Summary

Standing-pilot appliances — older gas fires, some water heaters, older non-condensing boilers, and many gas cookers with a pilot-lit oven — rely on a small permanent flame to prove combustion is safe before the main gas valve opens. If that pilot goes out, a Flame Failure Device (FFD) cuts the gas supply, which is the system working exactly as designed. The fault isn't that the gas cuts off — it's why the pilot won't stay alight in the first place.

This is one of the most common gas service calls on older housing stock, and it's frequently misdiagnosed as "just needs a new thermocouple" when the real cause is a draught, a fouled injector, or gas pressure. Fitting a new thermocouple without checking the other three fixes the symptom in maybe half of cases and leaves the customer calling back within weeks.

The distinction that matters for diagnosis is timing. A pilot that won't light at all is a different fault (no gas, no spark, blocked injector). A pilot that lights, burns for a few seconds and dies the instant the control knob is released is a thermocouple/FFD fault — the flame simply isn't generating enough millivolts fast enough to hold the safety valve open unassisted. A pilot that lights, stays alight fine for minutes or hours, then intermittently blows out is a draught, flame quality, or gas pressure fault. Getting this distinction right on the phone before a visit saves a wasted trip.

Key Facts

Quick Reference Table

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Symptom pattern Most likely cause Gas Safe test Typical fix
Pilot won't light at all Blocked injector, no spark, no gas Visual + gas supply check Clean injector, check ECV
Pilot lights, dies the instant knob released Thermocouple/FFD not generating enough mV, or held-in time too short Millivolt test at gas valve Hold longer first; replace thermocouple if <10mV
Pilot lights, dies after knob held full 30s+ Failed thermocouple or valve solenoid Millivolt test under load Replace thermocouple; if mV good, replace valve
Pilot burns fine for hours, dies with extractor fan or window open Draught / downdraught on open-flued appliance Spillage test at flue outlet Reposition, flue draught fix, ventilation review
Pilot flame lazy/yellow, unstable Fouled or part-blocked injector Visual flame inspection Clean or replace injector
Pilot dies only when boiler/other appliance also firing Low working gas pressure Working pressure test (20mbar static / 17-19mbar working) Contact gas network operator if upstream; check internal pipework if downstream
Pilot flame not touching thermocouple tip Bent/displaced bracket Visual inspection Realign bracket, reseat thermocouple
Intermittent, no pattern, multiple parts tested good Loose electrical connection at valve terminal Continuity/connection check Re-terminate, clean contacts

Detailed Guidance

Decision tree

PILOT LIGHT KEEPS GOING OUT
        |
        v
Does the pilot light at all when you try?
    NO  --> Different fault: see "boiler not lighting" / no-spark diagnosis
    YES --> Continue
        |
        v
Does it go out THE MOMENT the control knob/button is released?
    YES --> Was the knob held in for the full 15-30 seconds first?
              NO  --> Re-attempt: hold in fully, this may not be a fault
              YES --> THERMOCOUPLE / FFD SUSPECT
                      Gas Safe: measure mV output in flame
                      <10mV steady    --> replace thermocouple
                      10-15mV, marginal --> clean pilot + retest before replacing
                      >15mV but still cuts out --> suspect valve solenoid, not thermocouple
    NO  --> Continue
        |
        v
Does it stay lit for a while (minutes/hours) then intermittently die?
    YES --> Is there a pattern (extractor fan on, window open, windy day,
             another appliance firing)?
              YES --> DRAUGHT / VENTILATION SUSPECT (open-flued appliances)
                      Gas Safe: spillage test at flue outlet
                      Check room ventilation free area vs appliance rating
                      Check for competing extraction (fan, other flue)
              NO  --> Continue
        |
        v
Is the pilot flame lazy, yellow-tipped, small, or flickering
even when nothing else is happening?
    YES --> INJECTOR / FLAME QUALITY SUSPECT
            Gas Safe: remove and inspect pilot injector, clean or replace
            Check flame envelops thermocouple tip by 6-10mm
    NO  --> Continue
        |
        v
Does the pilot only die when another gas appliance
in the property is also running?
    YES --> GAS PRESSURE SUSPECT
            Gas Safe: test static (20mbar) and working pressure
            (17-19mbar) with all appliances firing simultaneously
    NO  --> Continue
        |
        v
Has the thermocouple already tested good (>15mV) and injector
is clean, no draught pattern found?
    YES --> Check bracket alignment (flame touching tip correctly)
            Check electrical connection at gas valve terminal for
            corrosion or looseness
            Consider gas valve solenoid replacement

Thermocouple and FFD testing (Gas Safe only)

Testing is done with a millivolt meter in series with the thermocouple circuit, or by using the meter across the gas valve's thermocouple terminals while the pilot is lit and the control knob is held/released as per the appliance's normal operating sequence.

