have a prepaid meter, ensure you have sufficient credit available.
  3. Press and hold the 'R' or Reset button for 5 seconds to see if the boiler restarts.

Parts commonly replaced

  • Ignition Electrode
  • Gas Valve
  • PCB (Printed Circuit Board)

Related codes

Full explanation for E01
E02

Your boiler has become too hot and has shut itself down as a safety precaution to prevent damage.

High DIY-safe £100-£250

What to check first

  1. Check that your radiator valves are open and not turned to zero
  2. Ensure there is enough water pressure in the system (usually 1.0 to 1.5 bar)
  3. Wait for the boiler to cool down and press the 'Reset' button

Parts commonly replaced

  • Overheat Thermostat
  • Circulation Pump
  • Main Heat Exchanger (if blocked)
Full explanation for E02
E03

Your boiler's safety sensors have detected a problem with how waste gases are being cleared from the system, preventing it from firing up.

High Engineer only £120-£280

Parts commonly replaced

  • Air pressure switch
  • Flue fan
  • Venturi tube

Related codes

Full explanation for E03
E05

Your boiler has detected an issue with the sensor that monitors the temperature of the water used to heat your radiators.

High Engineer only £90-£160

Parts commonly replaced

  • NTC Temperature Sensor (Flow)
  • Wiring Harness

Related codes

Full explanation for E05
E06

Your boiler's hot water sensor has stopped working correctly, which means the system cannot accurately measure how hot your water is.

Medium Engineer only £90-160

Parts commonly replaced

  • Domestic Hot Water (DHW) NTC thermistor
  • Wiring harness

Related codes

Full explanation for E06
E10

Your boiler has detected that the water pressure in the system is too low to run safely and has shut down as a precaution.

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. Open the filling valves slowly 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
  • Pressure sensor
  • Expansion vessel
Full explanation for E10
E25

Your boiler has shut down because it is getting too hot too quickly, usually because the water isn't moving through the system properly.

High DIY-safe £180-350

What to check first

  1. Check that your radiator valves and any external isolation valves are fully open
  2. Ensure there is enough water pressure in the system (typically 1.0 to 1.5 bar)
  3. Try resetting the boiler to see if the error clears

Parts commonly replaced

  • Circulation pump
  • Flow/Return temperature sensors (NTC thermistors)
  • Main heat exchanger (de-scaling)
Full explanation for E25
E35

Your boiler thinks it sees a flame even though it hasn't signaled for one to start, causing it to shut down for safety.

High Engineer only £120-300

Parts commonly replaced

  • Flame sensing electrode
  • Gas valve
  • Printed Circuit Board (PCB)
Full explanation for E35
Flame Failure

Your boiler has stopped working because it cannot light the gas flame or the flame has gone out unexpectedly.

High DIY-safe £100-280

What to check first

  1. Check if other gas appliances, like a hob, are working to ensure gas is reaching the property.
  2. If it is freezing outside, check that the white plastic condensate pipe leading outdoors isn't frozen.
  3. Press and hold the red 'Flame Failure' reset button for five seconds to see if the boiler restarts.

Parts commonly replaced

  • Ignition Electrode
  • Flame Sensing Probe
  • Gas Valve
Full explanation for Flame Failure
Flame Failure Reset Button

Your boiler has lost its flame and needs you to press a button to try and restart the ignition process.

High DIY-safe £0-250

What to check first

  1. Locate the reset button or selector switch on the front control panel.
  2. Press and hold the button for at least five seconds.
  3. Wait for the boiler to attempt to reignite; if it fails three times, call a Gas Safe engineer.

Parts commonly replaced

  • Ignition Electrode
  • Gas Valve
  • Flame Sensing Lead
Full explanation for Flame Failure Reset Button
Overheat

Your boiler has got too hot and has switched itself off as a safety precaution to prevent damage.

High DIY-safe £80-250

What to check first

  1. Wait for the boiler to cool down for 30 minutes
  2. Check that all radiator valves are open and not blocked
  3. Press the white Reset button on the front control panel

Parts commonly replaced

  • Overheat Thermostat
  • Circulation Pump
  • Main Heat Exchanger cleaning
Full explanation for Overheat
Pressure Gauge (below 0.5 bar)

Your boiler has stopped working because the water pressure inside the heating system has dropped too low.

High DIY-safe £0-120

What to check first

  1. Locate the filling loop (a flexible silver hose) underneath or near the boiler
  2. Slowly open the one or two taps on the hose while watching the pressure gauge
  3. Close the taps once the needle reaches between 1 and 1.5 bar

Parts commonly replaced

  • Filling loop
  • Pressure sensor
  • Automatic air vent
Full explanation for Pressure Gauge (below 0.5 bar)
Red reset button illuminated

Your boiler has failed to ignite and has locked itself out for safety, meaning you currently have no heating or hot water.

High DIY-safe £100-350

What to check first

  1. Check that your gas pre-pay meter has credit (if applicable).
  2. Ensure your gas stopcock is fully open.
  3. Press and hold the illuminated red reset button for at least 5 seconds.

Parts commonly replaced

  • Ignition Electrode
  • Flame Sensing Lead
  • Gas Valve
  • PCB
Full explanation for Red reset button illuminated
Safety Thermostat neon

Your boiler has switched itself off because it has reached an unsafe internal temperature and needs to cool down.

High DIY-safe £120-350

What to check first

  1. Check that your radiator valves (TRVs) are open and not all turned to zero.
  2. Ensure your system water pressure is between 1 and 1.5 bar on the pressure gauge.
  3. Allow the boiler to cool down for 30 minutes, then press the reset button or turn the selector switch to 'R'.

Parts commonly replaced

  • Circulating pump
  • Overheat thermostat
  • Main heat exchanger (flushing)
Full explanation for Safety Thermostat neon