Error A12, boiler indicates outside sensor fault, weather-compensated control not working

Safety: Turn off power before working on control wiring.

• 1. Locate outdoor sensor: The Brötje outdoor sensor is usually mounted on a north-facing exterior wall, away from direct sun.
• 2. Visual check: Inspect the sensor housing for damage or water ingress. Ensure the cable gland is tight.
• 3. Cable inspection: Follow the cable back to the boiler, checking for cuts, joints, or rodent damage.
• 4. Resistance measurement: Disconnect the sensor at the boiler and measure resistance. Compare to the Brötje outdoor sensor curve at the current ambient temperature. Replace the sensor if readings are wrong.
• 5. Terminal check: Clean and tighten the low-voltage terminals on the control board.