July 18, 2026 APM Plumbing & Electrical

Boiler Fault Codes and Error Code Diagnostics: A Gas Safe Engineer’s Troubleshooting Guide

How Boiler Fault Codes Work

Modern boilers use a combination of sensors, flow switches, pressure switches, and flame detection circuits to monitor safe operation. When a sensor reading goes out of range or a safety device trips, the boiler locks out and displays a fault code. Codes fall into broadly four categories:

Ignition failure — the burner failed to light within the trial-for-ignition period
Flame loss — the burner lit but flame was not sustained (or the ionisation probe did not confirm it)
Overheat / high limit — a safety thermostat or NTC sensor detected temperature above its limit
Water pressure / flow — low pressure, a blocked heat exchanger, or insufficient flow through the primary circuit

Before starting component-level diagnosis, always carry out a basic system check: water pressure (should be 1–1.5 bar cold), condensate pipe (blocked in cold weather causes many lockouts), gas supply (isolator open, meter not in test mode), and flue terminal (not obstructed).

Gas safety reminder: All gas work must be carried out by a Gas Safe registered engineer. Fault codes can be read by anyone, but rectification of gas-related faults requires registration. Always isolate the gas supply and prove dead before accessing burner or gas valve components.

Vaillant ecoTEC Fault Codes

Vaillant ecoTEC Plus and ecoTEC Pro are among the most widely installed boilers in the UK. Fault codes appear as F.XX or E.XX on the display.

F.22 — Water Pressure Too Low

Meaning: The boiler pressure has dropped below approximately 0.5 bar, triggering the pressure switch or low-pressure sensor.
Common causes: Expansion vessel failure or undercharged pre-charge pressure, leaking radiator valve or auto air vent, recent bleeding of radiators without re-pressurising, or a leak on a micro-bore circuit.
Check first: Top up to 1.0–1.5 bar via the filling loop, then reset. If pressure drops again within a few heating cycles, carry out a full leak search. Check expansion vessel pre-charge pressure (should be equal to static head or 0.75–1.0 bar on most domestic systems) with a tyre gauge when system is cold and pressure has been released to zero. See also: sealed system pressure guide.

F.28 — Ignition Failure (No Gas at Burner)

Meaning: The boiler attempted ignition and the ionisation probe did not detect a flame within the trial-for-ignition period.
Common causes: Gas supply interrupted (check other appliances), gas valve not opening (sticky valve or failed valve coil), spark electrode fouled or cracked, ignition lead tracking to earth, PCB ignition module failure.
Check first: Confirm gas supply to boiler with pressure gauge on test point. Listen/feel for gas valve click during ignition attempt. Check spark electrode gap (typically 3–4 mm) and clean or replace if carbon-fouled. Measure resistance of ignition lead — should be low Ohm; replace if cracked insulation. If gas is present and spark is good but no ignition, suspect blocked injector or cold gas valve that won't open under low inlet pressure. See: domestic gas pipework and supply requirements.

F.29 — Ignition Failure (Flame Not Established)

Meaning: Gas reached the burner and a spark occurred, but the ionisation probe did not confirm a stable flame.
Common causes: Ionisation/sensing probe dirty, corroded, or cracked; incorrect electrode gap; UV sensor fault on older models; burner assembly contaminated with condensate residue; gas/air mixture out of range due to partly-blocked injector or incorrect gas rate.
Check first: Inspect ionisation probe for deposits (clean with fine emery cloth), check probe insulation, and measure the ionisation current (should be >1 µA DC when flame is present). Carry out gas rate check at meter — compare to data plate. If gas rate is low, check for pressure drop on inlet or restriction in gas valve bypass.

F.75 — Pressure Sensor / Pump Fault

Meaning: On start-up the boiler expects to see a pressure rise from the pump, but the pressure sensor reading is outside expected range.
Common causes: Air-locked or seized circulator pump, failed pressure sensor (dry or faulty diaphragm), wiring fault between sensor and PCB.
Check first: Bleed pump via the bleed screw on the pump head (Grundfos UPS/Alpha/Wilo pattern). If pump shaft is seized, free with a flat-blade screwdriver on the motor shaft. Replace pressure sensor if pump runs freely but code persists.

