Chaffoteaux 705 Fault Code: Causes, Fixes & Repair Costs
What does the Chaffoteaux 705 fault code mean?
Code 705 appears on Chaffoteaux boilers fitted with multi-zone heating systems and signals that the control board has detected an invalid reading from one of the zone circuit NTC temperature sensors — either an open circuit (broken connection) or a short circuit within the sensor itself. The 700-series fault block as a whole covers zone flow and return sensor anomalies across zones 1 to 3: codes 701–703 relate to zone flow (outflow) probes, code 704 covers the zone 3 return sensor, and codes 705–707 extend to further zone sensor variants in the same group. When any of these sensors reports an out-of-range or absent signal, the boiler triggers a safety shutdown to prevent the heating circuit from running blind without reliable temperature data. This code is found on newer Chaffoteaux models with a numeric display — such as the Talia, Pigma, Inoa, and Niagara ranges — rather than older LED-flash models like the Britony Combi SE.
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 (temperature sensor) Common
The NTC thermistor responsible for monitoring zone flow or return temperature has degraded or failed outright. At 25°C a healthy sensor reads approximately 8.1–8.6 kΩ; a reading of zero or wildly outside this range confirms sensor failure. This is the single most common root cause of a 705 fault.
- Loose, corroded, or damaged wiring and connectors Common
The low-voltage wiring between the zone sensor and the control board is vulnerable to vibration, heat cycling, and general wear. A connector that has worked loose or a wire with internal corrosion can produce exactly the same open-circuit signal as a dead sensor, even when the sensor itself is perfectly functional.
- Moisture on sensor or PCB connectors Sometimes
Condensation or a minor water leak near the sensor connector or on the PCB connector block can cause resistance values to fluctuate or read incorrectly. Moisture-related faults often appear intermittently before becoming persistent.
- Power surge or unstable electrical supply Sometimes
A spike in the mains supply, or incorrect polarity during a previous installation or repair, can corrupt the control board's sensor readings and trigger 700-series errors even when the sensors themselves are undamaged. A one-off reset may clear this type of fault.
- PCB (control board) failure Rare
If all wiring and sensors check out correctly, the printed circuit board may have lost the ability to interpret sensor signals accurately. Physical signs such as burnt components, cracked solder joints, or visible heat damage indicate board failure as the root cause.
How to fix it
- Reset the boiler once or twice DIY safe
Press and hold the RESET button as described in your boiler's user guide. If the 705 code clears and does not return within a short period, the fault may have been a temporary electrical glitch. Do not reset more than two or three times — repeated resets without the fault clearing indicate a real underlying problem that needs professional attention.
- Check your mains electricity supply DIY safe
Confirm the boiler's fused spur is switched on and that no relevant circuit breakers have tripped in your consumer unit. An unstable or interrupted power supply can sometimes cause spurious sensor error codes on multi-zone systems.
- Call a Gas Safe registered engineer to inspect the zone sensor and wiring Gas Safe engineer
An engineer will isolate the boiler electrically, disconnect the suspect zone NTC sensor, and measure its resistance with a multimeter. A reading near 0 Ω or far outside the 8.1–8.6 kΩ range at room temperature confirms the sensor needs replacing. They will also inspect wiring harnesses and connector blocks for corrosion, moisture, or mechanical damage, reseating or replacing connectors as needed.
- Allow the engineer to replace the faulty NTC thermistor if required Gas Safe engineer
Chaffoteaux zone NTC thermistors (such as part reference 61000733) are relatively inexpensive at roughly £11–£40 depending on the exact variant. The engineer will fit the correct replacement, clear the fault code, and verify the zone sensor is reading accurately before restoring the boiler to full operation.
- Have the PCB inspected if the sensor and wiring are both sound Gas Safe engineer
If the zone sensor measures within specification and all wiring is intact but the 705 code persists, the control board itself is the likely culprit. The engineer will examine the PCB for burnt tracks, damaged components, or failed solder joints. PCB replacement or professional board repair may be required — see FAQs for cost guidance.
Parts you may need
- Zone NTC thermistor / temperature sensor (e.g. Ariston-Chaffoteaux ref. 61000733) · from £15
- Wiring harness / sensor connector loom · from £25
- Printed circuit board (PCB) · from £320
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 a Chaffoteaux 705 fault dangerous — will my boiler restart on its own?
Code 705 itself is not a gas-leak or combustion emergency; it is a sensor fault that causes the boiler to shut down as a precaution. The boiler will not restart automatically until the underlying cause is resolved — it is classified as a Safety Stop rather than a simple lockout that clears on reset alone. Keep the heating off until an engineer has investigated, but there is no immediate risk to your household.
What is the typical cost to fix a Chaffoteaux 705 fault in the UK?
For the most common repair — replacing a faulty zone NTC thermistor — you are typically looking at £100–£180 all-in, covering the call-out, an hour or so of labour, and the sensor part (usually £11–£40). If the PCB turns out to be the cause, board replacement typically adds £250–£400 in parts alone, pushing the total bill to £350–£550 or more. For this reason it is worth asking the engineer to confirm the sensor is genuinely faulty before authorising a PCB replacement.
Which Chaffoteaux models can show a 705 fault code?
Only Chaffoteaux models with a numeric fault-code display and multi-zone heating capability will show 705. This includes models in the Talia, Pigma, Inoa, and Niagara families. Older Chaffoteaux boilers such as the Britony Combi SE use a flashing LED pattern system and will never display a numeric code like 705.
Can I fix a Chaffoteaux 705 fault myself?
The only safe DIY actions are resetting the boiler (once or twice) and checking your electrical supply is intact. Testing the NTC sensor with a multimeter, inspecting wiring, and replacing any components are all tasks that must be carried out by a Gas Safe registered engineer — not because they involve the gas circuit directly, but because working inside a boiler without the correct competencies is unsafe and could invalidate your warranty or home insurance. Always verify your engineer's Gas Safe registration at gassaferegister.co.uk before they start work.