Chaffoteaux 105 Fault Code: Causes, Fixes & Repair Costs
What does the Chaffoteaux 105 fault code mean?
Fault code 105 on Chaffoteaux boilers indicates one of two closely related problems depending on your specific model and age. On most units — particularly those running the Expert Control system — it signals a flow check failure, meaning the boiler has detected that water is not circulating correctly around the heating circuit. On some other Chaffoteaux models, the same code points to a fault in the supply (flow) temperature sensor circuit, caused by a short circuit or open break in the NTC sensor wiring. In both cases the boiler will lock out as a safety measure. Note that codes 103, 104, 105, 106, and 107 are all part of the same flow-check family on the Expert Control range, so if you see any of these, the diagnosis process is broadly the same. Always cross-reference with your own model's manual to confirm which interpretation applies to your appliance.
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
- Air trapped in the heating circuit Common
Air locks in the pipework or radiators disrupt water flow, causing the boiler to detect a circulation failure and trigger fault 105. Gurgling radiators or cold spots at the top of radiators are tell-tale signs. Bleeding the radiators and checking system pressure usually resolves this.
- Low system pressure Common
If the system pressure has dropped below 1 bar — perhaps due to a small leak, a recently bled radiator, or a failing expansion vessel — the boiler cannot circulate water properly and will lock out with code 105. Check the pressure gauge on the boiler front panel.
- Circulation pump seized or failing Common
The pump is responsible for pushing water around the heating circuit. If it has seized — which can happen after a long period of inactivity, particularly in summer — or is running at reduced speed due to wear, the boiler will register a flow check failure. A seized pump often makes no noise at all, or produces a humming sound without any actual water movement.
- Closed or partially closed valves Sometimes
Isolation valves under the boiler or lockshield valves on radiators that have been inadvertently left closed will restrict circulation enough to trigger this fault. Worth checking before assuming a component has failed.
- Sludge or scale buildup restricting flow Sometimes
Years of use without a system inhibitor can lead to magnetite sludge accumulating in the heat exchanger, pump, or pipework, significantly reducing flow rates. This is more common in older systems or those that have never had a power flush.
- Faulty or failing expansion vessel Sometimes
A failed diaphragm inside the expansion vessel means it can no longer absorb pressure changes as the water heats and cools. This leads to pressure fluctuations — sometimes dropping too low, sometimes spiking too high — both of which can trigger a flow-related lockout.
- Defective flow temperature NTC sensor (some models) Sometimes
On Chaffoteaux models where code 105 relates to the supply temperature sensor circuit, the NTC thermistor itself may have failed, or its wiring may have developed a short circuit or open break. An incorrect plug connection to the sensor socket can also trigger this fault. The sensor's resistance reading will be outside the expected range when tested with a multimeter.
- PCB or control board fault Rare
If the sensor and wiring both test correctly, the electronic control board may be misreading the signal or has developed a fault of its own. Burn marks, swollen capacitors, or visible scorch marks on the PCB are indicators, though many PCB faults have no visible signs at all.
How to fix it
- Reset the boiler DIY safe
Press and hold the reset button on the boiler's front panel as described in your user manual. Wait for the boiler to complete its restart sequence. If the fault clears and the boiler fires normally, monitor it over the next few hours. If fault 105 returns within a short time, move on to the checks below rather than repeatedly resetting — resetting more than two or three times without investigating the root cause is not recommended.
- Check the system pressure gauge DIY safe
Look at the pressure gauge on the front of the boiler. Chaffoteaux recommend a cold system pressure of between 1 and 1.5 bar. If the needle is below 1 bar, the system needs repressurising. If it reads above 2.5 bar when cold, that is too high and you should not attempt to add more water — contact an engineer.
- Top up system pressure using the filling loop DIY safe
If pressure is below 1 bar, locate the filling loop — usually a silver or grey flexible braided hose with one or two valves beneath the boiler. Slowly open the valve(s) and watch the gauge rise. Stop at around 1.2–1.3 bar, then close the valve(s) firmly. Reset the boiler and check whether the fault clears. If you have to top up pressure repeatedly over days or weeks, there is likely a leak in the system that needs a Gas Safe engineer to locate and repair.
- Bleed the radiators DIY safe
Turn the heating on and allow it to run briefly, then switch it off. Starting with the radiator furthest from the boiler, use a radiator bleed key to open the bleed valve at the top of each radiator by half a turn. Hold a cloth underneath to catch drips. When water flows steadily with no spluttering, close the valve. Work your way back towards the boiler. After bleeding, re-check and adjust system pressure as above, since bleeding releases a small amount of water.