F.83 — NTC Sensor Fault (Temperature Difference Too Low)

Meaning: The flow and return NTC temperature sensors show too small a difference, indicating very low flow rate or that the burner ignited but the heat exchanger is not responding.
Common causes: Highly restricted primary circuit (sludged heat exchanger, closed zone valve or TRV bodies), air-locked system, pump failure at low output, faulty NTC sensors giving incorrect readings.
Check first: Confirm all zone valves are open and calling. Check NTC resistance values at cold (approximately 10 kΩ at 25 °C for most Vaillant sensors — verify against data sheet). If NTCs are correct, the fault is likely hydraulic: check for delta-T >20 °C which suggests restricted flow, and consider powerflushing the heat exchanger.

Worcester Bosch Greenstar Fault Codes

Worcester Bosch Greenstar i, CDi, and Si models display fault codes as C.XX, D.XX, or E.XX, though older models (Greenstar 24i/28i Junior) use a simpler LED pattern.

EA338 — No Flame Detected (Ignition Lockout)

Meaning: The most common Worcester lockout. The boiler attempted to fire but did not detect a flame within the safety period.
Common causes: As with Vaillant F.28/F.29 — gas supply, electrode, or sensing circuit. On Worcester, the ionisation and ignition functions are combined on a single electrode on many models, so a cracked ceramic is particularly common.
Check first: Inspect ignition/ionisation electrode assembly — the ceramic insulator cracks with age and allows tracking. Replace electrode assembly if cracked. Check gas inlet pressure at test point (should be 20 mbar static, 18 mbar working on natural gas). If pressure is correct and electrode is good, suspect PCB ignition module or gas valve coil.

E9 — Overheat Lockout

Meaning: The overheat thermostat (manually-resettable safety device) has tripped due to high temperature at the heat exchanger.
Common causes: Blocked or sludged heat exchanger restricting flow, air lock causing low-flow condition, failed pump, closed zone valves with no bypass fitted, or faulty overheat thermostat that trips at too low a temperature.
Check first: Allow boiler to cool fully (do not force-reset the overheat thermostat hot). Locate the overheat thermostat reset button (usually a small red button behind the boiler casing on the flow pipe). Investigate cause before reset — if the overheat trips again immediately, the heat exchanger is almost certainly sludged. A powerflush should precede any further operation.
Note: Resetting the manual overheat thermostat without investigating the cause can lead to BLEVE risk if the condition is a blocked heat exchanger with no flow. Always identify the root cause first.

E119 — Water Pressure Low

Meaning: System pressure has dropped below the boiler's minimum threshold (typically 0.3–0.5 bar on Worcester).
Common causes and checks: Identical to Vaillant F.22. Top up to 1.0–1.5 bar, reset, monitor. Persistent low pressure indicates a leak or expansion vessel failure. See: sealed heating system pressure guide.

E28 / E32 — Fan Fault

Meaning: The fan speed (measured by Hall-effect tachometer) is outside the expected range, or the pre-purge air pressure switch has not operated.
Common causes: Fan bearing failure (noisy or seized fan), blocked flue or flue terminal, faulty flue pressure switch (diaphragm split or tube kinked), PCB fan control fault.
Check first: Inspect flue terminal for obstruction (bird nests, ice, debris). Check condensate trap on flue — if blocked, back pressure can prevent pressure switch from operating. Listen to fan start-up — grinding or intermittent rotation indicates bearing failure. Measure fan supply voltage and tachometer feedback signal to distinguish PCB fault from fan motor fault.

Ideal Boilers Fault Codes

Ideal Logic, Vogue, and Evomax boilers use F codes for faults.

F1 — No Ignition / Flame Failure

Meaning: The equivalent of Vaillant F.28 — ignition lockout after failed trial-for-ignition.
Common causes and checks: As above — gas supply, electrode gap, ionisation probe, gas valve. On Ideal Logic models, the spark electrode and sensing probe are separate. Check both.

