Chaffoteaux 602 Fault Code: Causes, Fixes & Repair Costs
What does the Chaffoteaux 602 fault code mean?
Fault code 602 on Chaffoteaux boilers signals that the boiler has shut itself down as a safety measure. There are three closely related meanings depending on your model and installation type. Most commonly it indicates the safety thermostat has tripped due to overheating — the boiler's heat exchanger has reached an unsafe temperature and the unit has locked out to protect itself. On boilers installed in flats or apartments served by a shared mechanical ventilation and gas extraction system (VMC), code 602 can also point to a failure or interruption of that communal flue extraction system. On certain Chaffoteaux models, the same code flags a communication breakdown between the electronic control board (PCB) and the burner, which may be caused by loose or damaged wiring rather than a thermal event. In all cases the boiler will not fire until the underlying problem is resolved.
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
- Overheating — safety thermostat tripped Common
The most frequent trigger. If water cannot circulate freely through the heat exchanger — due to a failing pump, a partially closed valve, or a sludged system — heat builds up until the safety thermostat on the heat exchanger outlet cuts in and locks the boiler out. The code will keep returning until the root cause of the overheating is addressed.
- Faulty or seized circulation pump Common
A pump that is running slowly, making grinding noises, or has seized altogether will restrict flow through the heating circuit. Even if the pump appears to be energised, worn impeller bearings or a build-up of system debris can reduce output enough to cause localised overheating and trigger the 602.
- Shared VMC (communal flue extraction) system fault Sometimes
In apartment blocks where multiple boilers share a single mechanical ventilation and gas-extraction duct (VMC), any interruption to that system — a tripped circuit breaker, a blocked duct, or a failed extraction fan — will cause all connected boilers to lock out simultaneously. If neighbours are also without heating or hot water, this is the likely cause.
- PCB–burner communication fault Sometimes
On certain Chaffoteaux models the 602 code indicates the main control board has lost reliable communication with the burner assembly. Loose plug-in connectors, chafed wiring, or moisture ingress on the PCB can all produce this symptom, even when the boiler is not actually overheating.
- Dirty or faulty flame sensor (ionisation electrode) Sometimes
The flame sensor confirms to the PCB that a flame is present. A sensor coated in combustion deposits, or one with a cracked ceramic body, can send an unreliable signal that the PCB interprets as a fault, resulting in a lockout and the 602 code on some variants.
- Faulty safety thermostat or NTC temperature sensor Rare
If the thermostat itself has failed in a tripped position, or an NTC sensor is reading falsely high temperatures, the boiler will register an overheat event even when the water temperature is perfectly normal. This is less common but worth investigating once pump and flow issues have been ruled out.
How to fix it
- Check whether neighbouring properties are affected DIY safe
If you live in a flat or apartment block with a shared VMC gas system, ask a neighbour whether their boiler has also stopped working. A communal VMC fault will affect every connected boiler in the building at the same time. Report the issue to your building manager or landlord, as a qualified engineer will need to inspect and restore the shared system before any individual boiler will run again.
- Check the boiler's system pressure DIY safe
Look at the pressure gauge on the front of the boiler. It should read between 1.0 and 1.5 bar when the system is cold. If it has dropped below 1 bar, top it up using the filling loop — usually a braided flexible hose or a lever valve beneath the boiler. Add water slowly until the gauge reads around 1.2 bar, then close the filling loop fully. Low pressure alone may not trigger a 602, but it can compound overheating problems.
- Reset the boiler — once or twice only DIY safe
Press and hold the reset button (often marked with a flame symbol or labelled RESET) for around three seconds until the boiler attempts to restart. Allow a full firing cycle before judging whether the fault has cleared. If the 602 returns within a short time, do not keep resetting — repeated resets on an overheating fault can mask a serious problem and risk damage to the heat exchanger.
