Boiler Ignition Parts Fault Finding: Spark Electrode, Gas Valve, PCB, Flame Sensor and Ignition Lead Diagnosis

Quick Answer: Boiler ignition failures account for the majority of "no heating, no hot water" call-outs. The ignition sequence involves: PCB fires the spark electrode → gas valve opens → burner lights → flame sensor (ionisation probe) confirms flame → PCB holds gas valve open. Failure at any stage produces a lockout. All gas work, including diagnosis and parts replacement on gas boilers, is notifiable under the Gas Safety (Installation and Use) Regulations 1998 and must be carried out by a Gas Safe registered engineer.

SAFETY WARNING: Gas work is notifiable. Only Gas Safe registered engineers may work on gas appliances. If you smell gas, leave the property immediately, do not operate any switches, and call the National Gas Emergency Service on 0800 111 999. Never attempt to diagnose or repair a gas boiler unless you hold a valid Gas Safe registration and the relevant ACS competencies (e.g. CCN1, CPA1, CKR1).

Summary

A boiler that locks out on ignition but has a working gas supply, correct pressure, and no visible fault can be maddening to diagnose. The components in the ignition chain — spark electrode, ignition lead, gas valve, ionisation probe (flame sensor), and PCB — are all interdependent, and a fault in one can look exactly like a fault in another.

The approach used by experienced Gas Safe engineers is systematic: confirm gas supply and pressure first, then work through the ignition chain from PCB output to burner, testing each component in sequence. Replacing parts without this sequence wastes the client's money and delays resolution.

This article provides a structured fault-finding framework for Gas Safe registered engineers. It covers the most common failure modes, testing procedures, and a decision-tree diagnostic for the most frequently seen ignition faults on combi boilers.

Key Facts

• Gas Safety (Installation and Use) Regulations 1998 — all gas work must be done by a Gas Safe registered engineer. Notifiable work must be certified within 28 days on a Benchmark certificate.
• ACS competencies required — at minimum CCN1 (core) and CPA1 (pipework and appliance) for domestic gas boiler work. Additional competencies for commercial gas (CCCN1) and specific appliance types.
• Ignition sequence (typical combi): call for heat → pump runs → fan starts → air pressure switch confirms fan → spark fires (3–10 seconds) → gas valve opens → burner ignites → ionisation probe measures 3–8 μA DC → PCB confirms flame → fan modulates → boiler runs.
• Lockout vs soft lockout — a lockout requires manual reset; a soft lockout clears automatically after a timeout. Most modern boilers (post-2010) flash an error code before locking out.
• Spark electrode gap — typically 3–4mm. If gap is closed (carboned up) or open (cracked), spark will be weak or absent. Visual inspection is often sufficient.
• Ignition lead resistance — typically 5–20 kΩ. Out-of-spec resistance indicates lead failure. Measure with a multimeter on high resistance range (20 MΩ range).
• Ionisation probe (flame sensor) current — 1.5–8 μA DC in flame. Below 0.5 μA, most PCBs will lockout. Test with a DC microammeter in series with the probe lead; some PCBs allow reading via a diagnostic port.
• Gas valve coil resistance — varies by manufacturer. Typically 10–40 Ω for main gas valve solenoid; 80–200 Ω for pilot gas valve (if applicable). Check manufacturer's service data.
• PCB output voltage (ignition) — 230V AC to ignition transformer primary; secondary voltage to electrode typically 15,000–25,000V. Do not test secondary directly — infer from spark visible at electrode.
• Flue gas CO₂ reading — after ignition, CO₂ should be within manufacturer spec (typically 8.5–10.5% for natural gas at max fire, 7.5–9.0% at min fire). Low CO₂ can indicate intermittent air ingress, affecting combustion and ionisation.
• Common boiler lockout codes: E01/F01 (ignition failure — Baxi/Potterton); EA (ignition failure — Worcester Bosch Greenstar); 3H (ignition failure — Vaillant); E1 (ignition — Ideal); 110 (ignition — Viessmann Vitodens).
• Gas inlet pressure (natural gas) — 20 mbar at rest (standing), minimum 17 mbar working pressure at max rate. Below 17 mbar, gas valve may not open fully.
• Benchmark Checklist — mandatory completion for all boiler installations and first service after installation (Gas Industry Unsafe Situations Procedure 2020).

