Error E31

Safety first: do not work on gas components unless you are a qualified Gas Safe engineer. Before any checks isolate electrical power at the fused spur or consumer unit if you need to remove the boiler cover. If you suspect a gas leak or smell gas, do not attempt diagnostics — turn off the gas at the meter and call your gas supplier or an emergency Gas Safe engineer.

Initial homeowner checks (safe, non-invasive):

1) Try a simple reset: use the boiler reset button or switch the boiler off at the fused spur for 30 seconds and back on. If E31 clears and the boiler runs, monitor for recurrence. Repeated returns of the code mean a persistent fault. 2) Check visible signs of water leaks around the boiler, the flow pipework and the condensate trap. Moisture or leaks can cause short circuits. 3) Check system pressure on the pressure gauge; very low pressure can cause other faults and mask issues — bring pressure to the recommended range (refer to the boiler manual) only if you know how to do so safely. 4) Visually inspect any accessible wiring or connectors outside the boiler for loose, corroded or wet terminals. Do not open the boiler casing during this step.

If you are competent with basic electrical testing and comfortable with the risk, you can perform the following diagnostic checks, but only after isolating mains power and following safe isolation procedures. If you are unsure at any point stop and call a Gas Safe engineer. 1) Remove power (isolate the boiler at the fused spur) and remove the outer case only if you are confident and allowed to do so; otherwise leave this to an engineer. 2) Visually inspect the flow thermistor connector and wiring on the PCB for corrosion, burnt marks or water ingress. Dry and reseat any push-on connectors if they are visibly dirty or wet. 3) With power still isolated, unplug the thermistor plug from the PCB and measure resistance with a multimeter across the thermistor terminals: at typical room temperature the thermistor should show a finite resistance (not zero and not infinite). A reading near 0 ohms indicates a short; an open/infinite reading indicates an open circuit. If you have a multimeter with continuity to earth, also check there is no continuity between either thermistor wire and earth/ground. 4) If the thermistor measures as shorted (near zero ohms) or shows signs of water damage or corrosion, replacing the thermistor is the likely fix. Replacement steps per the Keston service manual: isolate power, remove outer case front panel, note and remove the push-on connector(s), unclip the thermistor from the flow pipe, fit the new thermistor and reconnect the push-on connectors in the correct positions, reassemble, restore power and test. Do not proceed with these steps unless you are competent and legally allowed to work on the appliance.

When to call a professional: If the reset does not clear the code, if you find water ingress, if wiring or PCB damage is suspected, if the thermistor replacement does not clear the fault, or if you are not confident performing the internal checks or swap, call a Gas Safe–registered heating engineer. Only a qualified engineer should perform internal repairs, replace sensors if removal of the casing is required, or carry out work that affects gas or sealed components. The engineer can confirm the sensor is faulty, replace the thermistor, repair wiring or replace the control box if necessary, and fully test and recommission the boiler.