Keston E31 Fault Code: Causes, Fixes & Repair Costs
What does the Keston E31 fault code mean?
The E31 code signals a problem with the flow temperature sensing circuit — the part of the boiler that monitors how hot the water is as it leaves the heat exchanger. Importantly, what E31 means depends on your specific Keston model: on the C36 Combi, it indicates an open circuit in the flow sensor (an interrupted electrical signal), while on the C40 and C55 Combis, it indicates a short circuit in the flow thermistor (an unwanted electrical connection, often caused by moisture). In both cases the boiler detects that it can no longer trust its temperature readings, locks out as a safety precaution, and will not restart until the Reset button is pressed. Left unresolved, you will have no heating or hot water.
General guidance only — not a substitute for professional advice, and not certified by a Gas Safe engineer. Always have your specific appliance assessed by a qualified professional. Any gas work must be carried out by a Gas Safe registered engineer. If you smell gas or suspect carbon monoxide, leave the property and call the National Gas Emergency line on 0800 111 999.
Common causes
- Failed flow thermistor or sensor Common
The thermistor itself has developed an internal fault — either an open circuit (C36) or a short circuit (C40/C55). These components degrade over time, particularly in older systems where water quality is poor, and are the single most common reason this code appears.
- Loose or corroded wiring connections Common
The push-fit connectors joining the thermistor to the wiring loom can work loose through vibration, or corrode due to age and damp conditions inside the boiler casing. A poor connection produces erratic or absent signals that the control board interprets as a sensor fault.
- Water ingress causing a short circuit Sometimes
Particularly relevant on the C40 and C55, where the fault description is specifically a short circuit. A small internal leak near the sensor or its wiring can introduce moisture that bridges the electrical contacts, immediately triggering E31. Your engineer should check for signs of any leak while inspecting the thermistor.
- Poor circulation or pump issue Sometimes
Restricted water flow through the heat exchanger — caused by a struggling pump, sludge build-up, or a partially closed valve — can produce unusual temperature readings that confuse the control board. While this is less commonly the root cause, it is worth ruling out during diagnosis.
- Faulty PCB (control board) Rare
If the thermistor, its wiring, and the circulation all test as healthy, the fault may lie with the PCB itself, which processes the sensor signal. PCB faults are the least likely explanation but should be considered once all other causes have been eliminated by a Gas Safe engineer.
How to fix it
- Reset the boiler once or twice DIY safe
Switch the boiler off at the mains, wait around 30 seconds, then switch it back on and press the Reset button. If the E31 clears and does not return, it may have been a transient electronic glitch. If it returns promptly or after a short time, the underlying fault needs proper diagnosis — do not keep resetting repeatedly as this can mask developing problems.
- Check the boiler casing for visible signs of a leak DIY safe
With the boiler switched off, have a look around the bottom of the unit and on the floor beneath it for any damp patches or water stains. If you spot evidence of a leak, note it down to tell your engineer — water near the sensor wiring is a likely cause on the C40 and C55 models.
- Inspect and test the flow thermistor and its wiring connections Gas Safe engineer
A Gas Safe engineer will remove the push-fit connectors from the thermistor, check for corrosion or moisture at the terminals, and test the thermistor's resistance with a multimeter. On the C36 an open-circuit reading confirms a broken sensor or wiring; on the C40/C55 a short-circuit reading points to the thermistor or water ingress. Loose or corroded connectors can often be cleaned and refitted rather than replaced outright.
- Replace the flow thermistor if found to be faulty Gas Safe engineer
If the thermistor tests outside its expected resistance range, your engineer will fit a replacement. The procedure involves disconnecting the push-fit connectors (noting their positions), removing the old thermistor, and fitting the new one in the same orientation. Keston thermistors are relatively inexpensive parts, so this is usually a straightforward repair.
- Investigate circulation and pump performance Gas Safe engineer
If the thermistor and wiring both check out correctly, your engineer should assess whether restricted water flow is contributing to abnormal temperature readings. This may involve checking the pump outlet thermistor, testing pump output, and looking for evidence of sludge or blockages in the system.
- Assess the PCB if all other components test as healthy Gas Safe engineer
As a last resort, if the sensor circuit and circulation are confirmed to be functioning correctly, the control board may be misreading the sensor signal. PCB diagnosis and replacement must be carried out by a qualified engineer and is the most expensive repair associated with E31.
- Call a Gas Safe registered engineer Gas Safe engineer
Beyond a single reset attempt and a visual check for leaks, all diagnostic and repair work on this fault requires a Gas Safe registered engineer. You can verify any engineer's registration at gassaferegister.co.uk before they begin work. Ask for a written fixed-price quote covering parts and labour before authorising the repair.
Parts you may need
- Keston flow thermistor (NTC sensor) · from £25
- Thermistor wiring harness / loom · from £35
- Keston PCB / control board · from £280
The exact spare depends on your boiler's GC number (on the data badge). Check this against the part before buying.
Typical repair cost
Expect to pay roughly £100–£220, depending on the underlying cause.
Frequently asked questions
Why does E31 mean something different on my Keston C36 compared to a C40 or C55?
Keston used the same fault code number across their range but applied it to slightly different electrical faults depending on the model. On the C36 it flags an open circuit in the flow sensor — meaning the electrical signal has been broken, suggesting a disconnection or failed sensor. On the C40 and C55 it flags a short circuit — meaning two parts of the circuit that shouldn't be touching are connecting, often because of moisture. Both point to the same component (the flow thermistor circuit), so the repair approach is similar even though the underlying electrical nature of the fault differs.
Can I fix Keston E31 myself?
The only thing a homeowner can safely do is attempt a reset once or twice and have a look for obvious water leaks around the boiler. All further work — testing the thermistor, checking wiring, or replacing parts — must be carried out by a Gas Safe registered engineer. It is a legal requirement in the UK that gas boiler repairs are performed by a registered professional.
How much will it cost to fix E31 on a Keston boiler?
For the most common repair — replacing the flow thermistor and sorting any wiring — expect to pay roughly £100–£220 all in, covering the call-out, labour, and the part itself (which typically costs £15–£40). If the PCB turns out to be at fault, costs rise considerably; a replacement control board for a Keston boiler can be £200–£350 for the part alone, bringing the total bill to £300 or more. Prices in London and the South East tend to run 15–25% higher than the national average. Always get a written fixed-price quote before work begins.
What happens if I keep resetting the boiler without fixing E31?
Resetting the boiler temporarily clears the lockout and allows it to attempt a restart, but if the underlying fault is still present it will lock out again — sometimes within minutes. Repeatedly resetting without a proper repair risks the boiler running in an abnormal condition and can potentially mask a developing problem such as a water leak near electrical components. Limit resets to one or two attempts and call an engineer if the code returns.