Overview
E40 on a Potterton Assure (Combi or System) indicates a fault with the central heating return NTC sensor (a negative temperature coefficient thermistor) or its wiring. The boiler monitors the temperature of the water returning from the heating circuit; if the control detects an implausible reading (open circuit, short circuit, or out-of-range value) it will log E40 and may lock out or limit operation to protect the boiler and maintain safe control of temperatures. This fault most commonly comes from a failed sensor, a loose/corroded connector, damaged wiring, or occasionally a PCB or connector fault. Severity is moderate: the boiler may refuse to fire for heating or may run incorrectly, causing loss of central heating and potential inefficient operation. It is not usually an immediate safety risk like a gas leak, but because it affects safe temperature control you should not ignore it. Some basic visual checks and a reset are suitable for a competent homeowner, but electrical and sensor replacement, measurements with meters, or any work inside the boiler must be carried out by a Gas Safe registered engineer. If the E40 clears after a simple reset and the system behaves normally, monitor it. If it returns, or if you are unsure about any step, arrange professional service. Incorrect repairs or isolated gas/electrical work can be hazardous and may void warranties.
Possible Cause: Central heating return negative temperature coefficient (NTC) fault
Troubleshooting Steps
Safety precautions:
- Do not attempt internal boiler repairs unless you are qualified. Work on sealed parts, gas trains, or PCB connections should be left to a Gas Safe registered engineer.
- Isolate electrical supply to the boiler before accessing any covers (turn off at the isolator or consumer unit). Allow the boiler to cool before touching pipework or components.
- Do not interfere with the gas supply or gas components. If you suspect a gas issue, turn off the gas supply at the meter and call a Gas Safe engineer.
Initial homeowner checks (safe, simple steps):
1. Note the conditions when the error occurred (cold start, after service, after power cut). Take a photo of the display and any other codes.
2. Try a basic reset: switch the boiler off at the programmer and the isolator, wait 30 seconds, then power back on and reset the boiler using the reset button per the manual. If E40 does not reappear and heating works, monitor for recurrence.
3. Check system pressure on the boiler gauge. If pressure is very low (<1.0 bar), repressurise to the recommended level (typically ~1.2–1.5 bar) following the boiler manual. Low pressure can cause circulation/temperature anomalies.
4. Inspect the visible pipework and sensor areas for obvious damage, corrosion, or water ingress. Look for loose connectors or unplugged sensor leads (only inspect visually; do not pull on wiring when live).
5. Ensure room/programmable thermostats and external controls are calling for heat correctly.
If the fault persists and you are competent with basic electrical measurements (only perform with boiler isolated):
1. Locate the central heating return NTC sensor. Refer to the boiler schematic in the manual for exact position (typically clipped to the return pipe or mounted in the hydraulic block).
2. With the boiler isolated from mains, disconnect the sensor connector at the wiring loom. Visually inspect the connector for corrosion, bent pins, or water damage.
3. Measure the sensor resistance with a multimeter. Check the boiler manual or part number for the nominal thermistor characteristic; many NTC sensors are around 10 kΩ at 25 °C, but confirm the exact value for your model. The resistance should change if you warm or cool the sensor slightly (hold briefly between fingers or apply a cold cloth) — resistance should decrease as temperature rises. An open circuit (infinite resistance) or a short (very low ohms) indicates a faulty sensor.
4. If the sensor wiring shows damage, trace the wiring back to the PCB and check connectors at both ends (with power off). Repair or replace damaged wiring only if you are qualified to do safe, compliant electrical repairs; otherwise call an engineer.
Replacement and restart steps (recommended to be performed by a qualified engineer):
1. If the sensor checks bad, replace it with the correct OEM part number for the Potterton Assure. Fit the new sensor to the return pipe/housing and secure the connector properly.
2. If connector or harness faults are found, replace or repair the connector and ensure proper strain relief and routing to prevent future chafing or water ingress.
3. After any work, restore power, repressurise the system if necessary, clear the fault code via the boiler reset, and run the boiler to confirm normal operation. Check for stable flow and return temperatures and that the E40 code does not reappear.
When to call a professional (must-call situations):
- The fault returns after reset or after replacing wiring/sensor attempts.
- You find water ingress, burnt connectors, PCB damage, or are unsure whether the sensor wiring is safe to repair.
- You are not competent with electrical testing, component replacement, or isolating the boiler correctly.
- The boiler shows additional faults (overheat, circulation faults, repeated lockouts).
Final notes:
- Keep a record of the fault code occurrences and any photos you took; this helps the engineer diagnose intermittent wiring or PCB faults.
- Do not attempt gas valve, PCB, pump, or internal wiring repairs yourself unless Gas Safe and competent in electrical work; incorrect repairs are dangerous and may make the appliance unsafe.
- A Gas Safe registered engineer should complete any replacement parts inside the boiler, test the system, and certify the work where required.
Helpful Resources
Boiler Manual
Download the official PDF manual for the Potterton Assure Combi and system.