Chaffoteaux 120 Fault Code: Causes, Fixes & Repair Costs
What does the Chaffoteaux 120 fault code mean?
Fault code 120 on a Chaffoteaux boiler most likely indicates an overheat or high-temperature lockout — the boiler has detected that water temperature has exceeded a safe limit and has shut itself down as a safety precaution. On some models with a two-part display, this may appear as '1-20' or a similar variant within the overheat code family (which includes related codes such as 1-01 for overheat and A41 for no circulation). Because Chaffoteaux fault code tables vary by model range — including the Britony, Minima HE, Talia, Pigma, and Expert Control series — it is important to cross-reference with your specific installation manual. The underlying cause is typically poor water circulation, a faulty pump, a sensor error, or a flue/fan issue preventing safe operation. Do not ignore a repeated lockout: the boiler is protecting itself and the system from damage.
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
- Circulation pump failure or blockage Common
The most common reason for an overheat lockout. If the pump has seized, slowed significantly, or is blocked by sludge, hot water cannot move away from the heat exchanger quickly enough. The boiler detects a rapid temperature rise and shuts down. You may notice some radiators are cold or that the boiler makes unusual noises before lockout.
- Sludge or scale restricting water flow Common
Over time, magnetite sludge and limescale accumulate in older systems — particularly in hard-water areas. Restricted flow through the heat exchanger causes localised overheating and can trigger the high-temperature safety cut-out. Noisy pipework, patchy radiator heating, and repeated lockouts are telltale signs.
- Flow or return thermistor fault Sometimes
Chaffoteaux boilers use thermistors on both the heat exchanger inlet and outlet to monitor water temperature. If one of these sensors develops an open circuit or a short circuit, the boiler's PCB may receive a false high-temperature reading and lock out even when actual temperatures are normal.
- Fan failure or flue restriction Sometimes
The fan is responsible for expelling combustion gases safely. If the fan motor is failing, the wiring is damaged, or the flue is partially blocked, the boiler locks out to prevent dangerous gas build-up. On Chaffoteaux models, fan faults also appear as code A40 on some ranges — a useful cross-check if you see additional codes.
- PCB fault Rare
The printed circuit board controls every function of the boiler. A failing PCB can misread sensor signals, fail to control the pump correctly, or trigger spurious lockouts. PCB faults often produce intermittent or inconsistent behaviour rather than a clean, repeatable lockout, making diagnosis more complex.
How to fix it
- Check your boiler's installation manual for the exact code definition DIY safe
Because Chaffoteaux fault code tables differ between model ranges, your specific manual is the most reliable source. Most Chaffoteaux manuals are available free of charge on ManualsLib.com — search your model name. Note the exact code display (e.g. '1-20', '120', or a light combination) and compare it against the fault table for your model before proceeding.
- Attempt a single boiler reset DIY safe
Locate the reset button on the boiler's front panel — usually marked with a flame or reset symbol. Press and hold for the duration specified in your manual (typically 3–5 seconds), then wait at least 60 seconds to see if the boiler fires successfully. If it locks out again immediately or within a short time, do not keep resetting: repeated resets on an overheat fault can mask a serious underlying problem. Stop after one or two attempts.
- Check the system pressure gauge DIY safe
Look at the pressure gauge on the boiler front panel. For most Chaffoteaux models, normal cold pressure should read between 1.0 and 1.5 bar. If the needle is below 0.5 bar, low pressure may be contributing to poor circulation. Top up using the filling loop beneath the boiler — slowly open both valves until pressure reaches around 1.2 bar, then close both valves. If pressure drops again within a few days, there is likely a leak or a failing expansion vessel that requires an engineer.
- Check for a frozen condensate pipe (winter months only) DIY safe
In cold weather, the white plastic condensate pipe that exits through an outside wall can freeze solid, causing a lockout. Locate the pipe — it is usually 21.5 mm white plastic running to a drain or outside wall. If it feels solid or you can see ice, carefully pour warm (not boiling) water along its length until it clears. Once thawed, reset the boiler. This is only relevant if the fault has appeared during a cold snap.
- Call a Gas Safe registered engineer for internal investigation Gas Safe engineer
If the boiler continues to lock out after your external checks, the fault requires professional diagnosis. A Gas Safe engineer will test the circulation pump for correct flow rate and current draw, measure the resistance of the flow and return thermistors, inspect the fan assembly and air pressure switch, check the PCB for fault signals or burn marks, and carry out a power flush if sludge is suspected. Do not attempt to open the boiler casing or work on any internal components yourself — this is a legal requirement as well as a safety one.
Parts you may need
- Circulation pump · from £85
- Flow thermistor (NTC sensor) · from £25
- Return thermistor (NTC sensor) · from £25
- Fan assembly · from £110
- PCB (printed circuit board) · from £180
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 £150–£420, depending on the underlying cause.
Frequently asked questions
Is fault code 120 the same on all Chaffoteaux boiler models?
No — and this is an important point. Chaffoteaux fault code tables are not standardised across all model ranges. The Britony, Minima HE, Talia, Pigma, and Expert Control series each use slightly different code structures. Code 120 most commonly points to an overheat or circulation-related lockout based on the Ariston Group code family that Chaffoteaux shares, but you should always verify against your own model's installation manual before drawing conclusions. If you no longer have the manual, it is available free on ManualsLib.com.
Why does my Chaffoteaux keep locking out on code 120 even after I reset it?
A lockout that returns shortly after resetting is a sign that the boiler is detecting a genuine fault rather than a one-off anomaly. Common culprits are a failing circulation pump, sludge restricting flow through the heat exchanger, or a faulty thermistor sending false temperature readings. Repeatedly resetting the boiler will not fix the underlying cause and may cause further damage. At this point you need a Gas Safe registered engineer to run proper diagnostic checks.
How much does it typically cost to fix a Chaffoteaux overheat lockout in the UK?
For the most common repairs — pump replacement, thermistor replacement, or a system power flush — most UK homeowners pay between £150 and £420 including parts and labour. A thermistor alone is a relatively inexpensive fix (often under £200 all-in), while a pump replacement typically falls in the £200–£350 range. Fan replacement usually costs £200–£350. If the PCB has failed, expect £300–£500 or more. A full heat exchanger replacement can exceed £600–£800, but this is uncommon and at that cost it is worth getting a replacement boiler quote for comparison, especially if your Chaffoteaux is over 12 years old.
Can I fix a Chaffoteaux fault code 120 myself?
Homeowners can safely carry out a small number of checks: attempting one or two resets, topping up system pressure via the filling loop, and thawing a frozen condensate pipe in winter. Everything beyond that — including replacing the pump, thermistors, fan, or PCB — must be done by a Gas Safe registered engineer. Working on gas appliances without Gas Safe registration is illegal and dangerous. If in any doubt, call a registered engineer straight away.