Gas Fire Not Lighting: Fault-Finding Guide

Summary

A gas fire that won't light is one of the most common gas appliance call-outs, and the fault almost always sits in one of a handful of well-understood places: gas supply, ignition, or the flame safety device. The diagnostic discipline is the same one used across all gas appliance fault-finding — work from the supply inward, confirm gas is present and at correct pressure before suspecting the appliance, and never bypass a safety device that has correctly shut an appliance down.

This is gas work. In Great Britain, all installation, servicing, diagnosis and repair of gas fires is restricted by law to a Gas Safe registered engineer holding the relevant appliance category (typically CF1/HTR1 or similar, depending on flue type). It is a criminal offence under the Gas Safety (Installation and Use) Regulations 1998 for anyone not competent and registered to work on a gas appliance. This guide is written for registered engineers diagnosing the fault on site, and for customers to understand what an engineer will check — it is not a DIY repair guide, and nothing in it authorises an unregistered person to open up or work on a gas fire.

The most dangerous mistake on this job is treating a fire that "won't stay lit" or "clicks but doesn't light" as a nuisance to be worked around by repeated relighting attempts. A flame failure device (thermocouple or thermopile) that shuts the gas off after the pilot goes out is doing exactly what it's designed to do — it has detected that the flame is not present and has cut the gas supply to prevent unburnt gas escaping. Repeatedly forcing a relight without establishing why the flame keeps failing risks releasing unburnt gas into the room and is how carbon monoxide incidents happen.

Key Facts

• Gas Safe registration is mandatory — no exemptions exist for "simple" gas fire faults; diagnosis and repair must be carried out by an engineer holding the correct appliance category for that fire type.
• Piezo ignition — a mechanical spark generator; failure to spark on repeated clicks usually indicates a worn piezo unit, a damaged ignition lead, or an incorrect electrode gap (the electrode must be close enough to the pilot burner to jump a spark reliably).
• Thermocouple — a safety device that generates a small millivolt current when heated by the pilot flame, holding the gas valve's safety solenoid open; if the pilot doesn't heat the thermocouple tip fully (misalignment, a weak pilot flame, or a failing thermocouple), the valve will close within seconds of releasing the ignition button and the pilot will go out.
• Thermopile — a higher-output version of a thermocouple used on some fires with electronic ignition or a remote control, generating enough current to power additional electronics as well as hold the main gas valve.
• ODS (Oxygen Depletion Sensor) — fitted to flueless gas fires; shuts the appliance down if room oxygen levels fall too low, which can present identically to an ignition fault if the room has inadequate ventilation.
• Spider/insect blockage — a very common cause of "no gas at the pilot" on fires left unused over summer; spiders build nests inside the pilot injector or burner ports, physically blocking gas flow.
• Gas supply checks — an empty prepayment meter, a closed emergency control valve, or a recently serviced/isolated system are the most common non-appliance causes of "won't light at all," and should be checked before assuming an appliance fault.
• Gas pressure — low working pressure at the appliance (checked with a manometer at the test point) will cause a weak or lifting flame, difficulty lighting, or a pilot that lights but won't sustain — this is a supply/installation issue, not necessarily a faulty appliance component.
• Electronic ignition fires — modern fires with a spark-generator box (rather than a manual piezo) can fail to ignite due to a flat or corroded battery, a faulty ignition module, or a loose connection at the ignition electrode.
• Balanced flue condensate — on some balanced-flue fires, condensate can pool and, in cold weather, partially block the flue path, affecting combustion air and causing ignition lockout on appliances with additional safety interlocks.
• CO alarm requirement is mandatory, not advisory — Building Regulations Approved Document J (as amended October 2022) requires a carbon monoxide alarm to BS EN 50291-1 in any room containing a new or replacement fixed combustion appliance (excluding some flueless appliances covered by their own specific rules) — this is a legal requirement of the installation, not a safety suggestion offered to the customer.
• Never bypass a safety device — a thermocouple, thermopile or ODS that is correctly cutting the gas supply is functioning as intended; the fault is upstream (why the flame isn't being detected), not in the device doing the cutting off.

