Chaffoteaux 5P3 Fault Code: Causes, Fixes & Repair Costs
What does the Chaffoteaux 5P3 fault code mean?
The 5P3 fault code on a Chaffoteaux boiler indicates that the flame was detected as having gone out unexpectedly while the boiler was mid-cycle — the burner lit and ran normally, but the flame then cut out before the heating or hot water cycle was complete. As a safety measure, the boiler shuts itself down entirely to prevent unburned gas from accumulating, which could otherwise cause a carbon monoxide or fire hazard. A note on the display: because the digit '5' on Chaffoteaux displays closely resembles the letter 'S', many users read this code as SP3 rather than 5P3 — both refer to the same fault. If 5P3 occurs repeatedly without being resolved, the boiler may escalate to fault code 504, which signals a burner ionisation anomaly after multiple successive 5P3 events. On Urbia and Pigma models, 5P3 can also appear as a general start-up or boot problem rather than a mid-cycle flame loss.
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 dirty ionisation (flame-sensing) electrode Common
The ionisation electrode monitors whether the burner flame is present by passing a tiny electrical current through it. If the electrode is coated with combustion deposits, corroded, or has worn away, it can falsely report that the flame has gone out, triggering a 5P3 lockout. This is the most frequent cause engineers find on this code. The ionisation current should measure above 1 µA; anything below that points to a probe issue.
- Gas supply problem or low gas pressure Common
If the gas supply pressure is too low, unstable, or interrupted entirely — whether due to a partially closed isolation valve, an empty LPG or propane tank, or a network supply issue — the flame will be weak and prone to lifting off the burner. Check that other gas appliances in your home are working normally as a first indication.
- Fan fault or flue/air-intake obstruction Common
The combustion fan draws air into the burner and expels flue gases. If the fan motor is failing, spinning too slowly, or if the flue terminal or air-intake is blocked by debris, leaves, or ice, the air-fuel mixture becomes incorrect and the flame becomes unstable or lifts away from the burner. The boiler then registers a 5P3 and locks out for safety.
- Poor earth connection Sometimes
The ionisation detection circuit relies on a solid electrical earth to measure the tiny current through the flame. A loose, corroded, or missing earth connection on the boiler's wiring can cause the control board to misread the flame signal and trigger a false 5P3 lockout even when combustion is normal.
- Incorrect combustion settings Sometimes
If the CO₂ and O₂ ratios in the flue gases are out of specification — for example following a gas valve adjustment or after a component replacement — the flame can be too lean or too rich to remain stable. This must be checked and corrected using a calibrated flue gas analyser by a Gas Safe engineer.
- Boiler casing not properly resealed after servicing Sometimes
If the combustion chamber cover or the boiler casing panels were not refitted correctly after a service or repair, the air-fuel balance inside the burner can be disrupted, causing flame instability. Gaskets and seals that are misaligned or damaged have the same effect.
- Faulty PCB or flame control module Rare
In a minority of cases where all other components test correctly, the printed circuit board itself — specifically the section that processes the ionisation signal — may be misinterpreting a good flame as absent. PCB failure is relatively rare and is normally only confirmed after other causes have been ruled out.
- Faulty gas valve Rare
A gas valve that does not open fully, sticks, or delivers inconsistent gas flow can starve the burner mid-cycle, causing the flame to drop out and a 5P3 to appear. Gas valve issues are less common than electrode or fan faults but should be tested if other components are found to be working correctly.
How to fix it
- Check that your gas supply is working DIY safe
Turn on a gas hob ring or check another gas appliance in your home. If nothing is working, the issue is with your gas supply rather than the boiler itself — contact your gas supplier or check your meter and any prepayment card balance. If you smell gas at any point, leave the property immediately and call the National Gas Emergency Service on 0800 111 999.
- Inspect the flue terminal from outside DIY safe
Look at the point where the flue exits your property (usually through an external wall or the roof). Check for obvious blockages such as bird nests, leaves, ice build-up, or other debris covering the terminal. Do not attempt to reach or dismantle the flue yourself — this is just a visual check from ground level.
- Check the boiler casing is properly closed DIY safe
If your boiler was serviced or worked on recently, verify that the outer casing panels are fully clipped or screwed back into place and that nothing is visibly out of alignment. A loose cover can upset the air balance inside the combustion chamber.
