Overview
Error code 10 on an ATAG boiler means the boiler has detected a fault with the outside (outdoor) temperature sensor. That sensor provides the boiler control with outdoor temperature information used by weather-compensated control algorithms; if the sensor is open, shorted, damaged or its wiring/connection is faulty the boiler will show this fault. Severity is generally low from a safety perspective — an outside sensor fault typically does not create a gas or combustion hazard — but it can cause poor heating performance, incorrect flow temperatures, reduced efficiency and possible boiler lockouts if the control logic relies on the sensor. Some causes are simple (loose connector, corroded contact, a rodent-chewed cable, frozen or physically damaged sensor), while others need electrical diagnosis (open/short circuit in cable, failed sensor element or a fault on the control PCB). Basic visual checks and a reset can sometimes clear a transient fault, but if the fault persists you should involve a qualified, Gas Safe registered heating engineer to diagnose and replace sensors or repair wiring because the work may affect system wiring, controls, boiler warranty and safe operation.
Possible Cause: Outside sensor error
Troubleshooting Steps
Safety precautions:
- Turn off the boiler and isolate mains power before touching wiring or opening covers. Do not work on gas or high-voltage parts unless you are a qualified Gas Safe engineer.
- Wear suitable PPE (gloves, eye protection) and follow basic electrical safety.
- If the boiler is under warranty, check the terms before attempting any internal work.
Initial homeowner checks (no tools or basic tools):
1. Note any other error codes or spanner/service icons on the display. If multiple codes are present, record them for the engineer.
2. Try a simple reset: switch the boiler off at the mains for 30–60 seconds, then power it back on to see if the code clears (transient faults sometimes reset).
3. Visually inspect the location of the outside sensor (usually mounted outside on an external wall or in a weather shield): look for physical damage, loose or disconnected sensor head, ice build-up, nests or chewed wires.
4. Follow the cable route where accessible and check for visible damage to the cable or connectors at entry points (e.g. where the cable enters the building). Do not force or pull wiring that is fixed.
If you are comfortable with simple electrical checks and have a multimeter (only perform these with the boiler powered off and isolated):
1. Locate the sensor wires at the boiler terminal or PCB. Refer to your boiler user/service manual for sensor connectors and labelling. Before testing, disconnect the sensor connector from the PCB so measurements reflect the sensor and wiring only.
2. Measure continuity between the two sensor wires. An open circuit (infinite resistance) indicates a broken wire or failed sensor. A short (very low ohms close to 0) indicates a short between wires. Intermittent readings when gently moving the cable suggest a damaged/fragile cable.
3. Measure the sensor resistance (NTC sensors change resistance with temperature). You can compare the reading to values in the official ATAG service manual or the sensor datasheet if available; if you don’t have reference values, look for obvious faults: open circuit or very low/near zero ohms is a failure.
4. Inspect and clean connector pins for corrosion; reseat firmly. Corroded or wet connectors outdoors can cause faults — if connectors are dirty, dry and clean them and use a suitable electrical contact protectant.
Basic fixes you can do safely:
- Reseat any loose connectors and ensure outdoor sensor is firmly mounted and protected from direct mechanical damage.
- Replace or re-route damaged external cable where it is accessible and you can make a safe, weatherproof repair (use proper outdoor cable and IP-rated junctions).
- If the sensor head is frozen/iced, thaw it and ensure it is in a weather-protected position.
When to call a professional:
- If the fault persists after reset and the simple visual/wiring checks above.
- If your measurements show an open or short and you cannot locate or repair the damaged section of cable.
- If you need to access the boiler PCB, replace the sensor inside the boiler or work with the boiler internals — this should be done by a Gas Safe registered engineer.
- If there are related faults (pressure, fan, ignition errors) or the boiler is locked out repeatedly.
What a qualified engineer will do:
- Use the ATAG service manual to check expected sensor resistances and PCB wiring diagrams.
- Test continuity and resistance with connectors disconnected, check the PCB input, and replace the outside sensor or repair the wiring as required.
- Verify correct operation after repair, clear faults and test the heating system.
Do not attempt to adjust gas components, internal PCB components, or live mains wiring unless you are a licensed engineer. If in doubt, stop and call a Gas Safe engineer to avoid safety risks and possible warranty invalidation.
Helpful Resources
Boiler Manual
Download the official PDF manual for the Atag ATAG Boiler.