- Check that all valves are fully open DIY safe
Inspect the isolation valves beneath or near the boiler — they should be fully open (lever handle in line with the pipe). Check the lockshield valves and thermostatic radiator valves (TRVs) on each radiator are not stuck closed. Gently try turning any TRVs that may be seized on their lowest setting.
- Check for visible leaks in the system DIY safe
Look beneath the boiler, behind radiators, and around any visible pipework for signs of drips, damp patches, or white limescale deposits where water has previously dried. Even a very slow weeping joint can cause the pressure to drop gradually and trigger fault 105 over time. Report any leaks to a Gas Safe engineer for repair.
- Have the circulation pump inspected and tested Gas Safe engineer
If pressure is correct, radiators have been bled, and valves are open but the fault persists, a Gas Safe engineer should test whether the circulation pump is running and delivering adequate flow. A pump may be seized internally and require freeing or full replacement. This is not a homeowner task.
- Have the NTC flow temperature sensor tested (where applicable) Gas Safe engineer
On models where code 105 relates to the flow sensor circuit, a Gas Safe engineer can disconnect the sensor, measure its resistance with a multimeter at a known temperature, and compare this to the manufacturer's specification. A reading of zero ohms (short circuit) or infinite resistance (open circuit) confirms sensor failure. Wiring continuity and plug connections should also be inspected at the same time.
- Have the expansion vessel inspected or replaced if needed Gas Safe engineer
A Gas Safe engineer can check the expansion vessel pre-charge pressure using a Schrader valve and tyre pressure gauge. If the diaphragm has failed, the vessel will need replacing. This is a straightforward job for an engineer but requires draining the system and working on the sealed heating circuit.
- Call a Gas Safe registered engineer if the fault persists Gas Safe engineer
If fault 105 has not cleared after completing the homeowner checks above, or if you are in any doubt, contact a Gas Safe registered engineer. Further diagnosis — including pump replacement, PCB testing, sensor replacement, or a power flush to clear sludge — must be carried out by a qualified professional. You can verify any engineer's registration at gassaferegister.co.uk.
Parts you may need
- Circulation pump (compatible with Chaffoteaux models) · from £110
- Flow temperature NTC sensor · from £22
- Expansion vessel (8–12 litre, boiler-side) · from £45
- Pressure relief valve · from £18
- System inhibitor (e.g. Fernox F1) · from £15
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–£380, depending on the underlying cause.
Frequently asked questions
Is fault 105 always the same thing on every Chaffoteaux boiler?
No — and this is important to be aware of. On models using the Expert Control system, code 105 is part of a family of flow check failure codes (103 through 107) and points to a circulation problem. On some other Chaffoteaux ranges, the same code is linked specifically to the flow temperature NTC sensor circuit. The research and repair steps overlap considerably, but if you want to be certain, check your model's manual or ask your Gas Safe engineer to cross-reference the code against your specific appliance documentation.
My Chaffoteaux keeps dropping pressure and showing fault 105 — what is causing this?
Recurring low pressure combined with fault 105 usually means there is a slow leak somewhere in the system, or the expansion vessel has failed. A small weeping joint behind a radiator or at a pipe fitting can be hard to spot. A failing expansion vessel is another common culprit — when the internal diaphragm splits, the vessel can no longer buffer pressure changes and the system pressure drops unpredictably. Both issues need a Gas Safe registered engineer to diagnose and fix properly.
Can I replace the NTC sensor myself to fix fault 105?
No. Even though an NTC sensor is a relatively inexpensive part (typically £15–£30), replacing it involves working inside the boiler's sealed waterway connections and on internal wiring. This must be done by a Gas Safe registered engineer. An incorrectly fitted sensor can result in further damage to the boiler or unsafe operation. The engineer will also be able to confirm that the sensor — and not the PCB or wiring — is actually at fault before ordering any parts.
How much does it cost to fix fault 105 on a Chaffoteaux boiler in the UK?
The most common repairs — topping up pressure, bleeding the system, and sensor replacement — typically cost between £100 and £250 including labour. A circulation pump replacement usually runs £150–£380 all-in, and an expansion vessel swap is generally £150–£250. If the fault turns out to be a PCB failure, replacement boards for Chaffoteaux models can cost £200–£400 for the part alone plus labour, but this is a less common outcome. Always get a written quote before work begins.