F2 — Flame Loss

Meaning: The burner lit but flame was lost during firing — typically nuisance lockout from combustion instability.
Common causes: Contaminated condensate affecting combustion chamber, incorrect gas/air ratio (check gas rate), partly-blocked injector, or draught reversal pulling flame off the burner. Also seen with gas valve that hunts (intermittent gas supply at part-load).
Check first: Carry out CO/CO₂ analysis at the flue terminal — high CO or poor combustion efficiency confirms gas/air mixture issue. Check that fan is achieving correct RPM at all firing rates. Clean burner assembly if contaminated.

F4 — NTC1 Sensor Fault (Flow)

Meaning: The flow (primary) NTC temperature sensor has given an out-of-range or open-circuit reading.
Common causes: Sensor has failed open-circuit or short-circuit, poor connector contact, wiring damage.
Check first: Measure NTC resistance — typically 10 kΩ at 25 °C, 2 kΩ at 60 °C. Replace if open or short circuit. Check connector pins for corrosion or moisture ingress.

F5 — NTC2 Sensor Fault (Return)

Meaning: Return NTC sensor out of range.
Common causes and checks: As F4 but on the return sensor. If both F4 and F5 appear in quick succession, suspect a wiring loom issue or moisture on the PCB sensor input circuitry rather than two simultaneous sensor failures.

F9 — Fan Speed Fault

Meaning: Fan tachometer outside expected range — same principle as Worcester E28/E32.
Check first: As Worcester fan fault above.

Baxi Fault Codes

Baxi 200, 400, 600, 800, Platinum, and Main boilers display E codes.

E110 — Overheating Safety Shutdown

Meaning: Boiler has detected an overtemperature condition — the safety thermostat has tripped.
Common causes and checks: As Worcester E9. Check flow rate through heat exchanger before resetting. If heat exchanger is sludged, carry out a powerflush before returning boiler to service.

E119 — Low Water Pressure

Meaning: System pressure too low — shared code with Worcester.
Common causes and checks: Identical to Vaillant F.22 and Worcester E119. See: sealed heating system pressure guide.

E125 — Pump / Flow Switch Fault

Meaning: The flow switch or pump has not provided expected feedback on start-up.
Common causes: As Vaillant F.75 — seized pump, air lock, or faulty flow switch.
Check first: Bleed pump, check for airlocks at high points in circuit. If pump runs but fault persists, test flow switch continuity.

E133 — No Ignition / Gas Valve Fault

Meaning: Ignition failure — boiler attempted to fire but gas valve did not open or flame was not detected. On some Baxi models, E133 specifically indicates a gas valve coil fault.
Check first: Measure resistance of gas valve coils — typically 800–2000 Ω depending on model. An open-circuit coil requires gas valve replacement. Confirm PCB output voltage to gas valve on ignition attempt (typically 24 V AC or 230 V AC depending on model).

Vokera / Riello Fault Codes

A01 — Ignition Failure

Standard ignition lockout — as above for other brands. Check electrode, gas supply, ionisation circuit.

A02 — Flame Failure During Operation

Burner lit but flame lost mid-firing — as Ideal F2 above.

A03 — Fan Fault

Fan speed outside expected range — as Worcester E28 above.

L2 — Low Water Pressure

System pressure below minimum — as Vaillant F.22 above. Vokera/Riello use L codes for lockout faults requiring manual reset. See: sealed heating system pressure guide.

Condensate Trap Blockage — a Cross-Brand Issue

In cold weather, the most common cause of boiler lockout across all brands is a frozen or blocked condensate pipe. The condensate drain runs from the condensate trap inside the boiler to the external drain or internal soil stack. When frozen or blocked, back pressure builds inside the combustion chamber and either:

Prevents the flue pressure switch from operating (triggering fan/air pressure fault codes)
Causes combustion gases to spill, triggering an overheat or CO detector
Causes gurgling/water sounds as condensate backs up into the heat exchanger

Temporary fix: Apply warm (not boiling) water to the external condensate pipe to thaw the blockage. Do not use boiling water as this can crack push-fit condensate fittings. Once thawed, reset the boiler.
Permanent fix: Reroute the condensate pipe to discharge internally (preferred) or insulate the external run. Minimum 19 mm pipe lagging on all exposed sections. Use 32 mm diameter external pipe rather than 22 mm to reduce freeze risk.