Quick Reference Table

Detailed Guidance

ASCII Decision Tree — Boiler Ignition Fault Diagnosis

BOILER LOCKOUT / WILL NOT IGNITE
          |
          v
Is there a gas supply to the property?
(Check gas hob if present, or meter supply)
          |
    NO ---+---> Call National Gas Emergency: 0800 111 999
          |
         YES
          |
          v
Check boiler fault code (manufacturer guide).
Is code an ignition fault?
(E.g. EA / E01 / F01 / 3H / E1 / 110)
          |
    NO ---+---> Follow fault-code specific guide (overheat, APS, pump, etc.)
          |
         YES
          |
          v
Check gas inlet pressure at valve test point.
Is working pressure ≥17 mbar?
          |
    NO ---+---> Low pressure fault — check meter regulator, pipework.
          |     Notify gas transporter (Cadent, Wales & West, SGN) if meter issue.
          |
         YES
          |
          v
Power boiler OFF. Remove burner cover.
Inspect spark electrode visually.
Is electrode cracked, carboned up, or gap outside 3–4mm?
          |
   YES ---+---> Clean/adjust/replace electrode.
          |     Reset and test. Does it ignite?
          |        YES → resolved.
          |        NO  → continue below.
          |
    NO (electrode looks OK)
          |
          v
Test ignition lead with multimeter (high-Ω range, 20MΩ).
Is resistance within spec (typically 5–20 kΩ)?
          |
    NO ---+---> Replace ignition lead. Test. Ignites? YES → resolved.
          |
         YES
          |
          v
Power boiler ON. Observe ignition attempt.
Is there a visible spark at electrode?
          |
    NO ---+---> Check PCB ignition output terminals.
          |     Is 230V AC present at ignition terminal during attempt?
          |        NO  → PCB not firing. Check APS first (see below).
          |        YES → Spark transformer failed (if separate) or
          |              electrode/lead fault not caught above.
          |
         YES (spark present)
          |
          v
Does gas flame appear at burner?
          |
    NO ---+---> Gas valve not opening.
          |     Check 230V AC at valve coil terminals during attempt.
          |        NO  → PCB not sending valve signal. Check APS, fan signal.
          |        YES → Check valve coil resistance. Within spec?
          |              NO  → Replace gas valve.
          |              YES → Valve mechanically stuck (rare). Replace.
          |
         YES (flame appears)
          |
          v
Does boiler run for >5 seconds then lock out?
          |
    NO ---+---> Ignition succeeds. Look for other fault (overheat, etc.)
          |
         YES
          |
          v
Ionisation/flame sense fault.
Test probe current with DC microammeter in series.
Is current ≥1.5 μA DC during stable flame?
          |
    NO ---+---> Probe dirty, cracked, or grounded. Clean/replace probe.
          |     Check earth reference for probe circuit.
          |     Still fails? Check PCB flame sense circuit.
          |
         YES
          |
          v
PCB failing to hold on adequate flame signal.
Consider PCB replacement (after checking all wiring, connectors,
and earthing — loose earths cause spurious ionisation readings).

If all components test correctly and fault persists:
→ Flue gas CO₂ reading — is combustion in spec?
→ Gas rate at meter (ft³/hr or m³/hr) — within boiler spec?
→ Check for intermittent gas supply (pressure surge/dip).
→ Consult manufacturer technical support.

