Overview
E44 on a Keston C36 combi means the boiler has detected an open circuit on the return temperature sensor (the thermistor that monitors the water coming back to the heat exchanger). The control electronics rely on both flow and return sensors to manage combustion, pump and safety limits; if the return sensor reading is missing (open circuit) the boiler will usually lock out or refuse to run because it cannot verify safe operating temperatures. This fault most often comes from a loose or disconnected connector, a broken or corroded wire, a failed thermistor, or less commonly a problem on the control PCB or wiring loom. Severity ranges from nuisance (loose connector) to requiring a parts replacement; the boiler is likely to be inoperative for heating and/or hot water while the fault is present. Some basic checks can be performed by a competent homeowner (reset, visual wiring check), but because the sensor is inside the boiler and the appliance involves gas, electricity and pressurised water, replacement or any work that requires opening the casing, draining the system or touching wiring should be carried out by a Gas Safe registered engineer.
Possible Cause: Return sensor open circuit – check connections to return sensor are in place
Troubleshooting Steps
Safety precautions:
1) If you are not competent with gas or electrical appliances, stop and call a Gas Safe engineer. Working on a boiler can be dangerous.
2) Before doing any checks isolate electrical supply to the boiler at the fused spur or consumer unit. If you will access internal components, isolate the gas supply as well and allow the boiler to cool fully.
3) Wear protective gloves and eye protection. Be aware of hot water and potential leaking when undoing pipework.
Initial homeowner checks (safe, no-boiler-opening steps):
1) Note the fault code and any other indicators. Press the boiler Reset button once and allow it to attempt to restart. If the fault returns immediately or within a short time, further investigation is needed.
2) Check for recent disturbances: has the boiler been serviced recently, has there been any work on pipework, or has the property experienced freezing or water ingress? These can break sensor wiring or the sensor itself.
3) Inspect any visible wiring and connectors outside the boiler (room thermostat, remote sensors). Make sure external connectors are seated and not visibly damaged.
If you are comfortable and competent with basic electrics and the boiler is isolated, further diagnostic steps (only if you understand electrical safety and local gas regulations):
1) Remove electrical supply to the boiler and, if you will access internal wiring, isolate the gas and allow the unit to cool. Remove the boiler case following the manufacturer’s instructions.
2) Locate the return thermistor: the manual/labels or the schematic inside the casing will show the return sensor position (usually clipped to the return pipe close to the heat exchanger and connected via push-on/two-pin connector to the PCB or wiring harness).
3) Visually inspect the sensor connector and wiring for loose pins, corrosion, chafing or breaks. Re-seat any push-on connectors firmly. If a connector is loose or visibly corroded, re-seating may restore operation.
4) If you have a multimeter and know how to use it safely, with the sensor disconnected you can check for continuity or resistance: an open circuit will show infinite/OL. A normal thermistor will show a finite resistance (not infinite); if you see OL/infinite, the sensor or its wiring is open and needs replacing. If you are unsure how to interpret readings, stop and call an engineer.
5) If wiring to the sensor is intact but the sensor reads open, the thermistor has likely failed. Replacement involves unclipping the sensor from the pipe, removing the push-on connector, fitting the correct replacement thermistor and reassembling. This typically requires isolation of the water circuit or careful working to avoid leaks and so is usually a job for a qualified engineer.
6) After any work, re-fit the casing, restore gas/electric supplies, repressurise the heating system if it was drained, purge air from radiators/boiler as required and reset the boiler. Observe whether the E44 returns.
When to call a professional and what to tell them:
1) Call a Gas Safe registered engineer if the simple re-seat/reset steps do not clear the fault, if you found damaged wiring, if you need the sensor replaced, or if the fault alternates with other codes. Tell them the boiler model (Keston C36), the displayed code E44, what checks you already performed (reset, visual wiring inspection, any multimeter result) and whether you had to drain or open the boiler.
2) Ask the engineer to check sensor wiring continuity from the sensor to the PCB, replace the return thermistor if faulty, and inspect the control board and connectors if the sensor and wiring test OK. Also ask them to perform a full commissioning check (repressurise, bleed air, run diagnostic tests) before leaving.
Important notes:
- Do not continue to operate the boiler if you suspect gas or electrical faults. - Do not attempt pipework, sensor replacement or PCB work unless you are certified and competent. - If in doubt or if the fault persists after basic checks, get a qualified engineer to prevent unsafe conditions and to restore correct operation.
Helpful Resources
Boiler Manual
Download the official PDF manual for the Keston C36 Combi Gas Boiler.