Gas Rate Check — the Overlooked Diagnostic Step

Many ignition and flame faults are caused by insufficient gas supply rather than component failure. Before condemning a gas valve or PCB, always carry out a gas rate check. See our guide to domestic gas pipework, sizing, and supply requirements for background on pipe sizing and meter capacity.

Isolate all other gas appliances.
Start the boiler at maximum firing rate (put into commissioning mode if required).
Time the rotation of the gas meter index wheel or read the pulse output — 1 rotation of the 1/100 ft³ dial = 0.0283 m³.
Convert to kW: kW = m³/h × calorific value (typically 10.97 kWh/m³ for UK natural gas at typical CV).
Compare to boiler data plate maximum output.

A gas rate significantly below the data plate figure indicates a supply restriction: check for undersized meter, long or undersized pipework run, shared riser with multiple appliances, or a sticky gas valve.

Combustion Analysis at Service

A combustion analyser should be used on every boiler service. BS 7967 Part 4 and Gas Industry Unsafe Situations Procedure (GIUSP) both require combustion analysis as part of the annual safety check. Target values for natural gas condensing boilers:

CO/CO₂ ratio: <0.004 (safe) — values above 0.008 are immediately dangerous
CO in flue: <200 ppm (well-adjusted boiler); >350 ppm = Immediately Dangerous (ID) under GIUSP
CO₂: 8.5–10.5% for natural gas at rated output
Flue gas temperature: varies — check efficiency calculation against label (should be >90% seasonal efficiency for condensing operation)

Poor combustion that cannot be corrected by adjusting the gas valve offset screw (on adjustable valves) requires further investigation: burner contamination, heat exchanger restriction, or gas/air ratio at part-load. Always re-analyse after any adjustment.

Useful Diagnostic Equipment for Fault Code Work

Having the right test instruments speeds up boiler fault diagnosis significantly:

Flue gas analyser — CO/CO₂/O₂ measurement to BS 7967, essential for every service
Digital manometer — gas inlet pressure, gas valve outlet pressure, flue draught measurement
Clamp meter with DC µA function — ionisation current measurement (confirm flame signal)
NTC resistance chart — printed reference for resistance vs temperature to diagnose sensor faults without brand-specific tools
Voltage testers and proving units — safe isolation before accessing live components
Multimeter — gas valve coil resistance, NTC sensor check, PCB output voltages

When to Condemn a Boiler

Fault codes assist diagnosis, but some faults are uneconomical to repair, particularly on older boilers (10+ years). Consider condemnation when:

The heat exchanger is cracked or corroded through — repair cost exceeds boiler value
The PCB is unavailable (obsolete boiler) or costs more than 40% of a replacement boiler
Multiple NTC sensors, gas valve, and pump have all failed simultaneously — indicative of underlying system problems (sludge, incorrect inhibitor, water quality) that will continue to cause failures
The boiler has repeated overheat lockouts and the heat exchanger cannot be restored by powerflush — blocked aluminium or stainless heat exchangers on condensing boilers often cannot be cleared and require replacement

When condemning a boiler, follow GIUSP: label the appliance, record the fault code and reason for condemnation on the Gas Safe register notification, and advise the customer in writing. If you are advising on a replacement, see our guide to combi vs system vs regular boilers for selection criteria. For landlord properties, see the CP12 and landlord gas safety requirements.

Summary: First-Five-Minutes Fault Code Checklist

Read the fault code and record it
Check system water pressure — top up if below 1.0 bar
Check condensate pipe — visual inspection, listen for gurgling
Check gas supply — confirm other appliances working, test point pressure
Check flue terminal — visual inspection for obstruction
Attempt reset — if boiler fires and holds, fault was likely transient (condensate, low pressure, brief gas interruption)
If fault returns: move to component-level diagnosis based on fault code category above

Working systematically from the most common causes saves time and prevents unnecessary parts replacement. The majority of boiler lockouts in the UK are caused by low system pressure, frozen condensate, or ignition component wear — not PCB or gas valve failure.

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