Chaffoteaux 202 Fault Code: Causes, Fixes & Repair Costs
What does the Chaffoteaux 202 fault code mean?
Fault code 202 on a Chaffoteaux boiler indicates that the control system has lost a reliable signal from the lower NTC temperature sensor (thermistor) fitted to the solar or hot water storage cylinder. The controller expects a resistance reading within a specific range from this probe at all times; when it receives either no signal at all (open circuit — broken wire or failed sensor) or a near-zero resistance reading (short circuit — wiring fault or shorted sensor), it logs code 202 and may lock out the appliance. In plain terms, the boiler cannot accurately measure the temperature at the bottom of the storage cylinder and has stopped operating as a safety precaution. This fault is specific to Chaffoteaux models equipped with a solar or indirect storage cylinder and will not appear on heating-only combi installations.
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
- Failed lower NTC thermistor (open or short circuit) Common
The lower NTC 10 kΩ probe on the storage cylinder degrades over time. At around 20–25°C the sensor should read approximately 9,000–11,000 ohms. A reading of infinite resistance (OL on a multimeter) confirms an open circuit; a reading close to zero confirms a short circuit. Either condition triggers code 202. This is by far the most frequent root cause.
- Loose, corroded, or damaged wiring connections Common
The low-voltage wiring between the storage cylinder sensor and the control board is routed through an environment that can be humid or subject to vibration. Over time, connectors oxidise or work loose, producing an intermittent or absent signal that the board interprets as a sensor fault. Cleaning and re-seating the connectors resolves a significant proportion of cases without any parts being replaced.
- Limescale or water chemistry degrading the probe Sometimes
In hard-water areas or where the water is unusually acidic, the immersion pocket or probe body can be attacked by scale or corrosion, accelerating electrical failure of the sensor element. Boilers in very hard-water regions (much of South East England, for example) tend to show probe failures earlier than average.
- Faulty PCB or control board Rare
If a new, verified-compatible sensor is fitted and the wiring is confirmed sound but code 202 persists, the input circuit on the control board itself may be faulty. The PCB may show visible signs such as burn marks, cracked solder joints, or swollen capacitors, though damage is not always visible to the eye. This is a relatively uncommon cause but should be investigated once all other possibilities are ruled out.
How to fix it
- Reset the boiler once using the front-panel reset button DIY safe
Press and hold the reset button for the duration specified in your user guide (typically 3–5 seconds), then release and allow the boiler up to 60 seconds to attempt a restart. If the fault clears and the boiler runs normally, monitor it over the next hour. If code 202 returns, do not keep resetting — repeated resets will not fix an underlying sensor or wiring fault and you should move to the next step.
- Check that the boiler's system pressure is within the normal range DIY safe
Look at the pressure gauge on the front of the boiler. Normal operating pressure when the system is cold is typically 1.0–1.5 bar. A reading below 0.5 bar or above 3 bar alongside a sensor fault may point to a broader system issue. If pressure is low, top up via the filling loop according to the instructions in your user manual. Note that low pressure alone will not cause a 202 fault — this step is simply good practice before calling an engineer.
- Visually inspect the wiring at the storage cylinder sensor (do not disconnect) DIY safe
Without touching any terminals or removing connectors, visually check the cable running from the lower probe position on the cylinder towards the boiler control board. Look for obvious physical damage such as chafing, kinking, rodent damage, or signs of moisture ingress at the connectors. Report anything you find to the engineer — it will speed up diagnosis.
- Have a Gas Safe engineer test the lower NTC sensor resistance Gas Safe engineer
An engineer will disconnect the sensor wiring at the control board and measure resistance across the probe leads with a calibrated multimeter. For a standard NTC 10 kΩ probe at approximately 20–25°C, the expected reading is roughly 9,000–11,000 ohms. An infinite reading (open circuit) or near-zero reading (short circuit) confirms the sensor has failed and needs replacing.
