Error E129

Safety first

1) If you smell gas: leave the property immediately, do not operate switches, do not use phones inside. Call the gas emergency service from a safe location.

2) If you have any carbon monoxide alarm warnings, evacuate and call emergency services and a Gas Safe engineer. Do not attempt repairs.

3) If you are unsure or uncomfortable, stop and call a Gas Safe registered engineer. Do not work on gas or live electrical parts yourself.

Initial visual and simple checks homeowners can do (no contact with live parts)

1) Note the exact error (E129) and any other codes or lights. Record what you were doing when it occurred (heating, hot water, after reset, etc.).

2) Check gas supply to the property: confirm other gas appliances (hob, gas fire) work, check gas meter isolation is on and meter credit is present if it’s prepay. If no gas supply, contact your gas supplier.

3) Check boiler pressure on the pressure gauge. Many faults arise if the system pressure is very low; pressure should typically be around 1–1.5 bar when cold. If pressure is low, top up using the filling loop following your boiler manual. Do not over-pressurise. After topping up, close the filling loop and recheck pressure.

4) Inspect external flue/air inlet visually for obvious blockages (bird nest, debris, snow). Do not dismantle the flue. If blocked, clear only if safe to do so from ground level; otherwise call an engineer.

5) Ensure adequate ventilation/air supply to the boiler and combustion area. Do not store items in front of the boiler intake or flue terminals.

Basic reset and observation

1) Try a single reset using the boiler reset procedure (press and hold the reset button for the time specified in your manual, commonly 5–10 seconds). Wait for the boiler to attempt ignition and observe behavior.

2) If the boiler restarts and runs normally, monitor it for a while. If the E129 returns within a short time or after a few cycles, do not continue to reset repeatedly.

3) If the boiler will not reset or the fault returns immediately, proceed to the next diagnostic steps.

Basic safe diagnostic steps you can safely perform (no internal repairs)

1) Turn off and isolate power to the boiler before doing any close inspection of internal components. If you are not competent, do not open the casing. Many electromechanical parts remain live when the boiler is powered.

2) With power isolated and only if you are comfortable opening the outer casing, visually inspect the flame sensing electrode (ion probe) and spark electrode for heavy soot, scale, or obvious damage and for secure connectors and wiring. Do not touch the electrodes while the boiler is live. Replaceable probes are a common cause of flame sensing faults but replacement should be carried out by a Gas Safe engineer.

3) Check wiring to the control PCB and the electrodes for loose connectors or damaged insulation. Re-seat any obvious plug connections (with power isolated) but do not attempt soldering or PCB-level repairs yourself.

4) If the electrode and wiring appear intact, make a note whether the boiler attempts ignition (you may hear the fan, spark, gas valve click). If you hear the spark and do not get a stable flame, that indicates either faulty sensing or gas delivery/pressure problem.

When to call a professional (must call a Gas Safe engineer)

1) If the E129 returns after a single reset or after topping up pressure and clearing any obvious external blockages. Repeated lockouts require professional diagnosis.

2) If visual inspection shows damaged sensors, loose wiring you cannot safely repair, or soot/carbon on electrodes.

3) If you suspect intermittent gas supply or low gas pressure. A Gas Safe engineer can measure and correct gas pressure safely.

4) If the flue shows signs of corrosion, water ingress, or you suspect flue recirculation causing flame instability.

5) For any replacement of the sensing electrode, spark electrode, gas valve, or PCB, or to run the boiler’s automatic calibration properly. The E129 message specifically suggests the calibration reached maximum correction — the automatic calibration should be enabled and interpreted by a trained engineer who can also replace components if required.

Important cautions

1) Do not repeatedly reset the boiler more than once or twice if the fault persists. Repeated resets can mask a dangerous underlying combustion or gas supply fault.

2) Never attempt to replace the gas valve, adjust gas pressure, or repair PCB/electronic parts yourself unless you are a registered Gas Safe engineer.

3) Keep a working carbon monoxide alarm near the boiler and ensure it is tested regularly.

Outcome

If after the safe checks and a single reset the boiler stays fault-free, continue to monitor. If E129 returns, contact a Gas Safe registered engineer and quote the E129 fault so they arrive prepared to test the flame sensing circuit, ignition system, gas supply/pressure, flue performance, and run the automatic calibration and any necessary replacements.