Chaffoteaux 251 Fault Code: Causes, Fixes & Repair Costs
What does the Chaffoteaux 251 fault code mean?
Fault code 251 on Chaffoteaux boilers signals a problem with the Fresh Water Station (FWS) domestic hot water outlet sensor. The boiler's control board monitors the NTC temperature probe fitted on the hot water outlet of the Fresh Water Station — a module used in certain Chaffoteaux configurations, including those with indirect cylinders or solar-assisted DHW systems. When the control board cannot obtain a reliable temperature reading from this probe, it raises code 251, locks the boiler out, and displays 'Err' on the screen to prevent unsafe operation. This is part of the 200-series fault range on Chaffoteaux boilers, which covers DHW and storage circuit anomalies — so if you see a code like 2 51 on your display, that is the same fault described here.
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 degraded NTC outlet sensor Common
The NTC (negative temperature coefficient) thermistor on the FWS DHW outlet is the most common culprit. These probes deteriorate over time, particularly in hard-water areas where scale can insulate the sensor element, pushing readings out of range. A multimeter resistance check compared against the manufacturer's resistance-temperature chart will confirm failure.
- Loose, corroded, or damaged wiring Common
The low-voltage signal cable between the outlet sensor and the PCB can suffer from loose connector pins, corrosion from moisture ingress, or cable damage — especially if the boiler is in a damp cupboard. The control board may clear the fault within seconds if the break was momentary, but if the connection is intermittent or permanently broken, the error will return each time.
- Short circuit in the sensor circuit Sometimes
An internal short within the sensor itself, or insulation breakdown in the cable, will cause the PCB to read an out-of-range resistance value. This can also be triggered if water temperature in the circuit has exceeded the recommended limit, stressing the sensor.
- Faulty PCB (control board) Rare
If the wiring and sensor both test within specification but the fault persists, the PCB may be misreading the sensor signal. Physical signs of board trouble include discolouration, swollen capacitors, or burnt track marks. This is a less common but more costly diagnosis.
How to fix it
- Check for a temporary glitch and reset the boiler DIY safe
Press the RESET button on the boiler's control panel. A transient wiring interruption can occasionally trigger the code without a permanent fault. Allow the boiler to restart and monitor it for 30–60 minutes. If the fault clears and does not return, no further action is needed. Limit yourself to two or three resets — if the code keeps coming back, move on rather than repeatedly resetting.
- Inspect wiring, connectors, and the sensor probe visually Gas Safe engineer
A Gas Safe registered engineer should examine the cable run from the FWS DHW outlet sensor to the PCB. They will look for loose or corroded connector pins, signs of moisture around the connector, chafed insulation, and any obvious physical damage to the probe body. Cleaning or reseating a connector is sometimes all that is needed.
- Test the NTC sensor resistance Gas Safe engineer
Using a calibrated multimeter, the engineer will measure the resistance of the NTC probe at a known water temperature and compare the reading against Chaffoteaux's resistance-temperature reference chart. Because NTC sensors decrease in resistance as temperature rises, a reading that falls significantly outside the expected range confirms sensor failure.
- Replace the FWS DHW outlet NTC sensor Gas Safe engineer
If the probe has failed, it must be swapped for a compatible Chaffoteaux replacement. Genuine Ariston/Chaffoteaux NTC temperature sensor thermistors (part number 61000733) are available from UK boiler spares suppliers for approximately £10–£15 + VAT. The engineer will drain down the relevant circuit section, fit the new probe, refill, and retest before clearing the fault code.
- Inspect and test the PCB if the fault persists Gas Safe engineer
Should the error return after a confirmed good sensor and sound wiring, the PCB is the next suspect. The engineer will check for visible damage and may carry out an electronic assessment of the board's sensor-input circuit. A PCB replacement is significantly more involved and costly than a sensor swap.
- Call a Gas Safe registered engineer Gas Safe engineer
If you have already reset the boiler without success, do not attempt any further internal work yourself. All diagnosis, wiring inspection, sensor replacement, and PCB work on a gas appliance must be carried out by a Gas Safe registered engineer under the Gas Safety (Installation and Use) Regulations 1998. Contact one to diagnose and resolve fault code 251 safely.
Parts you may need
- Chaffoteaux FWS DHW outlet NTC temperature sensor (e.g. part 61000733) · from £13
- NTC sensor connector / wiring harness section · from £18
- Chaffoteaux PCB / control board · 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
Can I fix Chaffoteaux fault code 251 myself?
The only step a homeowner can safely carry out is a boiler reset — press the RESET button up to two or three times to see whether the fault clears. Everything beyond that, including checking wiring, testing the sensor, and replacing any components, must be done by a Gas Safe registered engineer. Working inside a gas appliance without the correct registration is illegal and dangerous.
How much does it cost to repair Chaffoteaux fault code 251 in the UK?
For most households the repair involves replacing the FWS DHW outlet NTC sensor, which is an inexpensive part (around £10–£15). Adding a typical call-out fee and an hour of labour, expect to pay roughly £100–£180 all-in. If the fault turns out to be a PCB problem the bill rises considerably — PCB replacement commonly runs to £250–£450 — but this is a less common outcome. Prices in London and the South East tend to run 15–25% higher than the national average.
What is a Fresh Water Station (FWS) and why does it have its own sensor?
A Fresh Water Station is a heat-exchanger module used in some Chaffoteaux system configurations — particularly with indirect cylinders or solar thermal setups — to produce domestic hot water without direct contact between the primary heating circuit water and the drinking water. The DHW outlet sensor monitors the temperature of the hot water leaving the FWS so the control board can regulate it safely and efficiently. Fault code 251 means this specific sensor is reporting an out-of-range or unreadable signal.
Will fault code 251 affect my central heating as well as hot water?
Because code 251 relates specifically to the Fresh Water Station DHW outlet sensor, it primarily disrupts domestic hot water production. However, as the boiler locks out when this code is raised, central heating may also be unavailable until the fault is cleared. Once a Gas Safe engineer resolves the sensor fault and resets the boiler, both functions should return to normal.