Quick Reference Table

Detailed Guidance

Decision tree

GAS FIRE WON'T LIGHT — START HERE
(Gas Safe registered engineer only beyond this point)
         |
         v
IS THERE A SMELL OF GAS?
         |
    ----+----
    |        |
   YES       NO
    |        |
    v        v
STOP. Isolate at ECV,     Confirm gas supply present
ventilate, call National   (meter credit, ECV open,
Gas Emergency Service      isolation valve open)
(0800 111 999).                |
Do not attempt ignition.       v
                          Gas present at appliance?
                                |
                          -----+-----
                          |          |
                         YES        NO
                          |          |
                          v          v
                    Press/turn    Check meter, ECV,
                    ignition —    any isolation valve
                    does it       closed for other
                    spark?        work; resolve supply
                          |       issue first
                    -----+-----
                    |          |
                   YES        NO
                    |          |
                    v          v
              Does pilot   Inspect piezo, ignition
              light?       lead, electrode gap and
                    |      position; repair/replace
              -----+-----
              |          |
             YES        NO
              |          |
              v          v
        Does pilot   Isolate, inspect and clear
        STAY lit     pilot injector (check for
        when button  spider/debris blockage);
        released?    confirm gas reaching pilot
              |
        -----+-----
        |          |
       YES        NO
        |          |
        v          v
  Does MAIN     Pilot goes out = thermocouple/
  burner        thermopile not holding valve
  light?        open. Test millivolt output,
        |       check flame contact position,
  -----+-----   replace device if faulty
  |          |
 YES        NO
  |          |
  v          v
FIRE OK   Check main valve/solenoid,
now —     working gas pressure at
confirm   test point, and (flueless
CO alarm  fires) ODS status
fitted &
working

Confirming gas supply before suspecting the appliance

Before opening up any part of the fire, confirm gas is actually reaching it: check the meter (particularly prepayment meters, which can silently run out of credit), confirm the emergency control valve and any local isolation valve are fully open, and check whether other gas appliances in the property are working. A fire that "suddenly stopped working" after other trades have been in the property (plumbers, kitchen fitters) often turns out to be an isolation valve someone closed and forgot to reopen.

Ignition failure — piezo and electronic systems

A piezo igniter is a purely mechanical spark generator; if repeated clicks produce no spark, check the ignition lead for damage and the electrode gap and position relative to the pilot burner — the spark needs to jump reliably to the correct point, not just occur. On electronic ignition fires, check battery condition first (a surprisingly common cause dismissed as "the fire's broken"), then the ignition module and its connections per the manufacturer's fault-finding procedure.

The flame failure device — why it exists and how to diagnose it correctly

A thermocouple or thermopile is a safety device, not a nuisance to be defeated. It sits in the pilot flame and generates a small electrical current that holds a solenoid valve open, allowing gas to flow to the main burner. If the pilot flame doesn't properly heat the thermocouple tip — because the pilot flame itself is weak, poorly positioned, or the thermocouple has degraded — the valve will close within roughly 30–90 seconds of releasing the ignition button, and the pilot will go out.

Diagnosing this correctly means testing the thermocouple's actual millivolt output against the manufacturer's specification (typically using a millivolt meter connected in the circuit), not just replacing it on suspicion. A weak or misaligned pilot flame can produce the identical symptom to a genuinely failed thermocouple, so check flame position and quality first.

Blocked pilot injectors — the most common seasonal fault

Gas fires that sit unused for months (typically switched off over summer) are highly prone to insects — particularly spiders — building nests inside the small-bore pilot injector or burner ports. This physically restricts or blocks gas flow to the pilot, producing symptoms that look identical to a supply or valve fault. Isolating the appliance and inspecting/clearing the pilot assembly should be a routine early check, especially on a fire that worked fine at the end of the previous heating season.

Flueless fires and the ODS

Flueless gas fires include an Oxygen Depletion Sensor that will shut the appliance down — sometimes after it has run briefly — if oxygen levels in the room fall below the safe threshold. This can be mistaken for an ignition fault when it's actually a ventilation issue: check that the room meets the minimum volume and permanent ventilation requirements for that specific flueless appliance (see flueless and flue gas fire requirements) rather than treating it purely as a component fault.

