Error D.27

Safety precautions:

- If you are not a qualified Gas Safe registered engineer, do not open the sealed gas or burner compartments, do not attempt gas or PCB repairs, and do not work live on mains wiring. Electricity and gas are dangerous.

- Before doing any visual inspections turn off the boiler at the programmer and isolation switch. If you need to inspect wiring terminals, isolate the electrical supply at the consumer unit (mains) first.

- Make basic checks from a safe distance only (no internal access). If you smell gas, evacuate and call the gas emergency number immediately.

Initial homeowner checks you can do safely:

1) Note when D.27 appears: does it show when you demand heating or hot water, or at other times (e.g. during pump priming or frost protection)? If it coincides with a call for an external device it may be normal.

2) Check the external accessory: if the relay is supposed to run a pump or valve, confirm the accessory has power and isn’t at fault (is the pump fused, switched on, does it run when powered separately?).

3) Reset the boiler: switch off at the programmer, turn the boiler off for 1–2 minutes, then restart. See if the D.27 message reappears and whether any symptoms persist.

4) Visually inspect around the boiler (external wiring runs, connectors, and terminals you can safely access) for obvious damage, loose terminals, water ingress, corrosion or burn marks. Do not remove any covers or probe inside the boiler if you are not qualified.

Diagnostic steps a qualified engineer will carry out (for information):

- Verify the output function of accessory relay 1 in the boiler wiring diagram and confirm which external device is connected.

- With the system calling for the accessory, measure the coil/supply voltage to the relay output and check whether the relay contacts close correctly (voltage on switched terminals) using safe isolation and test procedures.

- Test the continuity/resistance of the external wiring from the boiler relay terminals to the accessory to rule out a short, open circuit or earth fault in the cable harness.

- Inspect the relay on the PCB for physical damage, sticking contacts or burn marks. Check the PCB for signs of moisture, corrosion or failed components.

- If the relay does not operate electrically but the coil supply is correct, replace the relay or the PCB as required. If the relay operates but the external device does not, fault-find on the external accessory (pump, valve, wiring, fuses).

- If relay switching is causing other fault codes or lockouts, the engineer will investigate related systems (pressure, sensors, flame detection) because multiple symptoms can be linked to PCB faults or water ingress.

When to call a professional:

- Call a Gas Safe registered engineer if the accessory does not operate when D.27 is displayed, if the message is accompanied by other fault codes, if you smell gas, or if you find signs of water damage or burnt electronics. Any internal checks, relay replacement, PCB diagnosis or work on mains wiring or gas equipment must be carried out by a qualified engineer.

Final note: D.27 often simply records the PCB switching accessory relay 1, but persistent unexpected behaviour, failure of the controlled accessory, repeated messages, or additional faults require a Gas Safe engineer to test and repair safely.