Overview
E45 on a Keston C36 combi indicates the boiler control has detected a short circuit on the flue temperature sensor (thermistor). The flue sensor measures the temperature of the flue gases and provides critical information to the control board to ensure safe combustion and to detect flue overheat conditions. A short circuit means the sensor wiring or the sensor itself is presenting a very low resistance (near zero), so the control cannot get a valid temperature reading. This is a safety-related fault and the boiler will usually lock out when it appears. Causes are typically damaged, chafed or water-damaged wiring, a failed sensor, poor connector contact, or less commonly a PCB wiring fault. Some basic visual checks can be done by a competent homeowner (power isolated), but because the fault affects the combustion safety system and may require access inside the boiler, replacement or in-depth electrical diagnostics should be carried out by a Gas Safe registered engineer. Do not attempt gas-system repairs yourself. If you smell gas or suspect a carbon monoxide risk, stop using the appliance, ventilate the area and call the gas emergency number immediately. If the error appears after a transient event (power surge, recent work, or moisture ingress) a simple reset may clear it temporarily, but recurring E45 faults need professional investigation. Continued operation with a faulty flue sensor is not recommended because the boiler cannot reliably monitor flue temperature and may not respond correctly to unsafe conditions.
Possible Cause: Flue sensor short circuit – check connections to flue sensor
Troubleshooting Steps
Safety precautions:
- If you smell gas or have any CO alarm activation, evacuate the property, ventilate where safe and call the gas emergency service immediately. Do not try to fix the boiler.
- Before opening the boiler, isolate the electrical supply at the boiler isolator or the consumer unit. Leave the power off while you access internal wiring.
- If you are not confident working with electrical wiring or opening the appliance casing, stop and call a Gas Safe registered engineer. Do not tamper with gas or combustion components.
- Keep children and pets away while working. Ensure the area is dry and well lit.
Initial homeowner checks (no tools or minimal tools):
1. Note the fault: record the error E45 and whether it is steady or intermittent. Take a photo of the display and any status LEDs for the engineer.
2. Try a simple reset: switch the boiler off at the programmer or isolator for 30 seconds, then switch back on and attempt to restart. Do not repeatedly reset more than once or twice; repeated lockouts can cause other issues.
3. Check the visible external flue and surrounding area for signs of damage, moisture or blocking. Also check for obvious water leaks near the boiler that may have dripped onto wiring.
4. Check that any condensate pipe is intact and not overflowing into the flue area (condensate leaks can cause moisture on sensors).
Diagnostic steps you can do if competent with basic tools and comfortable isolating electricity:
1. Isolate electrical supply to the boiler at the isolator or consumer unit and confirm the display is unpowered.
2. Remove the outer casing following the manufacturer instructions. Keep screws and panels safe. Only proceed if you are confident and do not disturb gas connections.
3. Locate the flue sensor: it is a small thermistor probe/wire located in or near the flue assembly/heat exchanger. Refer to the boiler manual or labelling if available.
4. Visually inspect the sensor wiring and connector: look for loose pins, corrosion, burn marks, crushed or chafed insulation, and moisture. If the connector is simply loose, re-seat it firmly and reassemble to test.
5. Dry any damp areas and connectors with a dry cloth and allow to air-dry. Do not use direct heat on components.
6. If you have a multimeter and know how to use it safely: with the boiler power still isolated, measure resistance between the two sensor wires. A short circuit will show very low resistance (approaching 0 ohms). You can also check for a short to earth/ground by measuring between each sensor wire and the metal chassis; a short to chassis is a fault. Note: exact resistance values for a thermistor depend on type and temperature; the important check for E45 is detecting an unexpected near-zero value or short to earth.
7. Inspect the cable route for pinched sections where wiring passes through panels or restraint clips; repair or replace damaged cable if you are competent to do so, otherwise stop and call an engineer.
8. If the connector pins are corroded or bent, gently clean and re-seat. If pins are damaged or melted, do not try to jury-rig; call an engineer.
If the sensor wiring and connector appear sound but the fault remains after reassembly and reset:
- The flue sensor itself may have failed and will likely need replacing, or there may be a fault on the PCB/control board. Both are tasks for a Gas Safe engineer.
- If the fault follows replacement of the sensor wiring or sensor and persists, the engineer will check the control PCB inputs and the rest of the combustion safety chain.
When to call a professional:
- If the fault remains after the basic visual checks and a single reset.
- If you found damaged wiring, corroded connectors, an intermittent fault, or are unable to safely test with a multimeter.
- Any work that requires removing gas-side or combustion components, replacing the sensor, replacing PCB, or further electrical testing should be done by a Gas Safe registered engineer.
Additional notes for the engineer visit:
- Provide the engineer with the error code (E45), when it started, any photos, and whether the fault is permanent or intermittent.
- Mention any recent works, power cuts, or water leaks near the boiler.
Do not operate the boiler for prolonged periods with this fault present. The flue sensor is part of the safety system that prevents unsafe combustion and overheating; a registered engineer should resolve persistent E45 faults.
Helpful Resources
Boiler Manual
Download the official PDF manual for the Keston C36 Combi Gas Boiler.