- Have the engineer inspect and clean all associated wiring connectors Gas Safe engineer
The engineer should check the connector block at the cylinder sensor end and at the control board end, cleaning away any oxidation with electrical contact cleaner and ensuring each terminal is firmly seated. In a proportion of cases this step alone resolves the fault without any parts being needed.
- Replace the lower NTC temperature probe Gas Safe engineer
If the sensor is confirmed faulty, the engineer will source a replacement NTC 10 kΩ probe compatible with the specific Chaffoteaux model (a commonly referenced part number for Chaffoteaux solar cylinders is 4758637929, though the engineer should verify compatibility for your exact appliance). It is important to fit a genuinely compatible probe — a sensor with incorrect resistance characteristics will give inaccurate temperature readings even if it clears the fault code.
- If the fault persists after sensor and wiring checks, have the PCB evaluated Gas Safe engineer
Should code 202 remain active despite a verified new sensor and sound wiring, the engineer should inspect the control board for signs of damage such as burnt components, cracked solder joints, or failed capacitors. If the PCB input circuit is faulty, the board will require repair by an authorised service centre or outright replacement. This is an uncommon outcome but should not be overlooked.
- Call a Gas Safe registered engineer if you have not already done so Gas Safe engineer
All work beyond the basic homeowner checks above — testing, disconnecting, or replacing the sensor, wiring, or PCB — must be carried out by a Gas Safe registered engineer. You can verify any engineer's registration at the Gas Safe Register website (gassaferegister.co.uk). Always ask to see their Gas Safe ID card before work begins.
Parts you may need
- Lower NTC 10 kΩ thermistor / storage cylinder probe · from £18
- Wiring harness / sensor cable (cylinder to board) · from £35
- Control board / PCB (Chaffoteaux Expert Control) · 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 £100–£280, depending on the underlying cause.
Frequently asked questions
Is Chaffoteaux fault code 202 dangerous — do I need to turn the boiler off immediately?
Code 202 relates to a temperature sensor on the storage or solar cylinder, not to the main gas burner or flue. The boiler locks out as a precaution because it cannot accurately monitor cylinder temperature, but the fault itself is not a gas or carbon monoxide hazard. You do not need to make an emergency call-out in the middle of the night, but you should arrange for a Gas Safe engineer to inspect it within a day or two. If you ever smell gas or suspect a carbon monoxide leak, leave the property, call the National Gas Emergency number (0800 111 999), and do not re-enter until declared safe.
Can I fix Chaffoteaux fault 202 myself?
The initial reset and a visual inspection of the cabling are safe homeowner steps, but testing the sensor with a multimeter, disconnecting wiring, and replacing the probe all require a Gas Safe registered engineer. Attempting this work yourself could invalidate your boiler warranty, your home insurance, and — more importantly — puts you in breach of the Gas Safety (Installation and Use) Regulations 1998. The sensor replacement itself is a relatively modest repair once the engineer is on site.
How much does it typically cost to fix a Chaffoteaux 202 fault in the UK?
Most people with this fault pay somewhere in the range of £100–£280 all in, covering the engineer's call-out, diagnosis, and sensor or wiring replacement. Labour rates in London and the South East tend to run 20–30% higher than the UK average. In the minority of cases where the control board (PCB) turns out to be the root cause, the bill can rise to £350–£500 or more, since PCBs for Chaffoteaux models typically cost £180–£250 for the part alone plus fitting time. If your boiler is over 12–15 years old and a PCB replacement is recommended, it is worth getting a new boiler quote at the same time for comparison.
My Chaffoteaux boiler keeps showing code 202 after reset — what does that mean?
If the fault clears on reset but returns within a short time, the underlying cause has not gone away. This pattern — sometimes called an intermittent fault — often points to a wiring connector that is borderline: it makes contact when vibration settles, then loses it again. A corroded or slightly loose connector at the cylinder sensor or at the control board is a common culprit. An engineer can reproduce the fault by lightly flexing the cable and connectors while monitoring the resistance reading, which quickly pinpoints the problem. Continuing to reset the boiler repeatedly is not a fix and may mask a worsening fault.