The carbon monoxide alarm is part of the job, not an add-on

Any fault-finding visit to a gas fire is an opportunity — and, on new or replacement appliance work, a legal requirement — to confirm a working CO alarm is fitted in the room, positioned to BS EN 50292 guidance (1–3 metres horizontal from the appliance, correct height below the ceiling). Under Approved Document J (as amended October 2022), a CO alarm to BS EN 50291-1 is mandatory for any new or replacement fixed combustion appliance — this is not a recommendation the engineer can leave to the customer's discretion. See co alarm positioning for exact siting rules.

Frequently Asked Questions

Can I relight the pilot myself if it's just gone out?

If a pilot light has simply gone out with no smell of gas and no other symptoms, following the appliance's printed relighting instructions (usually on a label inside the fire) is generally reasonable for a homeowner. However, if the pilot won't stay lit, keeps going out repeatedly, or there is any smell of gas, stop and call a Gas Safe registered engineer — repeated failure to hold a flame is the flame failure device correctly detecting a problem, and forcing repeated relights around a faulty safety device is dangerous.

Is it safe to keep pressing the ignition button if the fire won't light?

A handful of attempts following the manufacturer's instructions is normal. If it consistently won't light, or lights and immediately goes out, stop and get it looked at rather than continuing to try — repeated attempts can release small amounts of unburnt gas into the room, and the underlying fault won't resolve itself through persistence.

What does "clicking but no spark" mean?

This points to the ignition system rather than the gas supply — most commonly a worn piezo igniter, a damaged or disconnected ignition lead, or an electrode that has moved out of position or become fouled with soot/debris so the spark can't jump the gap reliably. This is a straightforward repair for a Gas Safe registered engineer but is not something to investigate by removing appliance covers without the correct competency.

Do I legally need a CO alarm fitted with my gas fire?

Yes, if the fire is a new installation or a replacement of an existing fixed combustion appliance. Approved Document J, as amended in October 2022, makes a CO alarm to BS EN 50291-1 a mandatory Building Regulations requirement for these installations — it is not optional or advisory. An engineer carrying out replacement or new gas fire work should fit and correctly position a compliant alarm as part of the job, not merely suggest one to the customer.

Regulations & Standards

• Gas Safety (Installation and Use) Regulations 1998 — the statutory basis restricting installation, servicing and repair of gas appliances to competent, Gas Safe registered persons.

• Building Regulations Approved Document J (as amended October 2022) — mandates a carbon monoxide alarm to BS EN 50291-1 for any new or replacement fixed combustion appliance.

• BS EN 50291-1:2018 — product standard for carbon monoxide alarms used to meet the Part J requirement (incorporating a January 2021 corrigendum; no further amendment currently in force).

• BS EN 50292 — siting and positioning guidance for carbon monoxide alarms relative to the appliance.

• BS 5871 series — installation requirements for gas fires and other domestic gas-fired appliances: Part 1 covers gas fires, convector heaters and fire/back boilers; Part 3 covers decorative fuel-effect appliances; Part 4 covers flueless appliances. The applicable part depends on the specific appliance category being installed or serviced.

• Gas Safe Register — registered engineer search and gas safety guidance

• HSE: Gas safety — statutory guidance on the Gas Safety (Installation and Use) Regulations 1998

• Approved Document J: Combustion appliances and fuel storage systems (gov.uk) — CO alarm and ventilation requirements

• National Gas Emergency Service — 0800 111 999, for any suspected gas leak or smell of gas

• gas fireplace — gas fire types (open-flued, balanced flue, flueless) and their distinct requirements

• flueless and flue gas fire requirements — room ventilation and sizing rules referenced by the ODS fault path

• co alarm positioning — exact siting distances and heights for the mandatory CO alarm

• gas safety certificate — annual landlord gas safety record requirements relevant to rented properties with gas fires

• gas fire installation pricing guide — pricing for replacement gas fire installation where repair isn't viable