Chaffoteaux 703 Fault Code: Causes, Fixes & Repair Costs
What does the Chaffoteaux 703 fault code mean?
Fault code 703 appears on Chaffoteaux multizone boilers and tells you that the control board has detected either an open circuit or a short circuit on the Zone 3 flow (supply) temperature probe. In plain terms, the board is trying to read the temperature of the water leaving the boiler into heating Zone 3, but the NTC thermistor sensor responsible for that reading is sending back either no signal at all or a signal that makes no sense. Codes 701 and 702 cover the same fault on Zones 1 and 2 respectively, so if you see any of those on the display, the diagnosis and remedy are essentially identical — only the affected zone differs. Until the board can get a reliable temperature reading from that probe circuit, the boiler will not operate normally.
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 probe Common
The Zone 3 flow probe contains a small thermistor whose electrical resistance changes with temperature, allowing the control board to calculate the water temperature. Over time these sensors degrade and can fail outright, producing either an open circuit (infinite resistance) or a short circuit (near-zero resistance). Either condition triggers the 703 code. This is by far the most frequent root cause.
- Loose or damaged wiring to the probe Common
The low-voltage wiring that runs from the control board to the Zone 3 probe can become loose at its connector, chafe against pipework, or suffer heat damage in the boiler casing. Any break in that circuit will present to the board as an open-circuit fault, even if the probe itself is perfectly healthy.
- Moisture or corrosion at the connector Sometimes
Condensation or a minor water leak near the Zone 3 probe connector can cause corrosion that increases contact resistance or bridges the two terminals with water, mimicking both open and short-circuit faults. This is more likely on older installations or where there has been a previous pipework leak.
- Control board (PCB) fault Rare
If the probe and all its wiring test within specification but the fault persists, the control board itself may have developed a fault on the input channel that reads the Zone 3 probe. PCB faults are the least likely explanation and are usually only diagnosed after everything else has been ruled out.
How to fix it
- Note the code and check the boiler display carefully DIY safe
Before doing anything else, write down the exact code shown. Codes 701, 702, and 703 all relate to zone flow probes for Zones 1, 2, and 3 respectively. Confirming which zone is affected helps the engineer locate the right probe quickly when they arrive.
- Attempt a single boiler reset DIY safe
Chaffoteaux boilers allow a reset after a fault — consult your user manual for the reset procedure on your specific model (typically holding the reset button for a few seconds). If the boiler restarts and runs normally without the code returning, it may have been a transient signal glitch. If the fault reappears, do not continue resetting repeatedly, as this will not fix an underlying probe or wiring fault and may mask the issue for your engineer.
- Check that your gas supply is active DIY safe
While a Zone 3 probe fault is not gas-related, it is worth confirming that other gas appliances in your home are working normally. If they are not, contact your gas supplier before calling a boiler engineer.
- Inspect probe wiring and connectors — engineer task Gas Safe engineer
A Gas Safe registered engineer will trace the wiring loom from the control board to the Zone 3 flow probe, checking for loose connectors, signs of burning, chafing, or moisture ingress. Connectors will be unplugged and inspected, and any corroded terminals cleaned or replaced. This step alone sometimes resolves the fault without needing a new probe.
- Test the NTC thermistor resistance — engineer task Gas Safe engineer
Using a calibrated multimeter, the engineer will measure the resistance across the Zone 3 flow probe at a known ambient temperature and compare it against the manufacturer's resistance-temperature curve. A reading well outside the expected range confirms the probe has failed and must be replaced. The Ariston/Chaffoteaux thermistor (part reference 61000733) is a low-cost part, so replacement is straightforward and affordable once the fault is confirmed.
- Replace the faulty probe or PCB as required — engineer task Gas Safe engineer
If the probe is confirmed faulty, the engineer will fit a new NTC thermistor sensor. If the probe and wiring both test correctly, the engineer will then assess whether the control board's input circuit has failed, which would require PCB replacement or repair. PCB work is significantly more involved and costly than a simple probe swap.
- Call a Gas Safe registered engineer Gas Safe engineer
All repair work on a gas boiler in the UK must be carried out by a Gas Safe registered engineer. You can verify an engineer's registration on the official Gas Safe Register website (gassaferegister.co.uk). Do not attempt to open the boiler casing, disconnect the probe, or interfere with any wiring yourself.
Parts you may need
- Chaffoteaux/Ariston NTC flow temperature probe (e.g. part 61000733) · from £13
- Wiring harness connector repair kit · from £8
- Chaffoteaux control PCB (model-specific) · 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 £110–£220, depending on the underlying cause.
Frequently asked questions
Can I fix Chaffoteaux fault code 703 myself?
The initial checks — noting the code, attempting one reset, and confirming your gas supply is on — are safe for a homeowner to carry out. However, actually diagnosing and replacing the Zone 3 flow probe requires opening the boiler casing, handling electrical components, and working on a gas appliance, all of which must be done by a Gas Safe registered engineer under UK law. The part itself is inexpensive (typically under £15), so the bulk of your bill will be labour.
What is the difference between fault codes 701, 702, and 703 on a Chaffoteaux boiler?
These three codes all describe the same type of fault — an open or short circuit on a zone flow temperature probe — but on different heating zones. Code 701 points to the Zone 1 flow probe, 702 to Zone 2, and 703 to Zone 3. The diagnosis and repair process is identical whichever code appears; only the physical location of the probe differs. If your boiler is a single-zone model and you see 703, check that you have the correct manual for your specific model, as some older Chaffoteaux ranges used different code schemes entirely.
How much does it cost to repair a Chaffoteaux 703 fault in the UK?
For most homeowners, the total bill for a Zone 3 probe replacement falls between roughly £110 and £220, including call-out, labour, and the part. The thermistor sensor itself typically costs under £15, so the majority of the cost is the engineer's time. If the fault turns out to be a wiring connector issue rather than a failed probe, the repair can be at the lower end of that range. In the less common scenario where the control board (PCB) has failed, costs can rise considerably — a model-specific PCB can cost £150–£250 for the part alone on top of labour — but a good engineer will exhaust cheaper options first.
Will a 703 fault stop my whole heating system or just Zone 3?
This depends on how your multizone system is configured and how the boiler's control board responds to the fault. In many cases the boiler will lock out entirely to prevent it operating without a valid temperature reading from Zone 3, which means all zones lose heating. Even if the boiler continues running in a limited mode, Zone 3 will not be controlled correctly. Either way, prompt repair is advisable, particularly in colder months.