Chaffoteaux 203 Fault Code: Causes, Fixes & Repair Costs
What does the Chaffoteaux 203 fault code mean?
Fault code 203 on a Chaffoteaux boiler indicates that the boiler has detected a problem with the hot water cylinder temperature sensor — known as the cylinder or 'ballon' probe. This NTC thermistor sits on or within the hot water storage cylinder and continuously reports the stored water temperature back to the boiler's control board. When the boiler receives a reading that is out of range — whether implausibly low (suggesting an open circuit or disconnection), implausibly high (suggesting a short circuit), or simply absent — it logs fault 203 and suspends normal operation to prevent the cylinder being heated incorrectly. Because this sensor only exists on systems that include a separate hot water storage cylinder, fault 203 is exclusively a system boiler issue and will not appear on Chaffoteaux combi boilers.
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 or out-of-spec NTC cylinder probe Common
The NTC thermistor itself has drifted outside its acceptable resistance range or failed entirely. This is the single most frequent cause of code 203. A faulty probe will either send no signal at all or a reading so far outside normal parameters that the control board rejects it and raises the fault.
- Wiring fault between cylinder probe and boiler PCB Common
The cable run from the cylinder probe back to the boiler can be long, and over time connectors can work loose, terminals can corrode, or the cable itself can be pinched or damaged. Even a high-resistance joint caused by mild corrosion is enough to distort the sensor reading and trigger the fault.
- Moisture or corrosion on sensor connectors Sometimes
Condensation or a minor water leak near the cylinder probe connector can introduce moisture into the plug, causing intermittent or permanently incorrect resistance readings. This often produces a fault that clears on reset but keeps returning.
- Short circuit in the sensor circuit Sometimes
A dead short across the sensor wiring — caused by damaged insulation, a connector bridge, or a waterlogged terminal — causes the control board to see an implausibly low resistance, which it interprets as an extremely high temperature reading. The board raises fault 203 to prevent overheating.
- PCB (control board) fault Rare
If the sensor and all wiring test correctly but the fault persists, the input circuit on the PCB itself may have failed. PCB faults are the least common cause of code 203 but must be considered once everything else has been eliminated.
How to fix it
- Check that the boiler and cylinder are both powered and that no other fault codes are present DIY safe
Confirm the boiler display shows only code 203 and that the cylinder is receiving power. If you see multiple codes, note them all before doing anything else, as they may point to a broader electrical issue.
- Attempt a single boiler reset DIY safe
Press and hold the reset button on the boiler's front panel as described in your user guide (usually 3–5 seconds). If the fault clears and the boiler runs normally for more than a few hours before returning, it may have been a transient signal glitch. Repeat the reset once or twice at most — persistent return of the code means an underlying fault needs investigation by an engineer.
- Visually inspect the cylinder probe cable where it is accessible DIY safe
Without touching any electrical connections, look along the visible section of the cable running from the cylinder to the boiler for obvious physical damage such as kinks, cuts, or signs of rodent damage. Note anything unusual and report it to the engineer.
- Engineer to inspect and test the cylinder NTC probe resistance Gas Safe engineer
A Gas Safe engineer will disconnect the probe and measure its resistance with a multimeter at a known water temperature, comparing the reading against the manufacturer's resistance-temperature curve. A reading significantly outside the expected range confirms the sensor needs replacing.
- Engineer to inspect all connectors and wiring in the sensor circuit Gas Safe engineer
The engineer should check both the probe-end connector and the PCB-end terminal for corrosion, moisture ingress, loose pins, or damaged insulation. Corroded connectors can sometimes be cleaned and re-sealed; damaged wiring sections will need replacing.
- Engineer to replace the cylinder NTC probe if found to be faulty Gas Safe engineer
Replacing the cylinder probe is a straightforward job for a competent engineer. The sensor is not a gas component, but work on the cylinder and associated wiring should only be carried out by a qualified professional to ensure correct installation and safe operation.
- Engineer to inspect and replace the PCB if all other components test correctly Gas Safe engineer
If the probe and wiring are both confirmed good but fault 203 persists, the control board's sensor input circuit is the likely culprit. PCB replacement is a specialist task and will significantly increase the overall repair cost.
- Contact a Gas Safe registered engineer if you have not already done so Gas Safe engineer
All diagnostic and repair work beyond a reset is beyond DIY scope. Book a Gas Safe engineer, quoting fault code 203 and your exact boiler model, so they can bring the most likely replacement parts to the first visit.
Parts you may need
- Cylinder NTC temperature probe (compatible) · from £20
- Sensor wiring harness / cable loom · from £35
- Connector repair kit / terminal pins · from £10
- Replacement PCB (control board) · from £250
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–£300, depending on the underlying cause.
Frequently asked questions
Will my hot water and heating still work with fault code 203 showing?
It depends on how the boiler handles the fault. Many Chaffoteaux system boilers will suspend cylinder heating when they cannot trust the probe reading, meaning you lose stored hot water. Central heating may continue to function normally, since it uses separate sensors. However, some models will lock out entirely. Either way, the fault should be investigated promptly to restore full operation.
Can I fix Chaffoteaux fault code 203 myself?
The only action a homeowner should take is a boiler reset — once or twice at most. Testing sensor resistance, inspecting internal wiring, and replacing components all require a Gas Safe registered engineer. Although the cylinder probe itself is not a gas component, working on boiler wiring and control systems without the right qualifications is unsafe and can invalidate your warranty or home insurance.
How much does it cost to fix fault code 203?
For the most common scenario — a faulty NTC probe and a straightforward swap — expect to pay roughly £100–£200 including parts and the engineer's call-out and labour. If the fault turns out to be a wiring loom issue, costs are similar. If the PCB needs replacing, the bill can rise to £350–£500 or more, since a Chaffoteaux PCB alone can cost £200–£300 before labour is added. Always ask for a written quote before authorising any work.
My boiler is over 12 years old and showing fault 203 — is it worth repairing?
Chaffoteaux boilers typically have a useful life of 10–15 years. If yours is approaching or past that age and the repair involves an expensive component such as the PCB, it is worth getting a replacement quote at the same time as the repair quote. A modern A-rated condensing system boiler will be significantly more energy-efficient, and the long-term savings on gas bills can offset a proportion of the installation cost.