- Reset the boiler once DIY safe
Locate the reset button on your Chaffoteaux boiler — it is typically a dedicated button on the control panel, sometimes requiring a few seconds to hold. Press it once and allow the boiler to attempt a restart. Watch whether the fault returns within the same heating or hot water cycle. If 5P3 comes back, do not keep resetting repeatedly — more than two or three resets without the fault clearing will not fix the underlying problem and may mask a worsening issue.
- Have a Gas Safe engineer test the ionisation electrode Gas Safe engineer
An engineer will measure the ionisation current at the electrode — it must exceed 1 µA for reliable flame detection. If the reading is low, the electrode will be cleaned with fine wire wool or replaced. This is the most common fix for a 5P3 fault and is straightforward for a competent engineer.
- Have the fan assembly inspected and tested Gas Safe engineer
The engineer will check fan speed, motor current draw, and wiring connections. A fan that is running too slowly or intermittently will cause flame instability. If the motor or fan housing is found to be faulty, the assembly will need to be replaced.
- Have a full combustion analysis carried out Gas Safe engineer
Using a calibrated flue gas analyser, the engineer will measure CO₂, CO, and O₂ levels in the flue gases. If the ratios are outside the manufacturer's specification, gas valve settings will be adjusted accordingly. Never ask an engineer to replace parts without performing this check first.
- Have the earthing and wiring connections checked Gas Safe engineer
The engineer will inspect all earth connections on the boiler, particularly those associated with the ionisation circuit and the control board. Corroded terminals will be cleaned or replaced, and loose connections will be re-secured.
- Have the gas valve tested and replaced if necessary Gas Safe engineer
If all other components are found to be functioning correctly, the engineer will test the gas valve's opening pressure and response. A valve that is not delivering consistent gas pressure will need to be replaced — this must only be done by a Gas Safe registered engineer.
- Consider PCB replacement as a last resort Gas Safe engineer
If the ionisation electrode, fan, gas valve, earthing, and combustion settings are all confirmed as correct and the 5P3 fault persists, the control board may be misprocessing the flame signal. PCB replacement is an expensive repair; request a written quote before proceeding and weigh it against the age of the boiler.
- Contact a Gas Safe registered engineer Gas Safe engineer
If the fault persists after your own basic checks, or if you are unsure at any stage, call a Gas Safe registered engineer. You can verify an engineer's registration at gassaferegister.co.uk. Do not attempt to inspect or adjust any internal gas components yourself.
Parts you may need
- Ionisation / flame-sensing electrode · from £25
- Ignition electrode (often replaced alongside ionisation electrode) · from £30
- Fan assembly (combustion fan / air pressure assembly) · from £120
- Gas valve · from £180
- PCB / main control 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 £100–£400, depending on the underlying cause.
Frequently asked questions
Why does my Chaffoteaux display show SP3 instead of 5P3 — are they the same fault?
Yes, they are exactly the same code. The digit '5' on Chaffoteaux boiler displays is styled in a way that looks almost identical to a capital 'S', so many homeowners and even some engineers write it as SP3. The correct designation in Chaffoteaux documentation is 5P3, but if you see SP3 on your display you are looking at the same flame-interruption fault and the same diagnosis and repair process applies.
Is it safe to keep resetting my boiler when the 5P3 fault keeps coming back?
Resetting once or twice to see whether the fault clears is reasonable, but repeated resets without the fault clearing will not fix the underlying problem. The boiler shuts down precisely to prevent unburned gas from building up — if it is locking out every cycle, there is a genuine combustion or detection issue that needs a qualified engineer. Continued resets can also escalate the fault to code 504 after six successive 5P3 events, which indicates a more serious ionisation anomaly.
How much does it typically cost to fix a Chaffoteaux 5P3 fault in the UK?
The most common repair — cleaning or replacing the ionisation electrode — typically costs £100 to £200 including parts and labour. If the fan assembly needs replacing, expect £250 to £400. A gas valve replacement usually falls in the £150 to £300 range. A PCB replacement is a more expensive outlier, often £450 to £700, and at that price point you should weigh the cost against the age of your boiler. Most 5P3 faults are resolved well within the £100–£400 range.
Can an annual boiler service prevent the 5P3 fault from appearing?
In most cases, yes. The majority of 5P3 faults stem from a dirty ionisation electrode, a partially blocked flue, or combustion settings that have drifted — all of which are checked and corrected during a proper annual service. Having your Chaffoteaux boiler serviced every year by a Gas Safe registered engineer is the most effective way to avoid mid-season lockouts.