Error D.28

Safety precautions:

1. If you are not competent with electrical or gas appliances do not remove covers or touch wiring. Contact a Gas Safe engineer.

2. Before any work that exposes live terminals, turn the boiler off at the mains isolator and, where applicable, isolate the gas and wait for the boiler to cool.

3. Do not operate the boiler if you smell gas — evacuate the property and call the gas emergency number.

Initial homeowner checks (safe, non-invasive):

1. Note exactly when the D.28 appears — during a demand for heating/HW, after powering up, repeatedly, or only when a particular accessory is operated (e.g. a timer, external pump or room controller). Patterns help diagnosis.

2. Try a simple reset: switch the boiler off at the mains, wait 30–60 seconds, then switch back on and observe whether D.28 returns. If the code does not return and the system runs normally, it may have been a transient event.

3. Check any external controllers, timers, thermostats or smart relays that are connected to the boiler. Turn them off or disconnect them (using their isolators) and see if the code persists — if the fault stops when an accessory is isolated, that accessory or its wiring is the likely cause.

4. Visually inspect any visible external wiring/controls (not inside the boiler) for damage, moisture, or loose connections.

If you are comfortable with basic, safe electrical isolation and the boiler is switched off at the mains, you can proceed with these diagnostic steps only if you have competence with low-voltage wiring:

1. With the boiler isolated from mains, remove the external wiring cover (if accessible from outside) or consult the user manual to identify the terminal labelled for accessory relay 2. Do NOT work inside the boiler cabinet unless you are qualified.

2. Check that the terminal screws and external connector are tight and free of corrosion. Re-tighten gently if loose.

3. If safe to do so, temporarily disconnect the accessory from relay 2 (with all power isolated). Re-apply mains power and see whether the D.28 message returns or whether normal operation resumes. If the code disappears when the accessory is disconnected, the accessory or its wiring is the fault.

4. If removing the accessory does not clear the message, or if the code appears with nothing connected to relay 2, the problem is likely internal (relay contact stuck, short on the PCB or PCB electronics fault).

When to call a professional and what they will do:

1. Call a Gas Safe registered engineer (and/or qualified electrician) if the code persists, if you had to open the boiler, if you found damaged wires or moisture, or if the boiler is not providing heating or hot water. Do not attempt to replace PCBs, relays, electrodes, gas valves or other internal components yourself.

2. The engineer will safely isolate and test the relay outputs and wiring with a multimeter, check the resistance and operation of the relay coil and contact, inspect the PCB for damage or moisture, and test the connected accessory under controlled conditions. They may swap or disconnect the accessory to confirm the fault path and will only replace internal parts if properly diagnosed.

3. When contacting an engineer, have the boiler model, serial number, and exact description of when D.28 appears ready, and tell them what preliminary checks you have already performed.

Notes and final advice:

- D.28 can be a simple diagnostic readout or the first indication of an accessory/wiring/PCB issue. If the boiler is otherwise operating normally and the message is occasional, monitor the system after a reset but arrange an engineer if it recurs. If the message coincides with loss of heating or hot water, call a professional immediately.

- Avoid guessing repairs or replacing electronic modules without proper testing — incorrect work can make the fault worse and may be unsafe. A qualified engineer will perform the safe electrical and gas-side checks needed to resolve the problem.