Error E129

Safety first

- If you smell gas, leave the property immediately, do not operate light switches or electrical items, and call the national gas emergency number. Do not attempt any further checks.

- If there is no gas smell, isolate electrical power to the boiler before visual checks by switching off at the fused spur or consumer unit. For most simple checks you do not need to open the boiler casing. Do not open the casing or touch internal components unless you are a Gas Safe registered engineer.

Initial homeowner checks you can safely do

1. Reset the boiler once. Use the boiler's reset function and wait to see if it relights and runs normally. Note whether E129 returns and how quickly.

2. Check other gas appliances. Confirm cookers, gas hob or gas fire will light. If other appliances are not working, you may have a gas supply issue for the property and should contact the gas supplier or a Gas Safe engineer.

3. Check boiler display and any recent fault history. Note exact sequence, times and whether code appears during hot water or heating demand, or during an auto-setup event.

4. Check system water pressure. Very low pressure can cause other faults. Repressurise to the normal operating range if required (typically about 1.0–1.5 bar) following the boiler manual, but only if you know how. Do not over-pressurise.

5. In cold weather check the condensate pipe to ensure it is not frozen. A blocked condensate can cause ignition and safety faults. If frozen thaw gently with warm (not boiling) water or towels.

6. Visually inspect external flue termination for obvious blockages such as bird nests or debris. Do not climb or disassemble the flue.

7. Check that any separate room thermostat or programmer is calling for heat correctly. Ensure the boiler is actually being asked to run when you test it.

Diagnostic notes and what an engineer will likely check

- If a single manual reset clears E129 and the boiler runs normally for a long period this may have been a one-off; continue to monitor. If it reoccurs, do not attempt internal repairs.

- Repeated E129 during calibration suggests the boiler cannot reliably detect flame during the auto-combustion or calibration sequence. An engineer will check gas pressure to the appliance, the gas valve operation, ignition electrode/spark, ionisation/flame probe, wiring and connectors, the ignition module and the PCB. They will also check mains voltage and for intermittent supply issues.

- The engineer may run combustion checks, re-seat or replace the flame detection electrode, replace faulty ignition components or replace a defective gas valve or PCB as required. They will also check for blocked flue, restricted air intake or condensate issues that may affect combustion stability.

When to call a professional

- Call a Gas Safe registered engineer if E129 returns after a reset, if the boiler locks out repeatedly, if other gas appliances are affected, or if you cannot identify a simple external cause like frozen condensate or blocked flue. Do not attempt to repair or replace gas, ignition or PCB components yourself.

What to prepare for the engineer

- Note the exact error code, how many times it has occurred, when it happens (on heating, hot water, at startup or during calibration), and any other codes shown. Take photos of the control panel and any wiring you can see externally. Note whether other gas appliances work and whether the condensate pipe or flue appeared blocked.

Important reminders

- Do not open the boiler casing or touch internal gas or electrics unless you are qualified. Internal inspection and repair of ignition, gas valve, ionisation probe, or PCB must be done by a Gas Safe engineer.

- If you ever smell gas take immediate action by evacuating and calling the emergency gas service. Repeated flame loss is a safety shut-down designed to protect you. Arrange professional diagnosis and repair rather than continuing to reset the appliance repeatedly.