- Listen and look for obvious circulation pump issues DIY safe
With the boiler running or shortly after a lockout, listen near the boiler for unusual sounds from the pump — grinding, rattling, or complete silence when the pump should be active. These are signs the pump has failed or is struggling. Do not attempt to open, adjust, or replace the pump yourself; note your observations to pass on to the engineer.
- Have a Gas Safe engineer inspect the safety thermostat and heat exchanger circuit Gas Safe engineer
An engineer will measure the flow and return temperatures across the heat exchanger to confirm whether genuine overheating is occurring, and will test the safety thermostat for continuity. If the thermostat has tripped and will not reset, or has failed electrically, it must be replaced by a qualified engineer.
- Have the circulation pump tested and replaced if necessary Gas Safe engineer
The engineer will check pump speed settings, test the motor windings, and assess flow rates. A pump running below specification will be replaced. On Chaffoteaux boilers parts availability can be more limited than on mainstream brands, so the engineer may need to source the correct pump before the visit.
- Have the PCB wiring and flame sensor inspected Gas Safe engineer
If overheating and pump issues have been ruled out, the engineer should check all wiring connectors at the PCB for corrosion or looseness, inspect the burner communication wiring, and clean or replace the ionisation electrode. A PCB replacement may be required if the board itself has failed.
- Call a Gas Safe registered engineer if the fault persists or returns Gas Safe engineer
If the 602 code returns after a single reset, or if you have been unable to identify an obvious external cause such as a shared VMC trip, book a Gas Safe registered engineer. Give them the exact model name, the fault code, and any observations (e.g. unusual noises, pressure readings, whether neighbours are affected). Do not continue to reset the boiler repeatedly.
Parts you may need
- Circulation pump (compatible replacement) · from £130
- Safety thermostat (high-limit thermostat) · from £35
- NTC flow or return temperature sensor · from £25
- Ionisation / flame sense electrode · from £20
- 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 £150–£420, depending on the underlying cause.
Frequently asked questions
Can I just keep resetting my Chaffoteaux boiler to clear the 602 fault?
You can try a single reset — that is a reasonable first step. However, if the code comes back, do not keep hitting reset. The 602 is fundamentally a safety lockout, and repeatedly overriding it while the underlying fault is present risks heat exchanger damage or, in the case of a VMC fault, combustion gases not being properly extracted. Two failed resets is a firm signal to call a Gas Safe engineer.
My Chaffoteaux boiler shows 602 and my neighbour's boiler has also stopped — what is going on?
This strongly points to a shared VMC (communal mechanical ventilation and gas extraction) fault rather than a problem with your individual boiler. All boilers connected to the same communal duct will lock out when the system is interrupted. Contact your building manager straight away. A qualified engineer will need to restore the VMC system — typically by resetting a tripped circuit breaker or clearing a blockage — before any of the individual boilers can be restarted.
How much does it typically cost to repair a Chaffoteaux 602 fault in the UK?
For the most common repairs — circulation pump replacement or safety thermostat replacement — most homeowners pay in the region of £150 to £420 including parts, labour, and VAT. A pump tends to cost around £150–£300 all-in, and a thermostat or sensor considerably less. If the fault turns out to be a PCB failure, expect to pay £300–£500 or more depending on parts availability; Chaffoteaux PCBs can be harder to source for older units, which may push costs up. Heat exchanger replacement is a larger job that can exceed £600, but this is rarely the primary cause of a 602 and would only be considered if the heat exchanger itself had been damaged by repeated overheating events.
Could a 602 fault mean my boiler needs replacing rather than repairing?
Not necessarily, but age is a factor worth considering. If your Chaffoteaux boiler is over 10–12 years old and the 602 fault is caused by a failed PCB or heat exchanger, spare parts may be expensive or difficult to source. In that situation a Gas Safe engineer may recommend replacement rather than repair. For newer boilers where the fault is a pump, sensor, or thermostat, repair is almost always the more cost-effective option.