Home › Boilers › Parts › Ignition electrode and spark generator
Boiler Ignition electrode and spark generator: What It Does and Signs It's Failing
The ignition electrode is a pair of metal pins (or one pin and the burner) set in a ceramic holder right at the burner face. The spark generator, also called the ignition transformer, is the small box that builds the high voltage and sends it down a lead to the electrode. Together they light the gas each time the boiler fires.
Who should replace it
Technician job
Related codes
8 error codes on this site point to this part.
How it works
When the board calls for heat, the fan starts, the gas valve opens and the generator fires a rapid string of sparks across a small gap, 3.5 mm (about 1/8 in) on Ideal's Logic Combi2 C. Once a flame is detected the sparking stops. On some boilers the same electrode then switches to sensing the flame.
Signs it is failing
- Repeated clicking from the boiler but no flame
- Ignition lockout code after several attempts to light
- Boiler lights only on the second or third try
Failed-ignition codes point here once gas supply and a frozen condensate pipe have been ruled out. Vaillant also mentions the ignition parts for sparking-related overheat and slow flame-out faults.
Can you replace it yourself?
The electrode sits inside the sealed combustion chamber, and the generator produces a dangerous voltage, so this is a Gas Safe registered engineer job (or an RGI in Ireland). From outside the casing, the most you should do is press reset once, as your boiler's manual shows. Electrodes are normally inspected at the annual service.
Buying the right part
Electrodes come with or without a lead and with a matching seal, which the engineer should replace too. They're specific to each burner design, so order by model and GC number.
Error codes linked to the ignition electrode and spark generator
- E133Gas supply interrupted or flame failed to establishBaxi
- E128Flame failureBaxi
- F.28Boiler failed to lightVaillant
- F.29Flame went out and the boiler couldn't relightVaillant
- F.62Flame took too long to go outVaillant
- EA / EA 227Burner didn't light (no flame detected)Worcester Bosch
- F7Flame signal with the burner offWorcester Bosch
- FAFlame still present after the gas shut offWorcester Bosch
Last reviewed 2026-10-21.