Chaffoteaux 104 Fault Code: Causes, Fixes & Repair Costs
What does the Chaffoteaux 104 fault code mean?
Fault code 104 on Chaffoteaux boilers signals that the boiler's control system has detected inadequate water circulation in the primary heating circuit. The boiler monitors how quickly the flow temperature rises: if the thermistor detects a temperature increase faster than roughly 20°C per second, it interprets this as a sign that water is not moving properly around the system — because without sufficient flow, heat builds up rapidly at the heat exchanger rather than being carried away. On some Chaffoteaux models, code 104 can also indicate a fault with the flow (feed) temperature sensor itself, either a short circuit or a broken sensor circuit. In both cases, the boiler locks out to prevent overheating or component damage. Always cross-reference with your specific model's manual, as Chaffoteaux have produced many different ranges over the years.
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
- Low system water pressure Common
If the system pressure has dropped below around 1 bar, the boiler cannot circulate water effectively. This is one of the most frequent triggers for code 104. Check the pressure gauge on the boiler front panel — it should read between 1.0 and 1.5 bar when the system is cold.
- Blocked or clogged system filter Common
A dirty inline filter or magnetic filter can restrict water flow enough to trigger the circulation fault. Debris, rust particles, and sludge accumulate over time and can partially or fully block the filter housing, starving the circuit of adequate flow.
- Faulty or seized circulation pump Common
The pump is responsible for pushing water around the heating circuit. If it has seized, failed electrically, or is running but producing insufficient pressure, the boiler will detect the lack of circulation and lock out with code 104. Pump failure is a very common root cause of this fault.
- Sludge, scale, or air in the system Common
Accumulated magnetite sludge, limescale, or trapped air pockets can disrupt water flow through radiators and pipework. Older or poorly maintained systems are particularly susceptible. Air in the circuit can also prevent radiators from bleeding down to the correct pressure zones.
- Faulty flow temperature sensor (thermistor) Sometimes
On certain Chaffoteaux models, code 104 is directly linked to the flow line thermistor. A short circuit will produce a resistance reading of zero; a broken circuit will read as infinite resistance. Either condition causes incorrect temperature data, prompting the boiler to lock out.
- PCB connector or wiring fault to the pump Sometimes
A poor solder joint or corroded connector on the PCB that supplies power to the pump can cause intermittent micro power cuts to the pump. The pump may appear to work but loses power momentarily under load, resulting in circulation failure. This has been confirmed as the root cause in documented real-world cases.
- PCB (control board) fault Rare
In rarer cases, the main printed circuit board itself is damaged — burnt components, broken tracks, or deformed connectors can prevent the pump from receiving the correct signals or power. A PCB fault is usually only suspected after other causes have been ruled out.
- Electrical supply issues Rare
An unstable or incorrect mains voltage supply can cause the pump or control electronics to behave erratically, leading to false circulation fault readings. Power surges are a recognised contributor to code 104 appearing intermittently.
How to fix it
- Reset the boiler once DIY safe
Press the RESET button on the front panel and wait at least 60 seconds. A single reset clears the lockout and allows the boiler to attempt a fresh start. If the boiler locks out again immediately or within a short time, do not keep resetting — this can mask an underlying problem and cause further damage. Move on to the next checks.
- Check the system pressure gauge DIY safe
Look at the pressure gauge on the boiler (usually a dial or digital reading on the front). It should read between 1.0 and 1.5 bar when the heating system is cold. If it reads below 1 bar, top it up using the filling loop — a silver or grey flexible braided hose, usually located beneath the boiler. Open the filling loop valve(s) slowly, watch the gauge rise to around 1.2–1.3 bar, then close the valve(s) firmly. If the pressure drops again quickly, there may be a leak in the system that a Gas Safe engineer will need to find.
- Bleed the radiators and check thermostatic valves DIY safe
Trapped air in the system reduces circulation and can contribute to code 104. Use a radiator bleed key on each radiator in turn, starting from the ground floor, until water (not air) flows freely from the bleed valve. Also check that at least one thermostatic radiator valve (TRV) in the home is set fully open — if all TRVs are closed, there is nowhere for water to flow, which can starve the pump and trigger a circulation fault.
- Attempt a second reset after correcting pressure and bleeding DIY safe
Once you have topped up the pressure and bled the radiators, press RESET again and allow the boiler to fire up. Monitor it for 10–15 minutes. If it runs successfully, the low pressure or air lock was the likely cause. If code 104 returns, stop resetting and proceed to calling an engineer.
