Boiler powers on but won't fire up
A boiler that lights its display, runs the pump, then stops short of ignition is usually failing at one of four points: gas supply, spark generation, flame detection or the PCB telling it to try. Working through them in order is quicker and cheaper than replacing parts on a hunch.
How we isolate the cause
We start with working gas pressure at the appliance, because a weak supply mimics almost every other fault. Then spark: electrode gap, lead condition and earthing. If it sparks and lights but drops out within seconds, that points at flame rectification — a dirty or cracked electrode rather than a failed valve. Only after those do we test the gas valve solenoid and the PCB.
What the visit covers
- Standing and working gas pressure recorded on a manometer
- Ignition electrode gap, ceramic condition and earth path checked
- Flame rectification current measured, not guessed
- Written outcome: repair cost vs. replacement advice if the boiler is old
Frequently asked
- It fires for two seconds then locks out — what's that?
- Almost always flame detection. The boiler lights, can't confirm a flame, and shuts the gas off as a safety response. A clean or replacement electrode usually resolves it.
- Is a PCB worth replacing?
- On a boiler under about ten years old with an otherwise sound heat exchanger, often yes. On an older unit the PCB cost is a large share of a new boiler, so we'll say so plainly.
- Can you fix it on the first visit?
- Electrodes, sensors and fans for common Worcester, Vaillant and Ideal models are carried on the van. Rarer parts are ordered next day.
Before you call
Note the exact fault code, whether you hear the fan spin up, and whether other gas appliances still work. Those three facts often halve the diagnostic time on site.
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