Why a boiler clicks when it tries to light
When your boiler gets a call for heat or hot water, it runs a start-up sequence. The fan starts to clear the combustion chamber, the gas valve opens and an ignition electrode sparks rapidly next to the burner. That spark is the fast clicking you hear. A flame sensing electrode (the same electrode on some models) then checks for a flame. If it does not detect one within a few seconds, the boiler shuts the gas off, tries again and, after a set number of attempts, locks out with a fault code.
So clicking without lighting means the spark is working but a flame is not being established or detected. Either the burner is not getting gas, the flame cannot burn properly, or the boiler cannot sense it.
A slower single click now and then is different. Relays on the circuit board click as they switch, and many boilers click once or twice as the diverter valve or pump starts. Occasional single clicks while the boiler is working normally are nothing to worry about.
Common causes
| Cause | Signs | Who fixes it |
|---|---|---|
| No gas reaching the property (meter, prepayment credit, supply fault) | The gas hob will not light either | You check, then your supplier or the gas emergency service |
| Gas isolation valve under the boiler closed | Recent service or work on the boiler or meter | You check |
| Frozen or blocked condensate pipe | Freezing weather, gurgling before the lockout | You thaw it, engineer if blocked inside |
| Worn, dirty or misaligned electrode | Clicks for several seconds, lights only sometimes | Gas Safe engineer |
| Gas valve fault | Spark is fine but no gas reaches the burner | Gas Safe engineer |
| Fan, air pressure switch or flue problem | Fan codes, unusual fan noise | Gas Safe engineer |
| Circuit board (PCB) fault | Erratic behaviour, repeated lockouts | Gas Safe engineer |
Safe checks you can do yourself
- Check other gas appliances. Try the gas hob if you have one. If that will not light either, the problem is the gas supply, not the boiler.
- Check the gas meter. On a prepayment meter, check you have credit and that emergency credit has not run out. On a smart meter, look for supply messages on the in-home display. Make sure the lever on the meter is in line with the pipe, which means open.
- Check the gas isolation valve under the boiler. The valve on the gas pipe below the boiler, often with a yellow handle, should have its handle in line with the pipe. If the handle is across the pipe, it is closed. This sometimes happens after a service or other work.
- Check the condensate pipe in freezing weather. If it runs outside and it has been below zero, thaw it with warm (not boiling) water or a hot water bottle. Our frozen condensate pipe guide shows how.
- Check the thermostat and timer. Make sure they are calling for heat, so the boiler is actually being asked to fire.
- Reset the boiler as described in the manual. If it still fails to light after 2 or 3 attempts, stop and call an engineer.
If you smell gas at any point, do not press reset or operate electrical switches. Turn off the gas at the meter if you can do so safely, open doors and windows, leave the property and call the National Gas Emergency Service on 0800 111 999 (0800 002 001 in Northern Ireland). See our guide on what to do if you smell gas near your boiler.
Do not keep resetting beyond this. Repeated resets can hide a fault that needs repairing, and some boilers block further resets after too many attempts in a short time.
Ignition failure codes by brand
Most boilers show a specific code when they lock out after failing to light. The code does not tell you which part has failed, but it confirms the boiler tried to ignite, found no flame and gave up.
| Brand | Ignition failure code | Notes |
|---|---|---|
| Vaillant | F28 | Failed to light on start-up (F29 is flame lost while running) |
| Worcester Bosch | EA 227 | No flame detected |
| Ideal | L2 | Ignition lockout after failed attempts |
| Baxi | E133 | Ignition failure or gas supply interrupted |
| Glow-worm | F28 | Ignition failure when starting up |
| Potterton | E133 | Interruption of gas supply or flame failure |
| Vokera | A01 | Ignition failure, flame not sensed |
| Viessmann | F4 | No flame signal |
Code meanings can vary between model ranges, so check the manual for your exact boiler. Write down the exact code before you reset, because it disappears once the boiler restarts and it helps the engineer bring the right parts. If the boiler clicks, fails and shows a different code each time, mention that too, as it can point to a control board fault rather than a gas or ignition problem.
When to call a Gas Safe engineer
If the gas supply is fine, the isolation valve is open and the condensate pipe is clear, the fault is almost certainly inside the boiler. Call a Gas Safe registered engineer if:
- the boiler still clicks and fails to light after 2 or 3 resets
- it lights sometimes but not others, which often points to a worn electrode
- it lights with a bang or a loud whoosh
- the clicking carries on continuously without the boiler locking out
- you see scorch marks, soot or melted parts around the boiler
The engineer will usually check the gas pressure at the boiler, inspect and clean or replace the electrodes, test the gas valve and fan, check the condensate trap and read the fault history on the control board. Never take the casing off to look at the electrode yourself. It sits in the sealed combustion chamber, and the gas valve and wiring around it must only be worked on by a registered engineer.
Typical repair costs in the UK
| Repair | Typical UK cost |
|---|---|
| Engineer call-out and diagnosis | Typically £60 to £120 |
| Ignition or flame sensing electrode replaced | Typically £100 to £250 |
| Ignition lead or spark generator replaced | Typically £100 to £250 |
| Condensate trap cleared or pipe rerouted | Typically £80 to £250 |
| Fan replaced | Typically £200 to £400 |
| Gas valve replaced | Typically £180 to £400 |
| Main circuit board (PCB) replaced | Typically £250 to £600 |
Electrode jobs sit at the lower end because the part is inexpensive and quick to fit on most boilers. Prices rise with London labour rates, evening and weekend call-outs, brand-specific parts (a board for a premium or discontinued model costs more than one for a common Baxi or Ideal combi) and boilers that are hard to reach. If your boiler is under warranty, contact the manufacturer before booking an independent engineer.
For how many times it is safe to reset a boiler before you call an engineer, see our guide on how to reset a boiler.