Gas Boiler Not Lighting: Gas Valve Test, Ignition Electrode, Thermocouple and When to Escalate to Gas Safe Engineer

Quick Answer: A boiler that fails to light shows no flame at the burner after the ignition sequence. Diagnosis requires Gas Safe registration — only a Gas Safe registered engineer can work on the gas supply, gas valve, or burner assembly. Non-gas-safe tradespeople can check external electrical controls, programmer settings, and gas supply to the meter. The most common causes are: failed ignition electrode, failed PCB, faulty gas valve, or incorrect gas pressure. Always verify gas supply is on and pressure is adequate before suspecting components.

Summary

A gas boiler that refuses to light is one of the most common heating service calls. The fault could be anywhere in a sequence that involves a gas valve opening, a spark ignitor firing, and a flame sensor (ionisation or thermocouple) confirming combustion. Any break in this sequence — electrical or gas-side — results in no flame and a lockout fault code.

The critical safety consideration: only Gas Safe registered engineers can work on the gas-side components of a boiler — the gas valve, burner, heat exchanger, gas supply connections, and any work that involves disconnecting or testing gas-carrying components. Non-Gas-Safe tradespeople who observe faults should describe findings to a Gas Safe engineer and hand off. Working on gas components without registration is illegal under the Gas Safety (Installation and Use) Regulations 1998 and can result in prosecution and invalidated insurance.

This article describes the diagnostic process for a Gas Safe registered engineer and the checks that any tradesperson can safely make before escalating. The diagnostic tree covers the most common failure modes on modern condensing combination and system boilers.

Key Facts

Diagnostic Decision Tree

Diagnosed the problem? Create a repair quote in minutes with squote.

Try squote free → 
BOILER DOES NOT LIGHT
        |
        v
Is there a gas smell?
    YES --> STOP. Open windows. Turn off meter. Call 0800 111 999
    NO  --> Continue diagnosis
        |
        v
Is the gas meter supply on?
    NO  --> Open the emergency control valve. Check credit on prepay meter
    YES --> Continue
        |
        v
Does the boiler show a fault code?
    YES --> Note code and refer to manufacturer fault code table
    NO  --> Continue
        |
        v
Does the ignition sequence attempt to run?
(Can you hear clicking and/or see sparking through the inspection window?)
    NO  --> Check electrical supply: is boiler powered? Check fuse, check
            programmer/room thermostat is calling for heat
    YES --> Continue
        |
        v
Is there a spark at the ignition electrode?
(Observe through sight glass or remove burner cover — GAS SAFE ONLY for removal)
    NO  --> Check electrode HT lead; check PCB output at HT terminal;
            replace electrode if cracked or fouled
    YES --> Continue
        |
        v
Does gas reach the burner?
(Listen for gas flow; check gas valve is opening — GAS SAFE ONLY)
    NO  --> Test gas pressure at test point upstream of valve;
            test PCB output to gas valve (24V AC);
            gas valve coil resistance test (typically 4–8 ohms)
    YES --> Continue
        |
        v
Does flame establish briefly then cut out?
    YES --> Flame detection fault:
            - Check ionisation electrode position and condition
            - Measure ionisation current (μA) with microamp meter
            - Check earth continuity (ionisation circuit needs good earth)
            - Thermocouple: test mV output in flame; replace if <10mV
    NO  --> Burner stays unlit after gas proves open:
            - Incorrect gas/air ratio (combustion analysis required)
            - Blocked burner injectors
            - Heat exchanger blockage restricting flue
        |
        v
PERSISTENT FAULT: Replace failed component or escalate

Detailed Guidance

Before Any Diagnosis — Safety Checks

  1. Smell test: never proceed if there is any smell of gas — even faint. Evacuate and call 0800 111 999.
  2. Visual check: no signs of sooting around the boiler casing, flue terminal, or heat exchanger access? Sooting indicates incomplete combustion — a safety issue requiring immediate Gas Safe investigation.
  3. Boiler history: how old is the boiler? What is the service history? A boiler that has never been serviced accumulates carbon deposits on the burner and heat exchanger; these cause lighting failures that a clean and service will resolve.

Gas Supply Verification (Gas Safe Required)

Inlet pressure test:

Gas valve operation test:

Ignition Electrode Inspection

The ignition electrode generates a high-voltage spark (typically 8–20kV) to ignite the gas. Failure modes:

Inspection (Gas Safe):

Ionisation/Flame Detection

Modern boilers use ionisation flame detection (a current flows through the flame between the ionisation probe and the earth path through the burner/heat exchanger) rather than the thermocouple used on older atmospheric boilers.

Ionisation current test:

Thermocouple (older atmospheric boilers and gas fires):

PCB Faults

The PCB (printed circuit board) controls the entire ignition and safety sequence. PCB failure manifests as:

PCB diagnosis is by elimination: confirm all inputs to the PCB are correct and all outputs from the PCB are incorrect → PCB fault. PCB replacement on most common boiler models costs £50–300 for the part.

Important: Before condemning a PCB, check the gas pressure and flue path. Many apparent PCB faults are actually low gas pressure causing repeated lockout, or a blocked condensate trap (common in cold weather — see boiler losing pressure) causing a high-pressure lockout that mimics PCB failure.

Frequently Asked Questions

I'm not Gas Safe but the client is asking me to diagnose their boiler — what can I do?

You can safely:

You cannot: remove any gas-carrying component, test gas pressure at gas valve test points, operate the gas valve manually, or work on the burner assembly. Doing so is illegal under the Gas Safety (Installation and Use) Regulations 1998.

The boiler lights then cuts out after 5–10 seconds — what is this?

This is a flame detection failure. The boiler lights successfully (gas valve opens, spark ignites) but the PCB cannot detect the flame — either because the ionisation current is too low, the earth is poor, or the thermocouple output is insufficient. Work through the flame detection section above. Most common single fix: clean and re-gap the ionisation probe, check earth continuity.

How do I know if it's a gas supply problem or a boiler problem?

If other gas appliances in the property are working (gas hob, gas fire) and the gas meter shows supply, then the gas supply to the property is fine — the fault is with the boiler specifically. If no gas appliances are working, or if the gas meter shows no supply, the fault is upstream of the boiler — contact the gas network operator.

Regulations & Standards