Common problems guide

The most common Potterton Prima 30B problems

The faults most likely to send a Potterton Prima 30B into lockout — with plain-English causes, what to check first, estimated repair costs and whether you need a Gas Safe engineer.

This model is discontinued. Parts can be harder to source and expensive. If repair costs are mounting, a new A-rated boiler may be the smarter choice.
42 documented codes
6 most common
1 DIY-safe checks
5 engineer needed

The 6 most common faults

01
Overheat Limit Thermostat High DIY-safe

Your boiler has shut itself down completely because the internal water temperature has become too hot.

Why it happens: This fault occurs when water inside the boiler reaches a dangerous temperature, triggering a mechanical safety switch to cut off the gas for your protection. The most common reasons are a failed pump failing to move hot water away from the heat exchanger, or a buildup of limescale and sludge creating 'hot spots' within the unit.

What to try first

Wait for the boiler to cool down for at least 30 minutesLocate the reset button on the underside of the boiler casingPress the button firmly to reset the thermostat and relight the pilot light following the instructions on the boiler door

Est. cost
£100-220
Parts
Overheat Limit Thermostat, Pump, Flow Thermostat
Full guide for Overheat Limit Thermostat
02
Lockout High Engineer

Your boiler has failed to start after three attempts, often due to a temporary interruption in gas supply or air in the lines, and has turned itself off for safety.

Est. cost
£120-250
Parts
Ignition lead, Ignition electrode, PCB (Printed Circuit Board)
Full guide for Lockout
03
Mains ON Only High Engineer

Your boiler has detected an internal fault that has caused it to stop heating your home and hot water for safety reasons.

Est. cost
£150-300
Parts
PCB, Ignition lead, Gas valve
Full guide for Mains ON Only
04
Flashing Red LED High Engineer

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

Est. cost
£120-250
Parts
Overheat thermostat, Pump, PCB
Full guide for Flashing Red LED
05
Red light flashing High Engineer

Your boiler has failed to start the flame after three attempts and has locked itself down for safety.

Est. cost
£120-250
Parts
Ignition electrode, PCB (Printed Circuit Board), Gas valve
Full guide for Red light flashing
06
Green light flashes rapidly High Engineer

Your boiler is attempting to start but failing to stay lit, which means it cannot provide heat or hot water.

Est. cost
£150-300
Parts
Ignition lead, Ignition electrode, PCB
Full guide for Green light flashes rapidly
Parts most often replaced

on the Potterton Prima 30B

Overheat Limit ThermostatPumpFlow ThermostatIgnition leadIgnition electrodePCB (Printed Circuit Board)

Based on parts cited in our fault code database. Your engineer will confirm what's actually needed after diagnosis.

Call a Gas Safe engineer if…

  • You can smell gas or see signs of a leak
  • The Potterton shows an Emergency or High severity code
  • The boiler keeps locking out after repeated resets
  • You've tried the DIY checks and the fault hasn't cleared
  • There's visible water leaking from the boiler
  • The flame is yellow or orange instead of blue
See the full 42-code list for the Potterton Prima 30B

Related guides

Boiler losing pressure → No hot water → When to replace → Symptom checker →