Intergas Fault Code 4: Flame Failure / No Ignition Signal
Fault code 4 on an Intergas boiler indicates that the printed circuit board (PCB) has been unable to detect a flame signal. When you call for heat or hot water, the PCB instructs the gas valve to open so the burner can ignite. If code 4 appears, the boiler cannot confirm that ignition has actually taken place — either the flame is present but going undetected, or it has failed to light altogether. As a safety precaution, the boiler will typically lock out until the underlying issue is resolved.
General guidance only — not a substitute for professional advice. Any gas work must be carried out by a Gas Safe registered engineer. If you smell gas or suspect carbon monoxide, leave the property and call the National Gas Emergency line on 0800 111 999.
What does the Intergas 4 fault code mean?
Fault code 4 on an Intergas boiler indicates that the printed circuit board (PCB) has been unable to detect a flame signal. When you call for heat or hot water, the PCB instructs the gas valve to open so the burner can ignite. If code 4 appears, the boiler cannot confirm that ignition has actually taken place — either the flame is present but going undetected, or it has failed to light altogether. As a safety precaution, the boiler will typically lock out until the underlying issue is resolved.
Common causes
- Faulty or dirty ignition electrode Common
Over time the ignition electrode accumulates carbon deposits or simply wears down, meaning it can no longer produce a reliable spark to light the burner. This is one of the most frequent triggers for code 4 on ageing Intergas boilers.
- Faulty flame sensor Common
The flame sensor monitors whether the burner has lit after ignition. If the sensor is degraded or has a poor electrical connection, it reports no flame to the PCB even when gas is actually burning, causing an immediate lockout.
- Gas supply issue Common
If the gas isolation tap adjacent to the boiler is partially or fully closed, or if there is a wider supply interruption affecting the property, the burner simply cannot ignite. This is worth ruling out before anything else.
- Faulty or seized gas valve Sometimes
The gas valve regulates the flow of gas to the burner. Mechanical wear can cause the valve to stick — most likely in a closed or partially closed position — starving the burner of fuel and preventing ignition.
- Loose or damaged ignition leads Sometimes
The ignition leads carry the high-voltage pulse from the ignition module to the electrode. If these leads have worked loose, chafed, or suffered insulation damage, the spark may be too weak or absent entirely.
- Carbon build-up on the burner Sometimes
In older boilers, progressive carbon fouling on the burner surface can prevent a stable flame from establishing or being maintained, eventually triggering a flame-failure lockout.
- PCB fault Rare
If the control board itself is malfunctioning, it may fail to send the correct signals to the gas valve or misinterpret the flame sensor reading. PCB faults are less common but can produce code 4 when all other components test as healthy.
How to fix it
- Check the gas supply is on DIY safe
Locate the gas isolation valve on the pipe feeding the boiler — the handle should be in line with the pipe to indicate it is open. Also check that other gas appliances in the home (e.g. a gas hob) are working normally, which confirms there is no wider supply interruption.
- Attempt a boiler reset (once or twice only) DIY safe
Press and hold the reset button on the boiler's fascia as described in your user manual, then wait for the boiler to run through its start-up sequence. If the boiler fires successfully and code 4 does not return within a short time, monitor it over the next few days. Do not reset the boiler more than two or three times — repeated resets without fixing the root cause can mask a genuine safety issue.
- Check system pressure DIY safe
While the boiler is cold, glance at the pressure gauge. It should read between 1.0 and 1.5 bar. Although low pressure is more commonly linked to other fault codes, confirm it is within range before calling an engineer. If it has dropped below 1 bar, top up via the filling loop following your boiler's user manual.
- Have the ignition electrode inspected and cleaned or replaced Gas Safe engineer
A Gas Safe engineer will remove the electrode, check for carbon fouling or physical wear, and clean or replace it as necessary. This is a relatively straightforward and cost-effective first repair if other checks are inconclusive.
- Have the flame sensor tested and replaced if needed Gas Safe engineer
The engineer will use a multimeter to verify that the sensor is producing the correct microamp signal when a flame is present. A failed sensor will be replaced; the engineer will also check the wiring connections back to the PCB.
- Have the ignition leads and connections checked Gas Safe engineer
The engineer will inspect each lead for insulation damage, loose terminals, or corrosion at the electrode and module ends, replacing any defective leads to restore a reliable ignition spark.
- Have the gas valve assessed by a Gas Safe engineer Gas Safe engineer
The engineer will test gas valve operation under working conditions. If the valve is seized, they may attempt to free it, but a valve that has stuck once is likely to fail again — in most cases replacement is the more reliable long-term solution. Only a Gas Safe registered engineer may work on gas valve components.
- Have the PCB inspected if all other components are healthy Gas Safe engineer
If the electrode, sensor, leads, and gas valve all test satisfactorily, the fault may lie with the PCB itself. The engineer will check wiring harnesses and connections first; if the board has failed it will need to be replaced, which is the most expensive repair outcome for this fault code.
- Call a Gas Safe registered engineer Gas Safe engineer
If the boiler remains locked out after the checks you can safely carry out yourself, contact a Gas Safe registered engineer (verify registration at gassaferegister.co.uk). Fault code 4 involves gas components and ignition systems that must only be worked on by a qualified professional.
Parts you may need
- Ignition electrode · from £35
- Flame sensor / ionisation probe · from £45
- Ignition leads · from £30
- Gas valve · from £180
- PCB (printed circuit board) · from £280
The exact spare depends on your boiler's GC number (on the data badge). Check this against the part before buying.
Typical repair cost
Expect to pay roughly £120–£350, depending on the underlying cause.
Frequently asked questions
Can I reset my Intergas boiler myself when fault code 4 appears?
You can try a reset once or twice — this occasionally clears a transient ignition hiccup. However, if the lockout returns promptly or the boiler will not fire at all, do not keep resetting it. Code 4 can indicate a gas supply or safety-critical ignition fault, and repeatedly resetting without diagnosing the cause is not safe practice. Call a Gas Safe engineer at that point.
How much does it cost to fix Intergas fault code 4 in the UK?
Most repairs for code 4 fall in the £120–£350 range, covering an engineer call-out plus the replacement of common parts such as the ignition electrode, flame sensor, or ignition leads. A gas valve replacement typically adds up to around £300 all-in. In the less common scenario where the PCB has failed, costs can reach £400–£500; at that level it is worth asking your engineer to assess the overall condition of the boiler, as a replacement boiler may offer better long-term value.
Why does my Intergas boiler lock out when fault code 4 shows?
The lockout is a deliberate safety feature. Because code 4 means the PCB cannot confirm a flame is present, the boiler shuts down to prevent unburnt gas from accumulating. The lockout will not clear permanently until the underlying cause — whether a worn electrode, faulty sensor, or gas valve issue — has been properly diagnosed and rectified.
How long does it take to repair an Intergas fault code 4?
Simple repairs such as an electrode or flame sensor swap can usually be completed within an hour or two during a single visit, provided the engineer carries common Intergas parts. A gas valve replacement typically takes most of a working day, and if the part needs to be ordered it can mean a wait of one to two days before your heating is restored. PCB replacements follow a similar timescale.