Chaffoteaux 205 Fault Code: Causes, Fixes & Repair Costs
What does the Chaffoteaux 205 fault code mean?
Fault code 205 on Chaffoteaux boilers signals that the control board has detected a problem with the temperature sensor monitoring the domestic hot water (DHW) inlet from a connected solar thermal system. The boiler expects a continuous, valid resistance reading from this NTC sensor; when that signal disappears entirely (open circuit — a broken wire or failed sensor) or collapses to near-zero (short circuit — wires touching or a sensor that has internally shorted), the board logs code 205 and may lock out to protect the system. The closely related code 206 refers specifically to the short-circuit version of the same sensor fault, so if you see 206 on your display the diagnosis and repair path is essentially identical. Because this fault sits within the solar circuit rather than the main gas-heating circuit, it will only appear on Chaffoteaux models plumbed into a solar thermal panel — if your boiler has no solar connection, double-check your model's manual, as code numbering can vary slightly across the Chaffoteaux range (many of which share fault-code logic with Ariston, given their shared parent group).
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 NTC temperature sensor Common
The solar DHW inlet sensor is an NTC (negative temperature coefficient) thermistor. Over time, internal element failure, water ingress, or thermal stress can cause it to develop an open circuit or short circuit, producing a reading the control board rejects as invalid. This is the most frequent root cause of a 205 fault.
- Damaged or disconnected wiring Common
The wiring loom running from the sensor back to the PCB can suffer from chafing, rodent damage, corrosion at connectors, or a plug that has worked loose — any of which can break or short the sensor signal circuit. This is particularly worth suspecting if the fault appeared after recent work near the boiler or a leak event.
- Moisture or corrosion at the sensor connector Sometimes
Because the sensor sits in the water circuit, a small leak or condensation can allow moisture to track along the cable and corrode the electrical connector. This can mimic both an open-circuit and a short-circuit condition depending on how the corrosion has bridged or interrupted the contacts.
- PCB (control board) fault Rare
If the sensor and its wiring both test as healthy, the PCB input channel that reads the sensor signal may itself have failed. This is a less common cause but should be considered once all other possibilities have been ruled out.
How to fix it
- Check that the boiler has not simply tripped on a transient glitch and attempt a reset DIY safe
Locate the reset button on your Chaffoteaux boiler's front panel (usually marked with a flame or reset symbol) and hold it as described in your user manual. Wait for the boiler to attempt a restart. If it fires up and runs normally, monitor it over the next day or two. Repeat the reset no more than two or three times — if the fault returns promptly, move on to the steps below rather than continuing to reset.
- Visually inspect the visible wiring around the solar circuit connections DIY safe
With the boiler powered off at the mains isolator, have a look at any wiring you can see without removing panels — in particular around the solar circuit pipework entry points. Look for obvious signs of chafing, burning, or a connector that has clearly pulled free. Do not remove any boiler casing or touch internal components. If you spot obvious external damage, note it to report to the engineer.
- Have a Gas Safe engineer test the NTC sensor resistance Gas Safe engineer
An engineer will disconnect the solar DHW inlet sensor and measure its resistance with a multimeter at a known water temperature, comparing the reading against the manufacturer's resistance-temperature curve. A reading that is open-circuit (infinite resistance) or short-circuit (near-zero resistance) confirms sensor failure. This step definitively separates a sensor fault from a wiring or PCB fault.
- Have the engineer inspect and test the wiring harness and connectors Gas Safe engineer
If the sensor itself tests within specification, the engineer will carry out a continuity check along the full wiring loom from the sensor back to the PCB connector, looking for broken cores, corroded pins, or moisture ingress. Damaged sections of loom or corroded connectors will be repaired or replaced as required.
- Replace the NTC sensor if it has failed Gas Safe engineer
A confirmed failed sensor must be replaced with the correct Chaffoteaux-compatible NTC part. The component itself is relatively inexpensive, so labour is the main cost here. After fitting, the engineer will clear the fault and run the boiler through a full hot-water cycle to confirm the new sensor reads correctly across its temperature range.
- Have the PCB investigated if the fault persists after sensor and wiring checks Gas Safe engineer
A persisting 205 code despite a healthy sensor and intact wiring points to the control board's sensor-input circuit. The engineer will assess whether the PCB can be repaired or requires full replacement. PCB replacement is the most expensive outcome for this fault, so confirm the diagnosis carefully before committing to the part.
- Call a Gas Safe registered engineer if you have not already done so Gas Safe engineer
All internal boiler diagnostics and component replacements must be carried out by a Gas Safe registered engineer. You can verify any engineer's registration at gassaferegister.co.uk. When booking, mention that your Chaffoteaux is showing fault code 205 and that you have a solar thermal system connected — this helps the engineer bring the right test equipment and, if needed, the correct sensor part.
Parts you may need
- Solar DHW inlet NTC temperature sensor · from £15
- Sensor wiring loom / harness section · from £25
- PCB (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 £100–£280, depending on the underlying cause.
Frequently asked questions
My Chaffoteaux boiler is showing code 206 — is that the same as 205?
Very nearly. Code 205 typically corresponds to an open-circuit condition on the solar DHW inlet sensor (the signal has been lost entirely), while code 206 points to a short-circuit on the same sensor (the signal has collapsed to near-zero). Both codes indicate a fault with the same component and wiring loop, so the diagnostic and repair process is identical. Whichever code your display shows, an engineer will test the sensor resistance and inspect the loom in exactly the same way.
My boiler does not have solar panels — why am I seeing fault code 205?
Code 205 is specifically associated with the solar DHW inlet sensor, so it should only appear on boilers connected to a solar thermal system. If yours has no solar connection and you are seeing this code, it is worth double-checking your model against the manual, as Chaffoteaux's fault-code numbering is not entirely consistent across all product generations. A Gas Safe engineer familiar with Chaffoteaux or Ariston equipment will be able to confirm the correct interpretation for your specific model.
How much does it typically cost to fix a Chaffoteaux 205 fault in the UK?
For the most common outcome — sensor replacement and any minor wiring work — most homeowners pay in the region of £100–£280 including parts and labour. A Gas Safe engineer's call-out and labour typically runs £50–£80 per hour, and the NTC sensor itself is an inexpensive part. If the fault turns out to be a PCB failure, the cost rises considerably; a replacement control board for a Chaffoteaux model can be £150–£200 for the part alone, pushing the total repair to £300–£450 or more. London and South East rates will generally sit toward the upper end of these ranges.
Can I keep using hot water while fault code 205 is showing?
It depends on how your boiler and solar system are configured and whether the boiler has locked out completely. In some cases the boiler will continue to provide heating and hot water with the solar circuit effectively bypassed; in others it will lock out entirely. Even if hot water is still available, you should book a repair promptly — running the system without a valid sensor reading means the control board cannot properly manage the solar input, which can reduce efficiency and, in some configurations, cause overheating in the solar circuit.