Error C4 237

Safety first: If you smell gas, leave the property immediately and call the gas emergency number. Do not attempt any checks. Before touching the boiler, switch off the electrical supply at the isolator and, if possible, at the consumer unit. If you are not comfortable opening the boiler or working with internal connectors, stop and call a Gas Safe registered engineer.

Initial homeowner checks you can safely do:

1) Note the exact error code and any cause code or flashing triangle on the display, and take a photo of the display and the boiler data plate (model and serial number). This information helps the engineer.

2) Attempt a single reset: use the boiler reset button or the reset procedure in your manual (hold reset for 3 seconds or follow the dial reset method). Wait to see if the code returns. If the code clears and the boiler runs, monitor for reoccurrence and arrange a service if it returns.

3) Check whether any recent service or part change has been carried out on the boiler. If yes, contact that installer — an incorrect HCM may have been fitted.

4) Check for other obvious faults (power supply on, mains fuse not tripped, and boiler water pressure roughly correct). Do not attempt to relight or force operation if other warnings exist.

If you are competent with basic electrical isolation and comfortable opening the boiler casing (only proceed if the boiler is isolated, power removed, and you understand the risks):

5) Isolate the boiler electrical supply at the isolator and allow a few minutes for capacitors to discharge. Remove the outer case following the manufacturer’s instructions.

6) Visually inspect the HCM (small plug-in module/code plug) and the main control box connectors for loose plugs, bent pins, corrosion, or debris. If a connector is visibly loose, gently but firmly re-seat the connector — do not force or twist. Photograph the connections before and after so an engineer can see what you did.

7) Check the HCM part number/code plug matches the boiler requirements. If you find a clearly wrong module (different part number) do not attempt to operate the boiler and contact the installer/manufacturer or a Gas Safe engineer.

8) Replace the casing, restore power and try a reset. If the fault persists or returns, switch the boiler off and book an engineer visit.

Why call a professional: Correctly diagnosing whether the HCM, the control box (PCB) or wiring is defective often requires specialist test equipment, spare modules, and safe handling of live components and gas systems. Replacement of the HCM or control box typically needs a Gas Safe registered engineer because work may affect gas safety and certification. If the HCM is found to be the wrong type, your installer or the engineer will need to swap it for the correct part. If the control box/PCB is faulty, it will require replacement and safe commissioning.

What to tell the engineer: provide the boiler model and serial number, the exact error code C4 237 and any cause codes or symptoms, any recent work on the boiler, and photos you took of the display and internal connections. This speeds diagnosis and helps ensure the engineer brings the right parts.

Do not attempt to bypass safety interlocks, run the boiler with its casing removed, or work on gas pipework unless you are a qualified Gas Safe engineer.