Spark Electrode Diagnosis

The spark electrode is the most commonly replaced ignition part, and also the most commonly misdiagnosed. Before replacing:

Visual inspection:

• Remove the electrode from the combustion chamber (usually 1–2 screws).
• Inspect the ceramic insulator for cracks — even a hairline crack causes tracking to earth and a weak spark.
• Inspect the electrode tip: carbon deposits reduce the spark intensity. Clean with fine emery cloth or a brass wire brush. Do not use steel — it leaves conductive particles.
• Measure the gap between electrode tip and earth (burner body or adjacent electrode): should be 3–4mm per manufacturer spec. Adjust carefully — the ceramic is fragile.
• Ensure the electrode body is not touching the combustion chamber (causes short to earth).

Electrical test: Using a multimeter in resistance mode (highest range), measure between the electrode tip and earth. In air (electrode removed from boiler), resistance should be >1 MΩ. If it is significantly lower, the insulator is tracking. Replace the electrode.

Ignition Lead Diagnosis

The ignition lead carries the high-voltage pulse from the ignition transformer to the electrode. Leads fail due to insulation breakdown, especially near the boiler casing where they are subject to heat cycling.

Test:

• Disconnect both ends of the ignition lead.
• Use a multimeter on a 20 MΩ range.
• Measure end-to-end resistance: should be 5–20 kΩ (check manufacturer spec — some specify much lower, e.g. 1 kΩ).
• A reading of several MΩ or an open circuit indicates a break in the lead.
• A reading of near 0 Ω indicates an internal short.

Visual: Check for insulation damage, kinks, burns, or a poorly seated push-fit connector at the electrode end. Corrosion at the connector can cause intermittent spark even when the lead itself is intact.

Gas Valve Diagnosis

Safety: Gas valve diagnosis involves working on the gas-carrying components of the appliance. Confirm the gas is isolated at the boiler service valve before testing coil resistance. Never bypass or manually force a gas valve open.

Solenoid coil test:

• Isolate gas supply.
• Disconnect the electrical connector from the gas valve.
• Using a multimeter in resistance mode, measure across the solenoid coil terminals.
• Compare to manufacturer's service data (typically 10–40 Ω for main gas valve solenoid).
• An open circuit (OL) indicates a burnt-out coil. Very low resistance indicates a short. Either requires gas valve replacement.

Live voltage test (Gas Safe engineer only):

• With the boiler attempting ignition, use a multimeter (correctly rated for 230V AC) to check for voltage at the gas valve terminals.
• If 230V AC is present and the valve coil is in spec, but gas does not flow, the valve body is mechanically stuck — usually due to scale or debris. Replacement required.
• If no voltage: PCB not energising the valve. Check air pressure switch, fan speed signal, and other preconditions.

Ionisation Probe (Flame Sensor) Diagnosis

The ionisation probe works by passing a small DC current through the flame. The flame conducts electricity (due to ionised gas ions), completing the circuit back to the PCB. A PCB typically looks for 1.5–8 μA DC.

Common failure modes:

• Dirty probe — combustion deposits on the probe reduce conductivity. Clean with fine abrasive and check function.
• Cracked probe insulator — allows leakage current to earth, giving a false low reading.
• Probe positioned incorrectly — if the probe is not in the flame envelope (e.g. after re-assembly), no current flows. Refer to manufacturer's diagram.
• Poor earth reference — the ionisation circuit relies on a reliable earth return via the boiler casing. A loose or corroded earth connection causes erratic readings. Check the boiler chassis earth connection.

Test: A DC microammeter connected in series with the probe lead (break the circuit, insert the ammeter) will show the actual probe current during a flame attempt. This is the definitive test. Most modern diagnostic tools (e.g. Fluke 87V with μA mode, or dedicated boiler diagnostic meters) can measure this directly.

PCB Diagnosis

The PCB is the last component to replace — it is the most expensive and most often incorrectly blamed. Before replacing a PCB:

1. Check all wiring connectors are fully seated — vibration from the fan loosens connectors over time.
2. Check the PCB for visible burn marks, swollen capacitors, or corrosion.
3. Verify all input signals are correct (fan speed signal, APS switch, NTC thermistor readings, pressure sensor).
4. Consult the manufacturer's wiring diagram and service manual — PCB outputs can be tested with a multimeter at the terminal block.

