Viessmann 10 Fault Code: Causes, Fixes & Repair Costs
What does the Viessmann 10 fault code mean?
Fault code 10 on Viessmann boilers signals that the boiler's control system has detected a short circuit in the outdoor temperature sensor circuit. Because the boiler can no longer read a reliable outdoor temperature, it switches into a fixed 'constant operation' mode — essentially running blind without weather compensation data. A closely related code, 18, triggers the same constant-operation fallback but for the opposite electrical reason: an open circuit or broken cable in the outdoor sensor rather than a short circuit. If you see code 18, the diagnosis and fix process is almost identical to code 10, so the information on this page applies to both. The practical effect for the homeowner is often underwhelming heating performance: the boiler may underestimate how cold it is outside and reduce its output accordingly, leaving radiators lukewarm even though the boiler appears to be running 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
- Corroded or damaged outdoor temperature sensor Common
The outdoor sensor is permanently exposed to the elements — rain, frost, UV light, and temperature swings all take their toll over time. Internal corrosion or moisture ingress can cause the sensor to short circuit, which is the direct trigger for code 10. Sensors fitted more than five to ten years ago are especially prone to this.
- Damaged or pinched sensor wiring Common
The cable running from the outdoor sensor to the boiler's control unit often passes through walls or under cladding. It can be accidentally cut or crushed during building work, gardening, or general maintenance activity. A pinched or chafed cable can create the short circuit the boiler detects.
- Loose or corroded connector at the sensor or controller Sometimes
The plug-in connectors at each end of the sensor cable can develop corrosion or work loose over time, particularly where they are exposed to damp. A poor connection can mimic a short circuit and trigger code 10.
- Faulty boiler control unit Rare
In rare cases where the sensor and its wiring check out fine, the fault may lie with the boiler's control unit itself misreading the sensor signal. This is uncommon but worth the engineer investigating if all other causes have been ruled out.
How to fix it
- Reset the boiler once DIY safe
A single reset can sometimes clear a transient fault. On most Viessmann models, turn the heating dial briefly to the reset symbol and back, or press 'Mode' and 'OK' simultaneously for a few seconds — check your manual for the exact method. If the code returns immediately or after a short time, move on to the next steps rather than repeatedly resetting.
- Check that your heating system is still performing DIY safe
Feel your radiators across the house over the next hour or so. If they are noticeably cooler than usual even though the boiler appears to be running, this is consistent with the weather compensation system operating in its fallback mode. Note this down to tell the engineer, as it helps confirm the fault.
- Visually inspect the outdoor sensor and its cable — without touching internals DIY safe
Locate the outdoor temperature sensor, which is usually mounted on a north-facing external wall. Look for obvious physical damage to the sensor housing or to the cable run — cuts, crush marks, loose connectors, or signs of heavy corrosion. Do not attempt to disconnect or rewire anything; just note what you observe for the engineer.
- Arrange for a Gas Safe registered engineer to test the sensor and wiring Gas Safe engineer
A qualified engineer will use a multimeter to measure the resistance across the sensor and check the continuity of the cable. A short circuit will show as near-zero resistance where there should be a specific reading. This confirms whether the sensor itself, the cable, or a connector is at fault.
- Engineer to replace the outdoor temperature sensor and/or repair the wiring Gas Safe engineer
If the sensor has failed internally, the engineer will fit a compatible replacement. If the cable is damaged, they will repair or re-route it. Once the new component is in place, the engineer will reset the boiler, verify the fault code has cleared, and confirm that weather compensation is working correctly again.
Parts you may need
- Viessmann outdoor temperature sensor (NTC type) · from £35
- Sensor cable/extension lead · from £18
- Weatherproof connector kit · from £8
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
Is fault code 10 dangerous — will my boiler shut down?
Code 10 is a warning rather than a lockout, so your boiler should continue to run in a constant operation mode. It is not an immediate safety risk in the way a gas leak or carbon monoxide fault would be, but you will likely notice reduced or inconsistent heating performance. You should still get it looked at promptly, as running indefinitely without weather compensation wastes energy and puts unnecessary strain on the system.
What is the difference between Viessmann fault codes 10 and 18?
Both codes indicate that the boiler has lost confidence in its outdoor temperature sensor reading and has switched to constant operation mode. Code 10 points specifically to a short circuit in the sensor or its wiring — two conductors making unwanted contact. Code 18 points to an open circuit or break — the signal path being interrupted altogether. The diagnostic and repair process is essentially the same for both: a Gas Safe engineer tests the sensor resistance and cable continuity, then replaces whichever component is at fault.
How much does it cost to replace a Viessmann outdoor temperature sensor in the UK?
For most homeowners, a straightforward outdoor sensor replacement — parts and labour combined — typically falls in the range of £100 to £200, depending on your location and the engineer's call-out rate. If the wiring run also needs repairing or re-routing, add a little more for the extra time. The sensor itself is a relatively inexpensive component, making this one of the more affordable boiler repairs. Always ask for a written quote before work begins.
Do I need a Viessmann-accredited engineer, or will any Gas Safe engineer do?
Any Gas Safe registered engineer is legally qualified to carry out this repair. However, Viessmann recommends using an accredited installer to protect your warranty — particularly if your boiler is still within its 10- or 12-year warranty period. You can find Viessmann-accredited engineers through Viessmann's own installer locator, or ask an engineer to confirm their accreditation before booking.