Viessmann 18 Fault Code: Causes, Fixes & Repair Costs
What does the Viessmann 18 fault code mean?
Fault code 18 on a Viessmann boiler signals that the boiler has detected a break or open circuit in the outdoor temperature sensor circuit. Rather than shutting down entirely, the boiler switches into a fallback 'constant operation' mode, treating the outside temperature as a fixed 0°C regardless of actual conditions. This means weather compensation — the clever modulation of boiler output based on how cold it is outside — is no longer working. The boiler will still run, but it won't vary its flow temperature intelligently, which can affect both comfort and efficiency. Note that the closely related code 10 points to a short circuit in the same sensor, whereas code 18 specifically indicates an open circuit or lead break: a useful distinction when diagnosing the fault.
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
- Broken or damaged outdoor sensor cable Common
The cable running from the outdoor weather compensation sensor back into the boiler is exposed to the elements year-round. UV degradation, frost, rodent damage, or physical disturbance during garden or building work can cause the cable to snap or lose continuity internally — triggering the open-circuit fault that code 18 describes.
- Failed outdoor temperature sensor Common
The sensor unit itself can fail internally over time, particularly after several winters of exposure to cold, damp, and condensation. When the sensor's internal resistance reading becomes infinite (as happens with an open circuit), the boiler's control unit registers a lead break and raises code 18.
- Loose or corroded cable connection Sometimes
At either end of the sensor cable — where it plugs into the sensor housing outside or where it terminates at the boiler's control board — a loose connector or corrosion build-up can interrupt the circuit. This is worth checking before assuming the cable or sensor has fully failed.
- Sensor not installed or intentionally disconnected Rare
If the outdoor sensor was never fitted, has been removed during maintenance, or was deliberately bypassed at some point, the boiler will raise code 18 because it is configured to expect a sensor signal and is receiving none.
How to fix it
- Check whether the outdoor sensor is physically present and connected DIY safe
Go outside and locate the outdoor temperature sensor — it is usually a small plastic housing mounted on a north- or east-facing external wall, away from direct sunlight. Check that it is still in place and that the cable entering it appears intact and undamaged. If the sensor is missing entirely or the cable is visibly severed, this confirms the source of the fault.
- Inspect the cable run for obvious physical damage DIY safe
Follow the sensor cable from the outdoor unit back towards where it enters the property. Look for signs of crushing, rodent chew marks, kinks, or weathered sections where the insulation has split. If damage is visible, make a note of where it is so you can describe it accurately to your engineer.
- Attempt a boiler reset DIY safe
A single reset can sometimes clear a transient fault. Locate the reset button on your Viessmann boiler's display panel, press and hold it for around 3 seconds, and allow the boiler to restart. If code 18 returns immediately, the fault is genuine and a physical inspection of the sensor circuit is required. Do not reset more than 2–3 times in total.
- Have a Gas Safe engineer test the sensor and cable continuity Gas Safe engineer
A qualified engineer will use a multimeter to measure the resistance across the outdoor temperature sensor and its cable. A correctly functioning sensor will give a resistance reading that matches the ambient temperature on a known curve; an open circuit will read infinite resistance, confirming the lead break. This step must be performed by a Gas Safe registered engineer as it involves accessing the boiler's wiring and control circuitry.
- Replace the outdoor temperature sensor or cable as needed Gas Safe engineer
If testing confirms the sensor has failed internally, the engineer will fit a compatible replacement Viessmann outdoor sensor. If the cable is the problem, they will either repair the damaged section or run a new cable. All work on the boiler's electrical terminals and control board connections must be carried out by a Gas Safe registered engineer.
- Call a Gas Safe registered engineer if the fault persists or you are unsure Gas Safe engineer
If the above checks have not identified the cause, or if you are not comfortable inspecting the sensor yourself, contact a Gas Safe registered engineer. Ask them to specifically reference Viessmann fault code 18 and mention that the outdoor temperature sensor circuit is suspected so they can bring the right test equipment and a compatible sensor if needed.
Parts you may need
- Viessmann outdoor temperature sensor (NTC) · from £45
- Sensor cable / extension cable (per metre) · from £8
- Weatherproof cable connector / junction box · from £12
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 £120–£280, depending on the underlying cause.
Frequently asked questions
Will my boiler still heat my home with fault code 18 showing?
Yes, in most cases the boiler will continue to operate in a fixed 'constant mode', running as though the outdoor temperature is 0°C. Central heating and hot water will still be produced, but because weather compensation is disabled, the boiler cannot modulate its output to match actual outdoor conditions. This means you may find rooms become too warm on mild days, and energy consumption can increase. It is worth getting the sensor repaired promptly to restore efficient, weather-compensated operation.
What is the difference between Viessmann fault codes 10 and 18?
Both codes relate to the outdoor temperature sensor, but they point to different electrical faults. Code 10 indicates a short circuit in the sensor or its cable — where two conductors are touching and creating an unintended path for current. Code 18 indicates an open circuit or lead break — the circuit is completely interrupted, so no signal reaches the control board at all. An engineer will test the sensor resistance with a multimeter to distinguish between the two and confirm which component needs attention.
Can I just remove the outdoor sensor to get rid of the fault code?
Removing the sensor will not clear the fault — in fact, a disconnected sensor is one of the reasons code 18 appears in the first place. The boiler is configured to expect a sensor signal, and the absence of one is what triggers the code. The correct fix is to repair or replace the sensor and its cable so that weather compensation can function as intended.
How much does it cost to replace a Viessmann outdoor temperature sensor in the UK?
The sensor itself typically costs around £40–£50 for a genuine Viessmann NTC outdoor sensor. Add an engineer's call-out and an hour of labour (typically £55–£80 depending on region and day of the week) and most people pay in the region of £120–£180 for a straightforward sensor swap. If a new cable run is needed — for example where the existing cable is damaged over a long run — labour costs can push the total towards £250–£280. If the fault turns out to involve the boiler's control board rather than the sensor itself, costs can be higher; discuss this possibility with your engineer before authorising any work.