Chaffoteaux 704 Fault Code: Causes, Fixes & Repair Costs
What does the Chaffoteaux 704 fault code mean?
Fault code 704 on Chaffoteaux boilers indicates a problem with the return temperature sensor (NTC thermistor) on heating zone 3. This sensor monitors the temperature of water coming back to the boiler after it has circulated through the zone 3 heating circuit, helping the boiler maintain safe and efficient operation. When the boiler's control board receives a reading from this sensor that falls outside the expected range — or receives no signal at all — it logs code 704 and may lock out. Because zone 3 is specific to multi-zone installations, this fault is most commonly seen on Chaffoteaux models fitted with external zone modules (such as the Alixia, Inoa, Talia, and Pigma ranges) rather than straightforward single-zone combis. Codes 701–703 cover flow probe faults on zones 1–3 respectively, so if you are also seeing those codes it can help narrow down whether the issue lies with wiring, a zone module, or the main PCB.
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
- Faulty Zone 3 Return NTC Sensor Common
The NTC thermistor itself is the most frequent culprit. Over time, sensors can drift out of their calibrated resistance range, suffer internal breakdown, or be damaged by water ingress or mechanical stress. At room temperature (around 25°C) a healthy sensor reads approximately 8.1–8.6 kΩ; a reading of zero or well outside that band points to sensor failure.
- Corroded or Loose Sensor Wiring Common
Oxidation on the sensor connector pins, a loose plug, or a chafed wire between the sensor and the zone module or PCB can cause intermittent or absent signals that the boiler interprets as a sensor fault. This is particularly common in installations where pipework is in a damp utility room or the wiring has never been inspected since installation.
- Power Surge or Electrical Supply Issue Sometimes
A voltage spike — from a nearby lightning strike, a power cut, or an unstable mains supply — can corrupt the sensor signal or damage the sensor circuit on the PCB. If code 704 appeared immediately after a power event, this should be investigated early.
- Failed Zone Module or PCB Rare
If the sensor and all associated wiring test correctly, the fault may lie with the zone expansion module or the main printed circuit board, which processes the sensor signal. PCB failures can manifest as false sensor fault codes across multiple zones and are usually confirmed only after everything else has been ruled out.
How to fix it
- Reset the boiler once or twice DIY safe
Press the reset button on the boiler's front panel and allow a full restart cycle. If the fault clears and does not return within a day or two, the trigger may have been a temporary electrical glitch. Do not reset more than two or three times in quick succession — repeated resets without a successful start can indicate a genuine fault that needs attention.
- Check that the mains power supply is stable DIY safe
Confirm the boiler's fused spur is switched on and the fuse has not blown. If you have a known problem with power surges in your property, mention this to your engineer, as a surge-protection device on the boiler's supply could help prevent recurrence.
- Visually inspect accessible sensor wiring (do not open the boiler casing) DIY safe
Without removing any boiler panels, look at the wiring loom running to the zone module (if externally mounted) for obvious signs of damage — scorching, pinched cables, or connectors that have worked loose. Note anything unusual and report it to your engineer. Do not disconnect or reconnect any wiring yourself.
- Arrange for a Gas Safe engineer to test the zone 3 return NTC sensor Gas Safe engineer
A qualified engineer will disconnect the sensor connector and measure its resistance with a multimeter. A healthy sensor reads around 8.1–8.6 kΩ at 25°C. A reading of zero, open circuit, or well outside the expected range confirms sensor failure. The engineer will also inspect the connector pins for oxidation and clean or replace them as needed.
- Have the engineer inspect and test the zone module and wiring harness Gas Safe engineer
If the sensor tests within spec, the engineer will trace the circuit back through the zone module (where fitted) and on to the PCB, checking for continuity, correct polarity, and signs of heat damage or moisture ingress in the connector block.
- PCB inspection and replacement if required Gas Safe engineer
Should both the sensor and wiring prove serviceable, the main PCB or zone expansion board must be assessed for burnt components, damaged tracks, or swollen capacitors. PCB repair or replacement is specialist work and must only be carried out by a qualified engineer using parts appropriate for the specific boiler model.
- Call a Gas Safe registered engineer if you have not already done so Gas Safe engineer
All internal boiler diagnostics and repairs on a gas appliance must be carried out by a Gas Safe registered engineer under the Gas Safety (Installation and Use) Regulations 1998. You can verify any engineer's registration at gassaferegister.co.uk. If fault code 704 persists after a reset, book a visit promptly rather than continuing to run the boiler.
Parts you may need
- Zone 3 Return NTC Temperature Sensor (Thermistor) · from £25
- Sensor Connector & Wiring Loom · from £20
- Zone Expansion Module · from £95
- Main PCB (Control Board) · from £260
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
Is fault code 704 dangerous — should I turn the boiler off?
Code 704 is an electrical sensor fault rather than a gas or combustion issue, so it is not an immediate safety emergency in the way that a carbon monoxide alarm or gas smell would be. However, the boiler will usually lock out automatically to protect itself. Leave it locked out and arrange an engineer visit — do not attempt to keep resetting it indefinitely, as this can mask an underlying problem.
How much does it typically cost to fix a Chaffoteaux 704 fault?
In most cases the repair involves replacing the zone 3 return NTC sensor, which is an inexpensive part. Including a call-out fee and labour you would typically expect to pay £100–£300 for a sensor swap. If the fault turns out to be the PCB or zone expansion module, costs rise considerably — a PCB replacement can run to £450–£700 all in — but this is a much less common outcome. Always get a firm quote before authorising any parts.
Can I replace the NTC sensor myself to save money?
Although NTC sensors are straightforward electrical components, replacing one inside a gas boiler requires access to the boiler's internals and work on components connected to a live gas appliance. In the UK this must only be done by a Gas Safe registered engineer. Attempting it yourself is illegal under the Gas Safety (Installation and Use) Regulations 1998 and would invalidate your boiler warranty and home insurance.
Why does my Chaffoteaux only show code 704 on a multi-zone system?
The 700-series fault codes on Chaffoteaux boilers relate specifically to zone temperature sensors. Codes 701–703 cover the flow probes for zones 1–3, while code 704 is the zone 3 return sensor. Single-zone combis do not have a zone 3 circuit, so this fault only appears on installations that use external zone modules to serve separate heating circuits — for example, a property where underfloor heating and radiators run as distinct zones.