- Have a Gas Safe engineer inspect and clean the system filter Gas Safe engineer
A registered engineer will locate the inline or magnetic filter (often near the boiler return pipe), isolate it, and clean or replace the filter element. This is a straightforward job but requires isolating parts of the heating circuit. If significant sludge is found, the engineer may recommend a full power flush of the system to remove accumulated debris from the pipework and radiators.
- Have the circulation pump tested and replaced if necessary Gas Safe engineer
The engineer will test whether the pump is running, spinning freely, and generating adequate pressure. A pump that has seized can sometimes be freed, but in most cases a faulty pump will need to be replaced. Pump replacement on a Chaffoteaux boiler typically costs £150–£300 supply and fit, and is one of the most common repairs for this fault code.
- Have the flow temperature sensor (thermistor) tested Gas Safe engineer
Using a multimeter, the engineer will measure the resistance of the flow thermistor. A reading of zero ohms indicates a short circuit; an infinite (open) reading indicates a broken circuit. Both require the sensor to be replaced. The part itself is inexpensive (around £15–£30), but labour is required to access and replace it correctly.
- Have the expansion vessel inspected Gas Safe engineer
A failed or under-pressurised expansion vessel can cause erratic system pressure behaviour, contributing to circulation problems. The engineer will check the vessel's charge pressure and either re-pressurise it or advise replacement if the diaphragm has failed.
- Have the PCB wiring, connectors, and control board inspected Gas Safe engineer
If all mechanical causes have been ruled out, the engineer will inspect the PCB for signs of burnt components, damaged tracks, or corroded connectors — particularly the connector feeding the pump. A faulty solder joint has been identified as a root cause in real cases and can sometimes be repaired. If the PCB itself is damaged, replacement will be required. Mention PCB repair costs in context — see the FAQ below for typical costs.
- Call a Gas Safe registered engineer if the fault persists Gas Safe engineer
If code 104 keeps returning after you have checked the pressure, bled the radiators, and attempted a reset, the fault requires professional diagnosis. Do not continue resetting the boiler repeatedly. Contact a Gas Safe registered engineer — you can find one at GasSafeRegister.co.uk — and describe the code and any steps you have already taken. This will help speed up the diagnostic process.
Parts you may need
- Circulation pump (suitable for Chaffoteaux) · from £75
- Flow temperature sensor / thermistor · from £22
- Magnetic system filter · from £55
- Expansion vessel · from £45
- PCB (model-specific — check compatibility) · from £220
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 fix Chaffoteaux fault code 104 myself?
There are a few safe checks a homeowner can carry out: topping up the system pressure via the filling loop to between 1.0 and 1.5 bar, bleeding the radiators to remove trapped air, and resetting the boiler once or twice. These steps resolve the fault in some cases, particularly if low pressure or an air lock is the cause. However, if the fault persists, the underlying issue — a blocked filter, failed pump, faulty sensor, or PCB problem — must be diagnosed and repaired by a Gas Safe registered engineer. Do not attempt to open the boiler casing or work on internal components yourself.
How much does it cost to repair Chaffoteaux fault code 104 in the UK?
For most households, the repair falls in the range of £120–£350. A simple pressure top-up or filter clean at the lower end; a circulation pump replacement (the most common fix) typically costs £150–£300 including parts and labour. A flow temperature sensor is an inexpensive part (around £15–£30) but requires engineer time to replace, so expect a total of £100–£180. If the fault turns out to be the PCB, costs rise to roughly £250–£450 depending on the model. A full system power flush — needed if heavy sludge is found — usually adds £300–£500 on top of other repairs.
Why does fault code 104 keep coming back on my Chaffoteaux boiler?
A recurring code 104 usually means the underlying cause has not been fully resolved. Common reasons for the fault returning include: a small leak causing pressure to drop repeatedly, a pump that is on the verge of failure and only works intermittently, ongoing sludge buildup that re-blocks a cleaned filter, or a PCB connector that has a poor solder joint causing intermittent pump power loss. If the code returns more than twice after resetting, stop resetting and call a Gas Safe engineer for a proper diagnosis rather than continuing to clear the fault temporarily.
Does fault code 104 mean I need a new boiler?
Not necessarily. In the majority of cases, code 104 is caused by something fixable — low pressure, a dirty filter, or a pump replacement. However, if your Chaffoteaux boiler is more than 10–12 years old and the fault turns out to require a major component such as a PCB or heat exchanger, it is worth getting a replacement boiler quote alongside the repair quote. Repair costs on older boilers can sometimes approach or exceed the cost of a new, more efficient boiler, and parts availability for older Chaffoteaux models can be limited.