Chaffoteaux 502 Fault Code: Causes, Fixes & Repair Costs
What does the Chaffoteaux 502 fault code mean?
Fault code 502 means the boiler's ionisation (flame detection) circuit is registering a flame signal at a point in the cycle when the gas valve should be fully closed. This is sometimes referred to as a 'ghost flame' or parasitic flame condition. Rather than a genuine flame burning where it shouldn't, this usually points to a faulty component sending a misleading signal — most commonly the ionisation electrode, the gas valve itself, or the PCB. Because the boiler cannot safely continue operating when its flame-detection logic is inconsistent, it shuts down and locks out until the underlying fault is resolved. Note that 502 is distinct from code 501, which indicates the opposite problem: a failure to detect a flame when one should be present. Both are lockout-type faults on Chaffoteaux digital-display models.
General guidance only — not a substitute for professional advice, and not certified by a Gas Safe engineer. Always have your specific appliance assessed by a qualified professional. 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.
Common causes
- Faulty or contaminated ionisation electrode Common
The ionisation electrode reads the electrical conductivity of the burner flame to confirm it is present. If the electrode tip is coated in carbon deposits, cracked, or has drifted out of position, it can send a spurious 'flame present' signal to the PCB even when the gas valve is shut. This is the most frequent trigger for a 502 fault and is relatively straightforward for an engineer to inspect and replace.
- Gas valve not closing fully Common
A gas valve that is worn, sticking, or suffering from a failed solenoid coil may allow a small trickle of gas through even when commanded to close. This residual gas can briefly ignite or sustain a small flame that the ionisation circuit correctly detects — but at the wrong point in the firing sequence. A Gas Safe engineer can test valve resistance and switching voltages to confirm whether the valve is the cause.
- Faulty ionisation cable or wiring connection Sometimes
The low-voltage signal cable running from the electrode to the PCB is susceptible to damage, chafing against metalwork, or loose terminal connections. Any current leakage along this cable can mimic a flame signal. An engineer will inspect the cable routing and measure insulation resistance to rule this out.
- PCB fault or incorrect flame signal interpretation Sometimes
If the ignition sequence appears to run normally but the boiler immediately shuts down with a 502, the control board itself may be misreading or generating a false flame signal. PCB faults are less common than electrode or valve issues but become more likely on older appliances. Diagnosis requires specialist testing equipment.
- Blockage or air pocket in the gas supply line Rare
An obstruction or air in the gas circuit can disrupt the normal combustion cycle and produce unusual flame behaviour that confuses the detection system. This is uncommon but worth ruling out, particularly if the boiler has been unused for an extended period or if other gas appliances in the property have also been affected.
How to fix it
- Check your gas supply is active DIY safe
Confirm that the gas isolation valve on the boiler's inlet pipe is in the open (inline) position and that other gas appliances in your home — such as a gas hob — are working normally. If you have a prepayment gas meter, check it has credit. If no gas appliances are working, contact your gas supplier rather than resetting the boiler.
- Reset the boiler once DIY safe
Press and hold the reset button (usually marked with a flame or key symbol) for around three seconds, then release it. Allow the boiler a full minute to attempt to restart. A single reset is reasonable to try; if the 502 code returns straight away or comes back within a short time, do not reset again — repeated resets on a flame-detection lockout can mask a potentially unsafe condition.
- Do not attempt to open or adjust the gas valve or burner components Gas Safe engineer
Any work on the gas valve, burner, or combustion chamber is notifiable gas work under UK law and must only be carried out by a Gas Safe registered engineer. Do not remove panels or interfere with internal components.
- Arrange inspection of the ionisation electrode and cable Gas Safe engineer
A Gas Safe engineer will remove the electrode, check its condition and gap, clean off any carbon fouling, and test the signal cable for continuity and insulation integrity. If the electrode is damaged or out of tolerance, replacing it is a cost-effective first repair that resolves the fault in many cases.
- Have the gas valve tested and replaced if necessary Gas Safe engineer
If the electrode checks out, the engineer will measure the resistance and switching voltages across the gas valve solenoids to verify it is closing fully and responding correctly to control signals. A valve that fails these tests must be replaced — this is not a serviceable or DIY item.
- Have the PCB tested if other components are healthy Gas Safe engineer
If both the electrode and gas valve are confirmed to be working correctly, the engineer will turn attention to the PCB. Control board faults are diagnosed by testing input and output signals under operating conditions. If the PCB is at fault it will need to be repaired by a specialist or replaced entirely.
- Call a Gas Safe registered engineer Gas Safe engineer
If the boiler locks out again after your single reset attempt, stop resetting and book a Gas Safe registered engineer to carry out a full diagnosis. You can verify any engineer's registration at gassaferegister.co.uk. Given the safety implications of a flame-detection fault, this is not a fault to leave unattended or to keep resetting in the hope it clears itself.
Parts you may need
- Ionisation electrode · from £25
- Ionisation signal cable · from £20
- Gas valve assembly · from £120
- Boiler PCB (control board) · from £200
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
Is Chaffoteaux fault code 502 dangerous?
The fault itself indicates a detection anomaly rather than a confirmed runaway flame, and the boiler has safely shut itself down as a precaution. However, a gas valve that is genuinely failing to close fully is a serious concern, so you should not ignore the fault or continue resetting the boiler repeatedly. Leave the boiler off and call a Gas Safe registered engineer promptly. If you smell gas at any point, do not reset the boiler — leave the property, avoid using electrical switches, and call the National Gas Emergency Service on 0800 111 999.
What is the difference between Chaffoteaux fault codes 501 and 502?
Code 501 means the boiler attempted to ignite but the ionisation circuit detected no flame — essentially a failed ignition. Code 502 is the opposite: the ionisation circuit is detecting a flame signal when the gas valve should be closed and no flame should be present. Both are lockout faults requiring engineer attention, but 502 tends to point more strongly toward the gas valve or a faulty electrode sending a false signal, rather than a straightforward ignition failure.
How much does it cost to fix a Chaffoteaux 502 fault in the UK?
Most homeowners pay somewhere between £120 and £350 all-in, depending on which component is at fault. Replacing the ionisation electrode — the most common fix — typically costs £100 to £200 including labour. A gas valve replacement usually falls in the £150 to £300 range. A PCB replacement is the most expensive common repair and can cost £300 to £500 or more, depending on parts availability for your specific model. In rare cases where other components are involved, costs can exceed these figures, which is worth weighing against the age and overall condition of the boiler.
My Chaffoteaux boiler keeps showing 502 after resetting — what should I do?
If the fault code returns after one or two resets, stop resetting the boiler. Repeated resets on a flame-detection lockout do not fix the underlying problem and could mask an unsafe condition. The fault needs hands-on diagnosis by a Gas Safe registered engineer who can test the ionisation electrode, signal wiring, gas valve, and PCB in sequence to find the root cause. Booking an engineer sooner rather than later also reduces the risk of being left without heating or hot water at an inconvenient time.