Common problems guide

The most common Gledhill boiler problems

The faults we see most often across the Gledhill range — with plain-English causes, what to check first, estimated repair costs, and whether it's a DIY job or needs a Gas Safe engineer. Based on fault-code data across 1 Gledhill boiler models.

63 unique codes
1 models covered
8 DIY-safe (top 8)
8 engineer needed

The most common Gledhill faults

01
P High DIY-safe

Your boiler has detected that the water pressure in the system is too low to run safely and has stopped working to protect itself.

What to try first

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

Est. cost
£0-120
Parts
Filling loop, Expansion vessel, Pressure relief valve
Full guide for GB35C P
02
D1 High DIY-safe

Your boiler has experienced a lockout, meaning it has switched itself off for safety after failing to fire correctly.

What to try first

Locate the reset button (labeled S1) on the control panel.Press and hold the reset button for a few seconds to see if the boiler restarts.If the boiler locks out again immediately, do not keep resetting it and call a professional.

Est. cost
£90-250
Parts
Ignition Electrode, Gas Valve, Flame Sensing Probe
Full guide for GB35C D1
03
Lockout error High DIY-safe

Your boiler has stopped working safely and needs a simple restart to clear a temporary system error.

What to try first

Locate the 'Reset' button (labeled S1) on the boiler control panel.Press and hold the Reset button for 2-3 seconds.Wait for the boiler to complete its startup sequence and check if the error light stops flashing.

Est. cost
£0-120
Parts
Ignition Electrode, Gas Valve, Flame Sensing Probe
Full guide for GB35C Lockout error
04
Low pressure safety trip High DIY-safe

Your boiler has stopped working because the water pressure inside the system has dropped too low to operate safely.

What to try first

Check the pressure gauge; it is likely below 1.0 bar.Locate the filling loop (braided silver hose) usually under the boiler.Open the valves slowly to top up the water until the gauge reaches 1.5 bar.Close the valves tightly and press the S1 button on the boiler to reset the system.

Est. cost
£0-120
Parts
Filling loop, Expansion vessel, Pressure sensor
Full guide for GB35C Low pressure safety trip
05
Overheat safety trip High DIY-safe

Your boiler has switched itself off to prevent it from getting too hot and causing internal damage.

What to try first

Wait for the boiler to cool down completelyLocate the S1 reset button on the control panelPress and hold the button to clear the fault and restart the unit

Est. cost
£100-250
Parts
Overheat Thermostat, Circulation Pump, Heat Exchanger (de-scale)
Full guide for GB35C Overheat safety trip
06
Ignition lockout High DIY-safe

Your boiler has tried to start three times but hasn't been able to light the flame, so it has shut down for safety.

What to try first

Check that your gas pre-payment meter (if you have one) has credit.Ensure your external gas emergency control valve is turned to the 'on' position.Press the central button (S1) beneath the boiler display to attempt a reset.

Est. cost
£100-£250
Parts
Ignition electrode, Gas valve, Spark generator
Full guide for GB35C Ignition lockout
07
C1 High DIY-safe

Your boiler has tried to light several times but has failed to create a flame, meaning your heating and hot water will not work.

What to try first

Check that your gas emergency control valve is in the 'on' positionIf you have a pre-payment meter, ensure you have credit and that the meter hasn't locked outPress the Reset button on the boiler control panel for 3 seconds

Est. cost
£120-350
Parts
Ignition electrode, Gas valve, Flame sensing probe
Full guide for GB35C C1
08
C13 High DIY-safe

Your boiler has experienced an internal software or communication error and has stopped working to protect itself.

What to try first

Turn the boiler's electrical power switch offWait for approximately 30 secondsTurn the power back on to see if the system clears the internal error

Est. cost
£100-£350
Parts
Main Printed Circuit Board (PCB), Display Interface
Full guide for GB35C C13

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