Chaffoteaux 420 Fault Code: Causes, Fixes & Repair Costs
What does the Chaffoteaux 420 fault code mean?
Fault code 420 on a Chaffoteaux boiler falls within the 400-series, which covers faults related to external zone temperature probes and the external PCB (printed circuit board). When this code appears, the boiler has detected that a zone temperature sensor is unavailable, reading an open circuit, or showing a short circuit — all of which prevent the boiler from accurately monitoring zone temperatures. Because Chaffoteaux is part of the Ariston Thermo Group, the fault logic is shared across several models in the range. Note that some Chaffoteaux models do not have a traditional display and instead use numbered light sequences; always cross-reference with your own model's manual to confirm the code meaning. Code 420 sits alongside neighbouring codes 411–413 (individual zone probe faults) and 430–433 (further external PCB/zone sensor errors) in the same fault category. This is typically a non-volatile, persistent error, meaning it will not clear on its own — some form of intervention is usually required.
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 zone temperature sensor (NTC thermistor) Common
The zone temperature sensor is an NTC thermistor that changes its electrical resistance in proportion to temperature. If it has deteriorated, cracked, or failed internally, it will read either infinite resistance (open circuit) or zero ohms (short circuit) rather than the expected 10–50 kΩ at room temperature. This is the single most common root cause of a 420 fault.
- Loose or damaged wiring to the sensor Common
The connection between the zone sensor and the external PCB can work loose over time due to vibration or thermal cycling. A broken wire, corroded terminal, or poorly seated connector plug can interrupt the signal and trigger the same error as a failed sensor itself.
- Moisture on sensor or PCB connectors Sometimes
Condensation or a nearby water leak can introduce moisture onto the sensor connector or the external board connector. This can cause intermittent resistance readings that the boiler interprets as a sensor fault, even if the sensor itself is still within specification.
- External PCB failure Sometimes
The external PCB that processes zone sensor signals can degrade over time through exposure to heat, moisture, or electrical stress. A cracked solder joint, burnt component, or corroded track can cause it to misread or fail to read a perfectly healthy sensor, producing the 420 fault code.
- Short circuit or open circuit caused by an upstream fault Rare
On rare occasions, a fault in another component — such as a zone valve or pump wiring — can create an unintended short or break in the circuit that feeds the sensor inputs, causing a false 420 reading without the sensor itself being at fault.
How to fix it
- Attempt a boiler reset DIY safe
Press and hold the reset button (labelled 'R' or shown as a circular arrow symbol on the boiler fascia) for 3–5 seconds, then release and wait two to three minutes. If the code clears and the boiler runs normally, monitor it closely over the next 24 hours. Limit resets to two or three attempts — repeated resets without a fix can mask a worsening fault. If the code returns promptly, move to the next step.
- Check that your heating system pressure is correct DIY safe
Glance at the pressure gauge on the boiler. It should read between 1.0 and 1.5 bar when the system is cold. Although low pressure alone would not cause a 420 code, confirming the boiler is otherwise healthy before calling an engineer avoids a wasted visit. Top up via the filling loop if needed, following your boiler manual's instructions.
- Visually inspect accessible wiring and sensor connectors Gas Safe engineer
A Gas Safe engineer should inspect the wiring loom running to the external zone sensor and the connector plugs at both the sensor end and the PCB end. They will be looking for signs of corrosion, moisture ingress, loose terminals, or damaged insulation. A simple re-seat of a connector sometimes resolves the fault entirely.
- Test the zone temperature sensor (NTC thermistor) with a multimeter Gas Safe engineer
The engineer will disconnect the sensor wires from the PCB and measure the thermistor's resistance at ambient (room) temperature. A healthy NTC thermistor typically reads 10–50 kΩ at around 20 °C. A reading of infinite resistance indicates an open-circuit failure; a reading near zero ohms indicates a short. Either result confirms the sensor needs replacing.
- Replace the faulty zone temperature sensor if confirmed defective Gas Safe engineer
If the sensor tests outside its expected resistance range, the engineer will fit a compatible replacement NTC thermistor. This is the most common and cost-effective resolution for a 420 fault. After fitting, the boiler should be powered up and the zone temperature monitored to confirm the code has cleared.
- Inspect and test the external PCB Gas Safe engineer
If the sensor and wiring both test as healthy, the engineer will turn attention to the external PCB. They will check supply voltages, inspect the board for burnt components, corroded tracks, or cracked solder joints. Minor damage such as a dry joint can sometimes be repaired on-site; more extensive board damage will require a full PCB replacement.
- Call a Gas Safe registered engineer to diagnose and complete the repair Gas Safe engineer
Because fault code 420 involves electronic components connected to the boiler's control system, all diagnostic work beyond a reset should be carried out by a Gas Safe registered engineer. They have the test equipment to distinguish between a sensor fault, a wiring fault, and a PCB fault — and only they can safely complete the repair. You can verify an engineer's Gas Safe registration at gassaferegister.co.uk.
Parts you may need
- NTC zone temperature thermistor (sensor) · from £25
- Sensor wiring loom / harness · from £35
- External PCB (zone control 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 £120–£350, depending on the underlying cause.
Frequently asked questions
Can I reset a Chaffoteaux 420 fault code myself?
Yes, a reset is a safe and sensible first step. Press and hold the reset button on the boiler for 3–5 seconds and wait a few minutes to see whether the code clears. If it does and the boiler runs normally, keep an eye on it. If the code returns within a short time, a component fault is present and you will need a Gas Safe registered engineer to investigate further. Do not reset the boiler more than two or three times without the underlying cause being identified.
How much does it cost to fix a Chaffoteaux 420 fault in the UK?
For most people, the repair involves replacing the zone temperature sensor (NTC thermistor), which is an inexpensive part. Including a call-out fee and labour, most homeowners pay in the region of £120–£200 for a sensor replacement. If the external PCB has also failed, the total cost rises, typically to £250–£350 once parts and labour are combined. PCB replacement alone can cost £250–£450 just for the part, so if your engineer confirms the board has failed on an older boiler, it is worth comparing that cost against the price of a new boiler before committing to the repair.
What is the difference between Chaffoteaux fault codes 411–413, 420, and 430–433?
All of these codes sit in the same 400-series category, which covers external PCB and zone sensor faults. Codes 411–413 point to problems with temperature probes on specific individual zones. Code 420 indicates a more general zone sensor or external PCB fault — the sensor may be open-circuit, short-circuit, or simply unavailable to the board. Codes 430–433 cover further faults within the same external PCB and zone sensor family. If your display shows any of these neighbouring codes, the diagnostic approach is very similar: check the relevant sensor and its wiring before suspecting the PCB itself.
My Chaffoteaux boiler has no display — how do I know if I have a 420 fault?
Many Chaffoteaux models indicate fault codes through a sequence of flashing indicator lights rather than a numeric display. The number of flashes or the combination of lights that are lit corresponds to a specific fault code. Consult the fault-finding section of your boiler's installation manual (or the user manual if it includes a fault table) to translate the light pattern into a code number. If you no longer have the manual, the model number on the boiler's data plate can be used to find a digital copy on the Chaffoteaux or Ariston website.