If a PCB is replaced and the fault persists, the original component was not the cause. A systematic approach at the start saves this outcome.

Frequently Asked Questions

The boiler sparks but the gas never lights — where do I start?

Confirm spark is present at the electrode (not just that the PCB is sending a signal). If spark is confirmed, the issue is with gas delivery to the burner — check gas pressure at the valve inlet test point, then check the valve solenoid coil resistance and whether the PCB is energising the valve during the ignition attempt. A gas valve that opens but does not allow adequate flow may have a blocked filter (replaceable on some valves) or a faulty actuator.

The boiler lights, runs for 3–5 seconds, then locks out — what is this?

This is almost always an ionisation probe (flame sensor) issue. The boiler lights successfully, but the PCB does not receive a strong enough flame-sense signal to hold the gas valve open. Clean the probe first. If that fails, test probe current with a DC microammeter. If current is adequate, the PCB flame-sense circuit may be at fault.

The boiler ignites fine on first call of the day but then locks out on subsequent calls — what causes this?

Intermittent ignition faults that worsen when the boiler is hot are often caused by a cracked electrode insulator (tracking worsens when hot), a heat-degraded ignition lead, or a PCB component that fails when warm. A gas valve that sticks when hot (thermal expansion of valve body) is less common but possible. Diagnose in the hot condition, not after allowing the boiler to cool.

Can I replace a gas valve myself if I am Gas Safe registered?

Yes, if you hold the relevant ACS competencies. Gas valve replacement is gas work within the scope of a Gas Safe registered engineer with CCN1/CPA1. The replacement valve must be the correct OEM part or an approved equivalent for that boiler. After replacement, a full tightness test, combustion analysis, and Benchmark record are required.

My client has a boiler under a service contract — should I still carry out the repair?

Check the terms of the service contract. Many contracts (British Gas HomeCare, etc.) require the client to use the contract provider for repairs. If you carry out repair on a boiler under a live third-party contract, the client may face issues with their contract warranty. Advise the client to check first.

Regulations & Standards

• Gas Safety (Installation and Use) Regulations 1998 (SI 1998/2451) — primary legislation for all gas work in the UK. Requires Gas Safe registration for all work on gas appliances.

• Gas Industry Unsafe Situations Procedure (GIUSP) — Gas Safe Register — procedures for handling unsafe gas situations discovered during work.

• Benchmark Scheme (Heating and Hotwater Industry Council, HHIC) — mandatory checklist for boiler installation and first service; completion required by boiler warranty conditions.

• BS 6798:2014+A1:2016 — Specification for selection, installation, inspection, commissioning, servicing, and maintenance of gas-fired boilers of rated heat input not exceeding 70 kW net.

• Building Regulations Part J (England and Wales) — combustion appliances and fuel storage systems.

• CIBSE Guide F — Energy Efficiency in Buildings — reference for combustion efficiency and gas boiler performance.

• HSE Approved Code of Practice L56 — safety in gas fitting — supporting guidance to Gas Safety (Installation and Use) Regulations 1998.

• ACS Competency Standards (Gas Safe Register) — CCN1 (core), CPA1 (pipework and appliances), CCCN1 (commercial core) — required competencies for gas work.

• Gas Safe Register — engineer registration, competency requirements, public safety resources

• HSE — Gas Safety — regulatory guidance and enforcement for gas safety

• Heating and Hotwater Industry Council (HHIC) — Benchmark — Benchmark checklist and scheme requirements

• National Gas Emergency Service — 0800 111 999, for gas escape emergencies

• Worcester Bosch Technical Support — manufacturer service manuals and fault code guides

• gas smell — gas escape emergency procedure

• boiler not lighting — broader boiler no-ignition fault tree

• no heating — systematic no-heating fault finding including controls

• cold radiators — fault finding for heat distribution issues post-ignition

• boiler servicing — annual service procedure covering combustion analysis and ignition component checks

• gas safe requirements — Gas Safe registration requirements and ACS competencies