BoilerFaultCodesThe 5 most common faults
01
Lockout lamp illuminating (Overheat Thermostat) High DIY-safe
Your boiler has switched itself off to prevent damage because the unit has become too hot.
What to try first
Check that all radiator valves are fully openEnsure the system pressure is between 1 and 1.5 barWait for the boiler to cool down and gently press the reset button
02
Phase reversed neon (behind the knob) lighting High Engineer
Your boiler has detected that the incoming electrical wires are connected the wrong way around, preventing it from safely starting the main burner.
Full guide for Phase reversed neon (behind the knob) lighting03
Boiler automatically stops functioning High DIY-safe
Your boiler has temporarily stopped working because the internal drain pipe that removes waste water is likely blocked, often due to freezing during cold weather.
What to try first
Locate the external plastic condensate pipe that leads outside from your boiler.Check if the pipe is frozen, blocked, or trapped.Pour warm (not boiling) water over the external pipe to thaw any ice and clear the blockage.
04
Lockout lamp illuminating (Flue Thermostat) Emergency Engineer
Your boiler has automatically shut down because the exhaust pipe has become dangerously hot.
Full guide for Lockout lamp illuminating (Flue Thermostat)05
Running lamp flashing Low DIY-safe
Your boiler is currently in a normal waiting period and will start back up automatically in a few minutes.
What to try first
Wait for approximately 3 to 5 minutes for the anti-cycle timer to finish.Ensure the room thermostat is turned up to call for heat.Check that the clock or programmer is set to 'On'.
on the Malvern 20/26
Overheat ThermostatCirculation PumpThermistorElectrical wiring (correction)External fused spurFlue thermostat
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 Malvern 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