Error 154

Safety first:

- If you are not competent with gas appliances, electrical work or pressurised heating systems, stop and call a Gas Safe registered engineer. Do not attempt gas valve, PCB or burner work.

- Before any visual checks, switch the boiler to off at the programmer and isolate electrical power at the fused spur. Do not isolate the mains if you need to reset or run the boiler for a controlled test — follow the manual’s reset procedure.

- Beware hot surfaces and hot water when bleeding radiators or inspecting pipes.

Initial homeowner checks you can safely do:

1) Reset the boiler once using the normal front-panel reset or the reset button, then allow a normal demand cycle. If the fault clears and does not return, monitor for recurrence. Repeated resets that clear then reappear usually indicate a real fault and need an engineer.

2) Check system water pressure on the boiler gauge and ensure it is within the normal operating range for your installation (consult the boiler manual for the correct value). If pressure is very low, follow the manual to repressurise or call an engineer.

3) Bleed radiators to remove airlocks that can reduce flow. After bleeding, re-check pressure and top up if required.

4) Listen at the boiler for pump operation when there is a central heating demand: you should hear/feel the pump running. Also check that any room/thermostat or programmer calling for heat is actually allowing the boiler to run and that zone valves (if present) are opening when called.

5) Visually inspect for obvious signs: blocked condensate is unrelated to this code, but look for closed isolation valves on the heating circuit, visibly kinked pipework, or recent work that could have introduced debris.

More detailed diagnostic checks (intended for competent DIYers or better carried out by an engineer):

1) Compare flow and return temperatures if the boiler display shows both. If return is higher than flow consistently, suspect sensor wiring/sensor position or a plumbing fault where hot water is returning into the return circuit.

2) Inspect the flow and return sensors/probes: check connectors for secure seating and corrosion. If you can safely access the probe connectors, isolate power and check continuity/resistance of the thermistors against expected values in the service manual. Dirty or displaced thermistors can give false readings — probes sometimes only need cleaning or re-seating.

3) Check wiring harnesses for damage, chafing or earth shorts. Unplug and re-seat connectors to ensure good contact. If readings change when harness is disconnected, the harness may be faulty.

4) Check the pump and flow switch: verify pump runs at full speed on demand and that any flow switch is operating correctly. Some ATAG/iR references state flow-switch switch-off around 7 l/min ±10% and switch-on a bit higher; a faulty or sticky flow switch can trigger flow/return anomalies. If the pump doesn’t run or runs intermittently, check the pump supply and consider that pump replacement/repair is a job for an engineer.

5) Check for blocked heat exchanger or hydraulic restrictions: partial blockage or a closed valve will reduce flow and increase delta T. If the system has a secondary heat exchanger or extra zones tied into the return, this can create expansion and flow anomalies — review the hydraulic layout and isolate secondary circuits where possible to test.

6) If the boiler has parameters for slope adaptation or gradient (documents reference parameter P5, P9 and parameter 475 auto slope adaptation), do not change these unless you are a trained service engineer. Incorrect P-parameter changes can make the fault worse or void warranty. Resetting parameter P9 has been reported to clear some 'sticking gradient' behaviours when the boiler left production test mode, but this should be done only by a qualified engineer following the service manual.

7) If you have access to service menus and know how to interpret probe values, monitor how quickly flow temperature rises during a call. ATAG documentation flags a fast increase (> ~5 K/second) as a protective trigger. If you can reproduce a rapid rise, that supports a flow/circulation issue or a probe reading error.

When to call a professional:

- If the fault does not clear after the basic checks (reset, bleed radiators, check pressure and that the pump/valves run), call a Gas Safe engineer.

- Call an engineer for any of the following: suspected sensor or wiring faults, pump replacement, flow-switch replacement, hydraulic blockages requiring flushing, PCB or parameter changes, repeated C154 lockouts, or any work involving gas or internal boiler components.

What the engineer will likely do:

- Verify flow and return probe readings and swap/prove sensors if required, test wiring and replace defective probes or harnesses.

- Check and test the pump, flow switch and hydraulic flow rates; flush or clear blockages and check zone valve operation.

- Review system pipework and any secondary exchangers or unusual zone arrangements that could cause return > flow, and correct the hydraulic layout if required.

- If needed, adjust controller parameters or perform firmware/service menu actions to clear production/test settings — note these changes should be documented and performed by the engineer.

Final note: Error 154 is a protective response to a real or sensed thermal hazard. Temporary resets may restore operation but recurring occurrences mean an underlying fault that must be diagnosed. For safety and compliance, persistent or unclear faults must be investigated and repaired by a qualified Gas Safe heating engineer.