Gas

Boiler Fault Finding: A Systematic Approach

Chasing a boiler fault by swapping parts costs money and rarely lands; working through the supplies, the ignition sequence and the symptom in order gets you to the cause.

Updated 20 Aug 20269 min read

A boiler that won't fire, keeps locking out or heats some things and not others is usually failing at one identifiable point in a chain. The engineers who fix them quickly aren't guessing — they follow the same order every time: prove the supplies, watch the ignition sequence, then read the symptom to split the fault down to a component.

This guide sets out a systematic approach: check gas and electrical supply first, understand ignition and flame sensing, separate no-heat from no-hot-water, and localise faults on the PCB, diverter valve and pump. Always work to the manufacturer's fault codes and wiring diagram; all gas work must be carried out by a Gas Safe registered engineer.

Key takeaways

Start with the supplies: correct gas working pressure and rate, and a sound electrical supply, earth and polarity — most 'boiler faults' are supply faults.
Understand the ignition sequence: call for heat, fan, pre-purge, spark, gas valve opens, flame established, flame sensed — a lockout tells you which step failed.
No flame sense despite a visible flame is a classic false lockout: check the sensing electrode, its lead and the earth/polarity.
Split the symptom: no heat and no hot water points at ignition or a global fault; hot water but no heat (or vice versa) points at the diverter valve or a zone control.
Read the fault code and use the manufacturer's flowchart — but verify with your meter, don't just swap the part it hints at.

Prove the supplies first

Before touching a component, prove what feeds the boiler. On the gas side, confirm the working pressure at the appliance at full rate and gas-rate it if in doubt — a boiler starved of gas will ignite weakly, lock out, or fail combustion, and no PCB swap fixes an undersized supply or a partly-closed valve. On the electrical side, confirm a sound supply with correct polarity and a good earth; reversed polarity alone will stop flame rectification working and produce a lockout on an otherwise healthy appliance.

This step catches a large share of 'boiler faults' that are really installation or supply faults. Get it wrong and every later reading is suspect.

Polarity and earth before the PCB

Flame sensing relies on rectification through a good earth and correct polarity. Reverse them and a perfectly good burner won't be 'seen' — you get a lockout that looks like a flame-sense or PCB fault but is really a wiring fault at the supply.

The ignition sequence

A fanned boiler follows a fixed startup sequence, and knowing it lets a lockout point at the failing step. A call for heat starts the fan; the air-pressure switch (or fan speed feedback) proves airflow and allows pre-purge; the spark generator fires; the gas valve opens; a flame is established at the burner; and the flame-sensing electrode confirms the flame within a few seconds, or the appliance goes to lockout.

Map the symptom to the step. Fan runs but no spark points at the ignition lead, electrode or PCB ignition circuit. Spark but no ignition points at gas supply, the gas valve or the injector. Ignition then immediate lockout points at flame sensing. No fan at all points back at the PCB, the call for heat, or the air-pressure/overheat safety circuit.

Flame sensing and false lockouts

Flame sensing usually works by rectification: the flame conducts a tiny current between the sensing electrode and earth, and the PCB reads that current as proof of flame. A dirty or misplaced electrode, a cracked ceramic, a poor sensing lead, or the earth/polarity issues above all break that signal — so the burner lights, but the PCB never 'sees' it and shuts the gas valve within seconds. That is the classic false lockout: a good flame, condemned by a sensing fault.

Check the electrode position and condition, its lead and connection, and the earth path before blaming the board. Where a flame-rectification current can be measured, compare it against the manufacturer's minimum — a low signal that just falls over on some cycles produces intermittent lockouts that are maddening to chase by parts-swapping.

Good flame, still locks out

If you can see a clean flame but the boiler locks out within a few seconds, suspect flame sensing: electrode, lead, earth and polarity — in that order — long before the PCB.

No heat vs no hot water

The split between heating and hot water tells you where to look on a combi. No heat and no hot water means the burner isn't firing at all (or a global fault) — work the ignition sequence and supplies above. Hot water fine but no central heating, or heating fine but no hot water, means the burner works and the fault is downstream in how the heat is being directed.

On a combi that points squarely at the diverter valve: it should send primary flow to the plate heat exchanger on a hot water demand and to the heating circuit otherwise. A stuck or failed diverter, or its actuator, produces exactly this 'one works, the other doesn't' pattern — lukewarm heating when a tap is off, or heating bleeding through during a hot water draw. On a system boiler the same split points at the zone valves and wiring centre instead.

PCB, diverter valve and pump

Localise before you replace. The PCB is the usual scapegoat but often innocent: prove its inputs (call for heat, sensors, air-pressure/flame signals) and outputs (fan, gas valve, pump, diverter) with your meter against the wiring diagram before condemning an expensive board. A missing output with correct inputs implicates the board; a missing input means look upstream.

The diverter valve causes the heating/hot-water split above and can also cause overheating or noise if it sticks mid-position. The pump causes overheat lockouts, kettling and poor circulation when it's failing, airlocked or seized — check it's running, not just powered, and that the system isn't sludged (a common hidden cause that mimics a pump or PCB fault). Reading the manufacturer's fault code narrows all of this down, but the code is a signpost, not a diagnosis: verify with the meter, then replace.

Frequently asked questions

Where should I start with a boiler that won't fire?

With the supplies. Prove correct gas working pressure and rate at the appliance, and a sound electrical supply with correct polarity and a good earth. A large share of 'boiler faults' are really supply or wiring faults, and they will corrupt every reading you take afterwards.

The boiler lights then locks out after a few seconds — why?

That is the classic flame-sensing failure. The burner lights but the PCB never confirms the flame, so it shuts the gas valve. Check the sensing electrode position and condition, its lead, and the earth and polarity before suspecting the board.

Hot water works but there's no heating — what's the likely cause?

On a combi that split points at the diverter valve or its actuator sticking, since the burner clearly fires for hot water. On a system boiler, look at the zone valves and wiring centre. Because the burner works, the fault is in how heat is being directed, not in ignition.

Is it usually the PCB?

Less often than it gets blamed. Prove the board's inputs and outputs with a meter against the wiring diagram first. Correct inputs but a missing output implicates the board; a missing input means the fault is upstream. Don't fit an expensive board on a guess.

From guidance to action

Gas rate calculatorRule the gas supply in or out before chasing the appliance.Gas service and maintenance recordRecord the fault, the readings and the remedial work on a signed service record.