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

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

• Gas Safety (Installation and Use) Regulations 1998: all gas work must be carried out by a Gas Safe registered engineer
• Gas Safe Register: the official list of registered engineers in the UK; check at gassaferegister.co.uk or call 0800 408 5500
• Common fault codes: F1/E28 = ignition failure; F28 = no gas detected; E9 = overheat lockout; varies by manufacturer
• Gas pressure (domestic): 20 mbar inlet static pressure minimum for most domestic boilers; 18–20 mbar working pressure at the gas valve
• Electrode gap: typically 2.5–4mm depending on manufacturer — incorrect gap causes weak or absent spark
• Ionisation current: minimum 2–4 microamps required to prove flame; below this the PCB locks out
• Thermocouple output: 15–30mV with good flame; below 10mV may not hold the valve open
• Gas valve operation: 24V AC signal from PCB opens the valve; check voltage at valve terminals if no gas is flowing
• First-time lockout vs persistent lockout: single lockout may be a transient fault; 3+ consecutive lockouts requires investigation
• Emergency action: if you smell gas, do not attempt to light the boiler — turn off supply at the emergency control valve and call the National Gas Emergency Service: 0800 111 999

Diagnostic Decision Tree

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:

• Remove the test point screw on the gas valve inlet (Gas Safe only)
• Attach a digital manometer
• Measure static pressure (no flow): should be ≥20 mbar
• Measure working pressure (boiler demanding gas, other appliances also running): should be ≥18 mbar
• Working pressure below 18 mbar → gas supply issue; contact gas distribution network (Cadent, Northern Gas Networks, etc.)

Gas valve operation test:

• The PCB sends 24V AC to the gas valve solenoids when it is satisfied that: (a) the ignition sequence is initiated and (b) pre-purge has completed
• Use a multimeter at the valve electrical terminals: check 24V AC is present when the boiler attempts to light
• If 24V is present and the valve does not open → faulty gas valve
• If 24V is absent → PCB fault or wiring fault preventing valve activation

Ignition Electrode Inspection

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

• Cracked ceramic insulator: high-voltage spark tracks to earth through the crack rather than jumping the electrode gap → no visible spark at burner
• Fouled tip: carbon deposits bridge the electrode gap → spark either absent or too weak
• Incorrect gap: if the electrode tip has moved due to vibration or previous servicing → spark misfires

Inspection (Gas Safe):

• Remove burner assembly (follow manufacturer service instructions — gas off, boiler isolated from mains)
• Inspect electrode for cracks, deposits, and correct gap (2.5–4mm typically; check manufacturer spec)
• Clean with fine wire wool if fouled; replace if ceramic is cracked
• Electrode is a low-cost consumable (typically £10–25) — replace if in doubt

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:

• Insert a microamp meter in series with the ionisation circuit (flame sensing wire)
• With flame lit, current should be ≥2μA (typically 2–6μA on a well-functioning boiler)
• Current below 1–2μA → PCB will lock out, interpreting as no flame
• Common causes of low ionisation: fouled probe, incorrect probe gap, poor earth continuity
• Check earth continuity with a continuity tester from the boiler casing back to the consumer unit earth terminal

Thermocouple (older atmospheric boilers and gas fires):

• Thermocouple generates a millivolt signal proportional to temperature in the pilot flame
• Output in pilot flame: 15–30mV on a good thermocouple; below 10mV will not hold the gas valve open
• Replacement thermocouple: universal type available, £8–20; clean the pilot injector before fitting new thermocouple

PCB Faults

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

• No response to heat demand (no fan, no spark)
• Persistent lockout with no component faults found
• Intermittent random lockouts
• Fault codes pointing to multiple simultaneous faults (often a PCB issue rather than multiple component failures)

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:

• Check that the gas meter emergency control valve is open
• Check that the programmer/room thermostat is set correctly and calling for heat
• Read and record the fault code from the boiler display
• Check if the condensate trap drain is blocked (clear this safely from outside — no gas work involved)
• Report your observations to a Gas Safe engineer or to the client to pass on

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

• Gas Safety (Installation and Use) Regulations 1998 (SI 1998/2451) — requires Gas Safe registration for all gas work

• Building Regulations Approved Document J — combustion appliances; installation requirements

• BS EN 677 — gas-fired condensing boilers; efficiency and safety requirements

• IGEM/UP/7 — gas pressure testing standards for domestic installations

• HSE INDG238 — Using gas appliances safely; general guidance

• Gas Safe Register — official register; verify engineer registration

• HSE: Gas Safety at Work — regulatory guidance

• Worcester Bosch Fault Codes — manufacturer-specific codes

• HHIC: Boiler Maintenance Guidance — Heating and Hotwater Industry Council

• boiler not firing — boiler firing then cutting out — pressure and thermistor faults

• boiler losing pressure — condensate blockage and pressure loss

• no heating — system-level heating faults

• thermostat not working — thermostat and programmer diagnostics