Chaffoteaux 711 Fault Code: Causes, Fixes & Repair Costs
What does the Chaffoteaux 711 fault code mean?
Fault code 711 appears on multi-zone Chaffoteaux boilers and indicates that the control board has detected a problem with the return pipe temperature sensor (NTC probe) on Heating Zone 1. The boiler cannot obtain a valid temperature reading from that sensor — because it is reading open-circuit, short-circuit, or outside its expected resistance range — and has locked out to protect the system. The 711 code is part of the 700-series zone probe family used on the Expert Control platform: 701/711 relate to Zone 1, 702/712 to Zone 2, and 703/713 to Zone 3. The x01–x03 variants cover flow (send) probes, while the x11–x13 variants (including 711) specifically target return probes. If your display shows 711, the fault is isolated to the Zone 1 return sensor circuit.
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 thermistor (return probe) Common
The NTC sensor itself has reached the end of its service life or has been damaged by moisture ingress or repeated thermal cycling. At around 20°C a healthy Chaffoteaux NTC should measure approximately 10 kΩ; a reading of near-zero (short circuit) or infinite (open circuit) confirms the sensor has failed and must be replaced.
- Loose or corroded wiring connection Common
The return probe connects to the zone control board via a low-voltage cable. Oxidised terminals, a partially unplugged connector, or a wire that has been pinched or chafed can break the signal entirely and trigger the same error as a dead sensor. The connector is a frequent culprit on older installations.
- Sensor drift from moisture or ageing Sometimes
Even without a clean failure, prolonged exposure to damp or many years of thermal cycling can push the sensor's resistance outside the acceptable tolerance window (roughly ±5% of its nominal value). The boiler interprets this drifting reading as an anomaly and raises the fault.
- Electrical supply disturbance or power surge Sometimes
A transient voltage spike or an unstable mains supply can confuse the control board's sensor monitoring logic and throw a spurious 711 code. A single reset may clear this if the hardware is undamaged.
- PCB or zone control board failure Rare
If the board itself has developed a fault, it may misread — or fail to read — one or more sensor inputs, producing zone probe fault codes even when the sensors and wiring are in good condition. This is the least common cause but should be considered if sensor and wiring checks prove inconclusive.
How to fix it
- Attempt a single boiler reset DIY safe
Press and hold the reset button as described in your boiler's user guide. A one-off power glitch can occasionally produce a spurious 711 code that clears itself. If the fault returns immediately or comes back within a short time, do not keep resetting — further resets will not fix an underlying hardware fault and may mask important diagnostic information for your engineer.
- Check that the boiler has power and that no other faults are displayed DIY safe
Confirm the boiler's display is lit and that there are no additional fault codes alongside 711. Note down any secondary codes before resetting, as this information will help a Gas Safe engineer diagnose the problem more quickly.
- Call a Gas Safe registered engineer to test the Zone 1 return NTC sensor Gas Safe engineer
An engineer will disconnect the return probe and measure its resistance with a calibrated multimeter. At roughly 20°C the expected value is around 10 kΩ. An open-circuit or short-circuit reading confirms sensor failure. A value deviating by more than around 5% from nominal also warrants replacement. The compatible Chaffoteaux NTC sensor part reference for Inoa, Mira, Talia, Urbia, and Serelia models is 65121175 — your engineer should confirm the correct part against your boiler's data plate before ordering.
- Inspect and remediate the sensor wiring and connectors Gas Safe engineer
The engineer will trace the wiring between the Zone 1 return probe and the zone control module, looking for signs of corrosion, looseness, chafing, or moisture ingress at terminals. Cleaning oxidised contacts or reseating loose connectors sometimes resolves the fault without any parts being needed, which is significantly cheaper than a component replacement.
- Replace the NTC return probe if confirmed faulty Gas Safe engineer
If the sensor is out of specification, the engineer will fit a replacement NTC probe of the correct type and resistance curve for your model. After replacement the boiler should be restarted and monitored through a heating cycle to confirm the 711 code has cleared and that zone temperatures are being read correctly.
- If the fault persists, have the PCB or zone control board assessed Gas Safe engineer
Should the code remain after a confirmed sensor replacement and a thorough wiring check, attention turns to the zone control board or main PCB. The engineer may use Chaffoteaux diagnostic tools to check eBus communication, verify module compatibility, and determine whether the board needs to be replaced. This is a more involved repair and the engineer will advise on cost versus the age of the boiler before proceeding.
Parts you may need
- Chaffoteaux NTC temperature sensor (ref. 65121175) · from £35
- Zone wiring harness / connector set · from £25
- Zone control board (external electronic 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–£300, depending on the underlying cause.
Frequently asked questions
Can I fix Chaffoteaux fault 711 myself?
Not beyond a single reset attempt. The 711 fault requires electrical testing of the NTC sensor with a multimeter and inspection of internal wiring — tasks that must be carried out by a Gas Safe registered engineer in the UK. Attempting to access internal components yourself could be dangerous and would likely void any remaining warranty on the boiler.
What is the difference between Chaffoteaux fault codes 701 and 711?
Both are Zone 1 sensor faults on the Expert Control platform. Code 701 points to the flow (send) probe — the sensor on the pipe carrying heated water out to Zone 1. Code 711 points to the return probe — the sensor monitoring water temperature as it comes back from Zone 1. The diagnostic approach is very similar for both, but the engineer will test the specific sensor indicated by the code.
How much does it cost to fix a Chaffoteaux 711 fault?
For the most common repairs — replacing the NTC sensor or rectifying a wiring fault — most homeowners pay somewhere between £100 and £300 including parts and labour. If the zone control board or main PCB turns out to be the underlying cause, costs can rise to £350–£500 or more. On an older boiler, an engineer may recommend weighing up a PCB repair against the cost of a new boiler, particularly if the unit is over ten years old.
Why does my Chaffoteaux boiler show fault 711 only in cold weather?
Cold weather can worsen marginal faults that are intermittent in milder conditions. A sensor with borderline resistance drift may tip outside the acceptable range only when it gets very cold, and moisture trapped in a connector can cause intermittent signal loss that worsens with temperature fluctuations. An engineer should still test the sensor and wiring — intermittent faults that are ignored tend to become permanent over time.