Error 129

Safety precautions:

1) If you smell gas, evacuate the property and call the gas emergency number immediately. Do not operate electrical switches or appliances.

2) Before any inspection, switch the boiler off at the isolator and turn off the mains power at the fusebox if you will be touching the appliance. Do not open or attempt internal repairs unless you are a qualified, Gas Safe registered engineer.

Initial homeowner checks you can safely do:

1) Try a single reset of the boiler following the manufacturer's reset procedure. If the code clears and the boiler runs normally, monitor closely; repeated resets indicate a persistent fault.

2) Check that the boiler has power: confirm the isolation switch is on and fuses/circuit breakers supplying the boiler have not tripped.

3) Visually inspect external air vents, the flue terminal and any intake grills for blockages (birds' nests, debris, snow/ice). A blocked flue or intake can prevent the fan from operating normally.

4) Listen for unusual noises at the start attempt that might indicate a seized fan bearing or obstruction. If you hear grinding or the fan trying to turn, switch off and call an engineer.

5) If you have external controls (smart thermostats, OpenTherm/Hive controllers, or volt‑free devices) try isolating them: set the room stat to call for heat or temporarily disconnect third‑party controls according to their instructions to rule out interference. Do not remove wires inside the boiler yourself.

Diagnostic and fix steps for a qualified engineer (do not attempt unless competent and Gas Safe registered):

1) Verify the fault history and observe the behaviour on power-up to confirm 129 is generated on initial fan-start check.

2) Measure supply to the fan: the fan typically has permanent 230V supply and is speed‑controlled by a PWM signal from the PCB. Check for 230V at the fan live/neutral terminals and check the PWM control and tachometer connections.

3) Check the tachometer (RPM feedback) wire and connector for continuity and correct signal. If 230V is present but no tachometer signal is received when the fan should be running, the control will register 129.

4) Remove or isolate the PWM control (engineer only) to allow the fan to run uncontrolled; if the fan runs at full speed without PWM connected then the issue is likely the PCB/PWM output or its configuration. If the fan still won't run with PWM removed but 230V present, the fan motor or mechanical obstruction is the likely fault.

5) Inspect the fan physically for seized bearings, damaged blades or foreign objects in the impeller and replace the fan motor/assembly if required.

6) Inspect wiring harnesses, connectors and earths between PCB and fan for corrosion, loose connections or shorts. Replace or repair damaged cables.

7) If the fan and wiring are good, investigate the PCB: test PWM output and any associated drive circuitry. A failing PCB can produce incorrect PWM or no tachometer reference and is a common cause; replace PCB if confirmed faulty.

8) After repairs, verify correct fan start and tachometer feedback; run the boiler through several start cycles and confirm no further 129 or related fan-speed errors (e.g. 151) occur.

When to call a professional:

- If the reset doesn't clear the code, or it returns repeatedly, contact a Gas Safe registered heating engineer. Any checks requiring the removal of the casing, electrical measurements, or replacement of the fan or PCB must be done by a qualified engineer. Because this fault prevents safe operation, leave the boiler off and seek professional service rather than attempting internal repairs yourself.