Keston E36 Fault Code: Causes, Fixes & Repair Costs
What does the Keston E36 fault code mean?
The E36 fault code on Keston boilers indicates that the control board has detected an open circuit on the flow (outlet) thermistor. In plain terms, the PCB is receiving no signal whatsoever from the sensor that monitors the temperature of water leaving the heat exchanger. Unlike a short circuit fault — where the signal is present but corrupted — an open circuit means the electrical path through the sensor has completely broken down. Because the boiler cannot safely monitor flow temperature, it will lock out and refuse to fire until the underlying problem is resolved and the reset button is pressed. Note for C36 and C37 owners: on those specific models, E36 and E37 relate to water pressure faults rather than a thermistor issue, so always cross-reference your model's documentation before proceeding.
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
- Loose or damaged wiring connections Common
The most frequent trigger. Vibration over time, corrosion, or a previous service where connectors were not pushed fully home can break the electrical circuit between the flow thermistor and the PCB. Even a partially dislodged connector can cause an open-circuit reading.
- Failed flow thermistor Common
The thermistor itself — a small temperature-sensitive resistor clipped to the flow pipework — can burn out or degrade, causing its resistance to rise beyond any usable range. A multimeter across the thermistor terminals will show no continuity when this has happened.
- Poor thermal contact from incorrect fitting Sometimes
When a thermistor is replaced without applying heat-sink compound between the sensor and the pipe, or is not clipped tightly, the reading can be absent or wildly inaccurate. A previous service carried out without following this step could trigger E36 intermittently.
- Water ingress near the sensor or wiring Sometimes
A slow leak close to the thermistor or along the wiring loom can cause corrosion, tracking, or short-circuit damage that ultimately results in an open-circuit condition at the PCB.
How to fix it
- Check you can safely access the boiler and note down the fault code DIY safe
Before doing anything else, confirm the display is showing E36. Take a photo if helpful. Make sure the boiler is not in a flooded or visibly damaged state — if there is any sign of a water leak around the boiler, turn off the water supply and call an engineer straight away.
- Attempt a boiler reset (once or twice only) DIY safe
Press and hold the reset button on the boiler's control panel as described in your user manual — switching the boiler off at the mains and back on will not clear a lockout. If the boiler fires and runs normally, monitor it closely. If E36 returns within a short time, do not keep resetting; a persistent fault needs an engineer's attention.
- Call a Gas Safe registered engineer to inspect the wiring loom Gas Safe engineer
The engineer will trace the wiring from the flow thermistor back to the PCB, checking every connector and section of cable for corrosion, heat damage, or loose terminals. This step alone resolves the fault in many cases and costs very little if no parts are needed.
- Engineer to test the flow thermistor with a multimeter Gas Safe engineer
By measuring resistance across the thermistor terminals at room temperature and comparing the reading against the manufacturer's specification, the engineer can confirm whether the sensor has failed. An open-circuit (infinite resistance) reading means the thermistor must be replaced.
- Engineer to replace the flow thermistor if faulty Gas Safe engineer
The replacement part is inexpensive. Critically, the engineer should apply a thin film of heat-sink compound between the new sensor and the pipe before clipping it firmly in place — this ensures accurate thermal contact and reduces the chance of the fault recurring after the repair.
- Engineer to inspect for water ingress and any leak sources Gas Safe engineer
If corrosion on the wiring or connector suggests moisture has been present, the engineer should trace and repair the source of any leak before closing up the boiler. Leaving a leak unaddressed could damage the replacement thermistor or, worse, the PCB.
- Engineer to reset, fire the boiler, and confirm the fault has cleared Gas Safe engineer
After all repairs, the engineer resets the boiler, allows it to run through a full heating cycle, and verifies that E36 does not return. System pressure, flow temperature readings, and general operation should all be checked before signing off the job.
Parts you may need
- Flow NTC thermistor (Keston C40/C55 compatible) · from £18
- Thermistor wiring harness/loom · from £25
- Heat-sink thermal compound (for thermistor fitting) · from £5
- Keston PCB (control board — if water damage is found) · from £290
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–£280, depending on the underlying cause.
Frequently asked questions
Can I fix the Keston E36 fault myself?
No. While you can safely attempt a single reset of the boiler, the diagnostic and repair work — testing wiring continuity, measuring thermistor resistance, and replacing components — must be carried out by a Gas Safe registered engineer. Keston also specifically recommends that only qualified engineers work on their appliances.
How much does it cost to fix a Keston E36 fault in the UK?
For the most common scenario — a failed thermistor or loose wiring — expect to pay roughly £100–£280 in total, covering the engineer's call-out, an hour or so of labour, and the part itself (typically £10–£25). Rates are higher in London and from large national firms. If the fault has caused water ingress that has damaged the PCB, a replacement board can add £200–£350 to the bill, though this is an uncommon outcome.
Why does my Keston boiler show E36 after a recent service?
E36 sometimes appears shortly after a service if the flow thermistor was disturbed and then refitted without heat-sink compound, or without being clipped tightly enough to the pipe. It can also occur if a wiring connector was not fully pushed home. Contact the engineer who carried out the service — this kind of post-service fault is usually quick and inexpensive for them to correct.
Is the Keston E36 the same fault on all Keston models?
No — this is an important distinction. On the C40 and C55, E36 means a flow thermistor open circuit. However, on the older C36 and C37 models, the E36 code refers to a water pressure fault, not a thermistor issue. Always check your specific model's manual or ask a Keston-experienced engineer to confirm the correct interpretation before authorising any repair work.