  1. Light the pilot and hold the control knob in for the manufacturer's specified time (commonly 20–30 seconds).
  2. With the pilot burning steadily, measure mV output — expect 15–30mV on a healthy single thermocouple, or 300–750mV on a thermopile system.
  3. Release the knob and watch whether the pilot stays lit. If mV was healthy but the pilot still drops out on release, the fault is more likely the gas valve's holding solenoid than the thermocouple itself.
  4. If mV reads below roughly 10mV, replace the thermocouple — this is a low-cost, quick fix and should be tried before condemning the gas valve.
  5. Always re-check flame quality and pilot injector cleanliness at the same visit — a fouled injector reduces flame contact with the thermocouple tip and gives a false-low reading that a new thermocouple won't fix on its own.

Draught and downdraught diagnosis

Open-flued appliances (most older gas fires, some older boilers, multipoint water heaters) draw combustion air from the room and rely on a natural-draught flue to remove combustion products. Anything that disturbs this balance can pull the pilot flame sideways enough to trigger the FFD without a true flame failure:

A spillage test (smoke match or purpose-made spillage tester held at the draught diverter or flue outlet with the appliance running and any extract fans operating) confirms whether products of combustion are spilling into the room rather than being drawn up the flue — this both diagnoses the draught problem and identifies a potential CO hazard requiring immediate action.

Injector cleaning and flame quality

A part-blocked pilot injector restricts gas flow, producing a smaller, less stable flame that may not sit correctly against the thermocouple tip. Signs: yellow tipping (incomplete combustion, often dust or lint contamination), a flame that "dances" rather than burning steady, or a flame visibly not touching the thermocouple.

Cleaning: remove the injector per the manufacturer's service instructions, clear with a proprietary injector cleaning tool or compressed air (never wire pick, which can enlarge the orifice and change the gas rate) and refit. If pitted, corroded, or the flame remains poor after cleaning, replace the injector — these are inexpensive parts.

Gas pressure checks

Where the pilot only misbehaves under specific conditions — for example when the central heating boiler and a gas fire are both calling for gas — suspect working pressure. Test static pressure (no appliances firing, expect 20 mbar) and working pressure with all gas appliances in the property running simultaneously (expect the appliance manufacturer's stated minimum working pressure, typically 17–19 mbar for natural gas). A pressure drop under load points to either an undersized internal pipe run, too many appliances on one supply, or an upstream issue with the meter or the gas network's supply — the last of these requires escalation to the gas distribution network, not appliance repair.

When adjustment isn't the answer

It is never acceptable to "tune" a fault away by bending the FFD permanently into the pilot flame path so it reads flame presence regardless of the real flame condition, or to bypass/disable the FFD to stop nuisance shutdowns. This defeats the safety device's purpose and is a serious compliance and safety breach. If a genuine intermittent fault cannot be resolved with the checks above, escalate to component replacement (gas valve, full pilot assembly) rather than defeating the safety mechanism.

Frequently Asked Questions

I'm not Gas Safe registered — what can I safely check before calling an engineer?

You can observe and report: whether the pilot lights at all, how long the customer holds the control knob in before release, whether the outage correlates with an extractor fan or open window, and the flame's appearance (colour, size, steadiness) viewed through any sight glass or grille — without removing covers or touching gas components. Pass these observations to the Gas Safe engineer; they narrow the diagnosis significantly before the engineer even arrives.

The pilot lights fine and stays lit when I hold the knob, but dies within a second of release — is this always the thermocouple?

Usually, but not always. It's the classic thermocouple symptom, and a millivolt test in flame will confirm it in most cases. However, if the thermocouple tests within the healthy 15–30mV range and the pilot still drops out on release, suspect the gas valve's holding solenoid or coil rather than replacing a thermocouple that's actually fine.

Can a dirty pilot injector cause the same symptom as a bad thermocouple?

Yes, indirectly. A part-blocked injector produces a weaker, smaller flame that may not fully envelop the thermocouple tip, generating a lower millivolt reading even though the thermocouple itself is healthy. Always inspect and, if needed, clean the injector and confirm flame contact with the tip before condemning the thermocouple.

Is a pilot light going out ever an emergency?

Not on its own — the appliance shutting off gas supply when the pilot fails is the safety system working correctly. It becomes urgent if there are signs of spillage (soot staining, a smell of combustion products in the room, symptoms of carbon monoxide exposure) or if anyone has attempted to defeat the flame failure device to keep the appliance running. In either case, stop use of the appliance immediately and arrange a Gas Safe engineer without delay.

Why does my gas fire's pilot only go out on windy days?

This points to a draught or downdraught issue at the flue terminal — wind over certain terminal positions or types can create a pressure differential strong enough to disturb the pilot flame. A Gas Safe engineer should check the flue terminal position, type, and condition, and may recommend a terminal upgrade (e.g., an anti-downdraught cowl) where the appliance and flue design allows.

Regulations & Standards