Chaffoteaux 701 Fault Code: Causes, Fixes & Repair Costs
What does the Chaffoteaux 701 fault code mean?
Fault code 701 appears on Chaffoteaux boilers that are set up in a multizone heating configuration. It tells you the boiler's control board has detected something wrong with the NTC thermistor monitoring the flow (departure) temperature on Zone 1 — the pipe carrying hot water out to your first heating zone. NTC thermistors work by changing their electrical resistance as temperature rises or falls; the PCB reads those resistance values continuously to control heating output. When the signal falls outside the expected range — due to an open circuit, a short, or a wiring break — the boiler logs code 701 and may shut down that zone or lock out entirely. This fault belongs to a family of multizone sensor codes: 701 covers the Zone 1 flow sensor, 702 the Zone 2 flow sensor, and 703 the Zone 3 flow sensor. Codes 711, 712, and 713 cover the equivalent return sensors for each zone. Note that code 701 only applies to newer Chaffoteaux models with a digital display and multizone capability — such as the Urbia, Serelia, Mira C, and Expert Control ranges. Older models like the Britony Combi SE use a flashing LED system rather than numeric codes.
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 NTC flow sensor Common
The Zone 1 flow sensor itself has failed — either a short circuit (resistance reads close to zero ohms) or an open circuit (resistance is excessively high or infinite). At around 25°C a healthy sensor should measure roughly 8.1–8.6 kΩ. A reading well outside that range points to a failed component that needs replacing.
- Loose or damaged wiring to the sensor Common
The wiring loom running from the Zone 1 flow sensor back to the PCB may have a loose plug, a broken wire, or corroded contacts. Corrosion at the connector pins is a particularly common culprit, as it increases resistance in the circuit and mimics the signature of a failed sensor.
- PCB fault Sometimes
If the sensor and all associated wiring test within normal limits, the problem may lie with the boiler's control board itself. Burn marks, cracked solder joints, deformed components, or oxidised tracks on the PCB can cause it to misread or lose communication with the sensor entirely.
How to fix it
- Reset the boiler once DIY safe
Locate the reset button on your Chaffoteaux boiler — usually a dedicated button or a turn-and-hold on the control dial. Press it once and wait for the boiler to attempt a restart. If the fault clears and stays clear, monitor the boiler over the next day or two. If code 701 returns promptly, do not keep resetting; repeated resets without fixing the underlying cause can mask the problem and may damage components.
- Check that your heating system has adequate pressure DIY safe
Look at the pressure gauge on the boiler front. It should read between 1.0 and 1.5 bar when the system is cold. If it is below 1 bar, top it up via the filling loop until it reaches about 1.2–1.5 bar, then reset. Low pressure alone will not cause a 701 fault, but it is a quick check worth ruling out before calling an engineer.
- Visually inspect accessible wiring and connectors DIY safe
With the boiler powered down, take a look at any wiring connectors you can see without removing panels. Look for obvious signs of damage — scorching, fraying, or green-white corrosion on metal contacts. Do not unplug or probe any connectors yourself; simply note what you see and report it to the engineer. Do not remove boiler casing panels.
- Engineer to test the Zone 1 flow sensor resistance Gas Safe engineer
A Gas Safe engineer will disconnect the Zone 1 flow sensor and measure its resistance with a multimeter set to resistance mode. At 25°C the reading should fall within approximately 8.1–8.6 kΩ (±0.2 kΩ). A reading near zero indicates a short circuit; an open-line or very high reading indicates a broken sensor. The engineer should also verify the other zone sensors (used in codes 702, 703, 711–713) are within range, as multizone systems can develop faults on more than one sensor.
- Engineer to check wiring continuity back to the PCB Gas Safe engineer
If the sensor itself tests healthy, the engineer will trace the wiring loom from the sensor connector all the way to its terminals on the PCB, checking for continuity and inspecting each plug for corrosion or pushed-back pins. A corroded or poorly seated connector is often all that needs attention, keeping the repair cost low.
- Engineer to replace the Zone 1 flow sensor if confirmed faulty Gas Safe engineer
Where the multimeter confirms a failed sensor, the engineer will fit a replacement NTC thermistor matched to the boiler model. This is typically a low-cost part. Once fitted, the boiler should be restarted and monitored through a full heating cycle to confirm the fault has cleared.
- Engineer to inspect and assess the PCB if sensor and wiring are sound Gas Safe engineer
If sensor resistance and wiring continuity both check out, the control board becomes the prime suspect. The engineer will inspect the PCB for visible damage — burnt components, cracked solder joints, deformed tracks, or oxidised contact areas. A faulty board will need either specialist repair or replacement. Given the cost of a new PCB, the engineer may also advise on whether replacement of the whole boiler is more economical, particularly on older units approaching 12–15 years of age.
- Call a Gas Safe registered engineer if you have not already done so Gas Safe engineer
If resetting the boiler has not resolved fault 701 or the fault keeps returning, contact a Gas Safe registered engineer. All internal diagnosis and component replacement on this fault must be carried out by a qualified professional. You can verify an engineer's Gas Safe registration at gassaferegister.co.uk.
Parts you may need
- NTC flow temperature sensor (Zone 1) · from £20
- Sensor wiring loom / connector harness · from £35
- PCB (control board) — model-specific · from £280
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–£250, depending on the underlying cause.
Frequently asked questions
What is the difference between fault codes 701, 702, 703, 711, 712, and 713 on a Chaffoteaux boiler?
These are all multizone sensor faults. Codes 701, 702, and 703 relate to the flow (departure) sensors on Zones 1, 2, and 3 respectively — they monitor the temperature of water leaving the boiler for each zone. Codes 711, 712, and 713 are the equivalent faults for the return sensors on each zone — they monitor water coming back from each zone. The diagnosis and repair process is essentially the same across all six codes; only the specific sensor location differs.
Can I fix Chaffoteaux fault code 701 myself?
A small number of checks are safe for a homeowner: a single boiler reset, topping up system pressure, and a visual look at any obviously accessible wiring for signs of corrosion or damage. Beyond that, diagnosing and replacing sensors, testing wiring continuity, or inspecting the PCB requires a Gas Safe registered engineer. Working inside the boiler without Gas Safe registration is both unsafe and illegal in the UK.
How much does it typically cost to fix a 701 fault on a Chaffoteaux boiler?
If the Zone 1 flow sensor itself is the cause, the part is inexpensive — usually £10–£40 — so a total bill including labour tends to come in around £100–£200. If the problem turns out to be the PCB, a replacement board for a Chaffoteaux model typically costs £180–£350 for the part alone, pushing the total repair to £300 or more. If your boiler is over 12 years old and the PCB has failed, it may be worth getting a new boiler quote alongside the repair quote.
Will code 701 stop my whole boiler from working or just Zone 1?
It depends on the boiler's programming and how the fault develops. In many cases the boiler will lock out entirely rather than continue running with a faulty sensor reading, since it cannot safely control flow temperatures without accurate data. In some configurations it may just disable Zone 1 and continue heating other zones. Either way, the fault needs attention promptly — running with unreliable temperature sensing can lead to overheating or inefficient operation.