Overview
E154 on a Potterton Assure combi or system boiler indicates the boiler has detected a problem with the flow/return temperature sensing — essentially a flow/return sensor temperature test has failed or the control is seeing an unexpected mismatch. The boiler constantly monitors the temperature of the water leaving (flow) and returning (return) the primary heat exchanger. If the sensors disagree, read out impossible values, or the expected temperature difference is missing, the boiler will flag E154 to protect itself and prevent unsafe operation. Common reasons for E154 are a faulty NTC temperature sensor on the flow or return pipe, damaged or loose sensor wiring or connectors, poor primary circulation (jammed pump, airlock, blocked heat exchanger or closed valves), or an intermittent PCB/sensor interface fault. Severity ranges from moderate to high: this fault often prevents hot water and central heating from working correctly and can cause the boiler to lock out to avoid overheating or running dry. Some basic checks and resets can be safely done by a homeowner, but accurate diagnosis and replacement of sensors, pump work or any internal wiring and gas-side components should be carried out by a Gas Safe registered engineer. If E154 clears after simple steps (reset, repressurise, bleed), monitor the boiler; repeated or persistent occurrences should be treated as a professional service call. Do not attempt to work on the gas, burner, sealed hydraulic components or replace internal electrical parts unless you are qualified — these are safety-critical tasks.
Possible Cause: Flow/return sensor temperature test
Troubleshooting Steps
Safety precautions:
- If you smell gas, leave the property immediately and call the gas emergency number — do not operate electrical switches or the boiler.
- Before touching any electrical parts or wiring, isolate the boiler at the mains isolator and/or switch off the consumer unit.
- Do not open sealed combustion or gas components.
- If there is any significant water leakage, isolate the water supply to the boiler and turn off the electricity.
Initial homeowner checks (safe, simple steps):
1) Note the exact error code, when it appears and any symptoms (no CH, no DHW, intermittent faults). Take photos of the display if helpful for later.
2) Reset the boiler once using the manufacturer reset button or switch. Wait for the boiler to attempt a normal restart. If it locks out again with E154, proceed with checks. Do not reset repeatedly more than a couple of times.
3) Check system pressure on the boiler gauge. If below about 1.0–1.2 bar, repressurise the system using the filling loop to around 1.2–1.5 bar, then try the boiler again. Low pressure can cause poor circulation and temperature errors.
4) Call for heating and feel the flow and return pipes at the boiler (careful: they can be hot). The flow pipe should be noticeably hotter than the return when the boiler is running. If both pipes are cold while the boiler is calling, there may be no circulation.
5) Listen for the pump: when the boiler is firing there should be a low vibration/hum from the pump. If you hear loud noises or nothing at all, note this for the engineer.
6) Bleed radiators to remove airlocks in the system. Air in the circuit can cause incorrect flow/return temperatures.
7) If you have a magnetic filter or inline filter, check it is not obviously blocked and cleanable by you (isolate system and follow manufacturer guidance). Do not dismantle sealed boiler parts.
Specific diagnostics and further safe checks:
1) After switching off mains power, remove the boiler outer cover only if you are comfortable and confident (many homeowners should not). Inspect visible sensor wiring and connectors on the flow and return thermistors for loose plugs, corroded connectors or damaged insulation. Re-seat any loose connectors and ensure they are clipped in securely. Replace the cover and restore power, then restart and check if the fault persists. If you are unsure, skip this and call an engineer.
2) If your boiler or external controller shows flow and return temperatures in its service or display menus, note the values. Abnormally low or very high readings, or identical values with no expected difference, point to sensor or circulation faults. Accessing service menus varies by model and may be best left to an engineer.
3) If the pump appears seized (no sound but you can see power is present), do not attempt complex pump repairs unless experienced. Some pumps can be freed manually but this is a job for a qualified engineer.
4) Check the zone valves, thermostats and any external controls to confirm the boiler is actually being asked to heat; a permanently closed valve or cancelled demand can create confusing symptoms.
When to call a professional and what to tell them:
- Call a Gas Safe registered heating engineer if E154 persists after the basic checks above, or if you find loose/failed sensors, seized pump, repeated lockouts, or you are uncomfortable opening the boiler.
- Tell the engineer: boiler model and serial, exact error code (E154), what symptoms you see (no CH/DHW, pump noise, pipe temperatures), what checks you have already done (reset, repressurised, bled radiators, inspected connectors) and any readings you noted.
- A professional will use diagnostic equipment to read sensor resistances/voltages, check the NTC thermistors and wiring, test pump operation and check flow through the heat exchanger, and if necessary replace the faulty sensor, free/replace the pump or repair wiring/PCB. These are regulated tasks requiring Gas Safe qualification.
Final notes:
- Don’t continue to use the boiler if it is locking out repeatedly; this is a safety feature.
- Sensor replacement, pump work, wiring repairs, or PCB work should be done by a Gas Safe engineer. Keep a record of the fault code, times and any tests you performed to speed up diagnosis when the engineer attends.
Helpful Resources
Boiler Manual
Download the official PDF manual for the Potterton Assure Combi and system.