Ariston 407 Fault Code: Causes, Fixes & Repair Costs
What does the Ariston 407 fault code mean?
Fault code 407 on an Ariston boiler points to a problem with the room temperature sensor circuit. The boiler's control board has detected either an interruption (open circuit) or a short circuit in the wiring that connects the room sensor to the boiler. Because the boiler can no longer receive a valid temperature reading from that sensor, it logs this fault and may restrict or stop heating operation until the circuit is restored. The '4' prefix in Ariston's fault-code numbering system groups this with room and zone sensor faults, so any 4xx code on your display is pointing you toward the room-sensor side of the system rather than the boiler's internal components.
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
- Loose or disconnected wiring at the room sensor Common
The most frequent trigger. Vibration over time, a knock during decorating, or a poorly seated connector at the room thermostat or its wall-plate can break the circuit just enough for the boiler to log a fault. Visually inspecting the wires at the room thermostat and checking they are firmly seated costs nothing and takes minutes.
- Damaged or broken sensor cable Common
The two-core cable running between the room sensor and the boiler can develop a break or chafe — especially where it passes through walls, door frames, or cable clips. A break anywhere along the cable will register as an open circuit at the PCB, triggering code 407.
- Failed room temperature sensor Sometimes
If the cable and all connections check out, the sensor element itself may have failed. Room sensors contain a thermistor whose resistance drifts or breaks completely with age. A Gas Safe engineer can confirm this quickly with a multimeter resistance check before ordering a replacement part.
- Short circuit in the sensor circuit Sometimes
A pinched or moisture-damaged cable can cause the two conductors to touch, creating a short circuit. The boiler's PCB registers this as an abnormal signal from the room sensor and raises code 407 just as it would for an open circuit.
- PCB fault misreading the sensor input Rare
Rarely, the fault lies not with the sensor or its wiring but with the input terminal on the PCB itself. Corrosion, a micro-fracture on the board, or a surge can cause the PCB to report a sensor fault even when the sensor is healthy. This is a diagnosis of exclusion — only considered after the wiring and sensor have been ruled out.
How to fix it
- Check that your room thermostat or smart controller is powered and undamaged DIY safe
Look at the room thermostat on the wall. If it has a display, confirm it is lit and showing a temperature reading. If it is battery-powered, try fitting fresh batteries. A completely dead thermostat can break the circuit and trigger code 407 without any fault in the boiler itself.
- Inspect visible wiring and connectors at the room thermostat DIY safe
Carefully remove the room thermostat from its wall-plate and check that both wires are firmly secured in their terminals. Look for any obvious damage — frayed insulation, a wire that has pulled out, or signs of moisture. Re-seat any loose connections and refit the thermostat. This is low-voltage room-wiring, not gas work, so a competent homeowner can safely check it.
- Reset the boiler once DIY safe
Press and hold the reset button for approximately 5 seconds until the boiler attempts to restart. If code 407 clears and does not return within a few hours, the problem may have been a temporary connection issue. If the fault reappears after one reset, do not keep resetting — repeated resets will not fix an underlying wiring or sensor fault and may mask other problems.
- Have a Gas Safe engineer test the sensor circuit with a multimeter Gas Safe engineer
An engineer will disconnect the room sensor wiring at the boiler and measure the resistance across the sensor terminals. A healthy NTC thermistor has a predictable resistance at room temperature (typically around 10 kΩ at 25 °C). An open circuit (infinite resistance) or short circuit (near-zero resistance) confirms a faulty sensor or cable. This is the definitive diagnostic step.
- Have the engineer repair or replace the wiring if a cable fault is found Gas Safe engineer
If the multimeter test reveals a break or short in the cable run rather than the sensor itself, the engineer will trace and repair the damaged section or re-route a replacement cable. This is straightforward work but must be carried out correctly to avoid a recurrence.
- Have the room temperature sensor replaced if the sensor element has failed Gas Safe engineer
If wiring and connections are sound but the sensor's resistance reading is outside specification, the sensor needs to be replaced. Room temperature sensor parts are inexpensive (typically £15–£40), and replacement is a short job. The engineer will ensure the new sensor is properly configured and that the fault code clears after installation.
- If the fault persists after sensor and wiring replacement, have the PCB inspected Gas Safe engineer
In the rare situation where a new sensor and fully intact wiring still produce code 407, the PCB's sensor-input circuit may be faulty. PCB diagnosis and replacement must be carried out by a Gas Safe engineer. Discuss costs before proceeding, as a PCB is a more significant investment than a sensor.
- Call a Gas Safe registered engineer if you have not already done so Gas Safe engineer
If the checks in steps 1–3 have not resolved the fault, book a Gas Safe registered engineer to complete the diagnosis. You can verify any engineer's credentials at gassaferegister.co.uk. Attempting to probe or repair internal boiler wiring or components yourself is unsafe and may invalidate your warranty.
Parts you may need
- Room temperature sensor (NTC thermistor) · from £25
- Room thermostat two-core cable (per metre) · from £2
- Replacement room thermostat / wall-plate · from £35
- Boiler PCB (if sensor input circuit damaged) · 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–£280, depending on the underlying cause.
Frequently asked questions
Can I fix Ariston fault code 407 myself?
The only safe DIY steps are checking that your room thermostat has power (or fresh batteries), inspecting the low-voltage wiring at the thermostat wall-plate for loose connections, and performing a single boiler reset. Any work beyond that — testing the sensor with a multimeter, repairing cables inside the boiler, or replacing components — should be carried out by a Gas Safe registered engineer. The boiler runs on gas, and UK law requires a Gas Safe engineer for any work on a gas appliance.
How much does it cost to fix a room sensor fault on an Ariston boiler in the UK?
For most homeowners, the total bill lands between £100 and £280. If the fix is a loose wire or a replacement sensor (a low-cost part at £15–£40), you are typically paying mostly for the engineer's call-out and labour. If the fault has damaged or points to the PCB, costs can rise to £350–£500 or more, but that scenario is uncommon. Getting two quotes is worthwhile for anything beyond a straightforward sensor swap.
Will my Ariston boiler still heat water with a 407 fault?
It depends on how the boiler is configured. If your boiler uses the room sensor as the primary control signal for central heating, it may suspend heating until the fault is cleared. Hot water from the domestic circuit may still be available in many configurations, as that function is often controlled separately. Check your boiler's display to see whether heating and hot water are both affected, and note this when you speak to an engineer.
What is the difference between a room sensor fault (code 407) and a boiler temperature sensor fault?
Ariston uses the first digit of its fault codes to indicate which part of the system is affected. Codes starting with '4' relate to the room or zone sensor — the external thermostat on your wall. Codes in other number groups (for example, those starting with '1' or '5') typically relate to sensors inside the boiler itself, such as the flow or return temperature sensors. Code 407 specifically means the external room sensor circuit has an open or short circuit, so the fault investigation begins at the room thermostat and its wiring, not inside the boiler.