Boiler displays A21, frequent short cycling, poor temperature control

This repair may involve working with high voltage components or water connections. Always unplug the appliance before removing any panels.

If you are not confident in your ability to perform this repair safely, we strongly recommend contacting a professional technician.

Possible Causes

Faulty outdoor temperature sensor, broken sensor cable, incorrect sensor placement

Safety: Isolate power before altering sensor wiring.

Check sensor location: The outdoor sensor should be on a north or northwest wall, away from direct sun or heat sources. Relocate if incorrectly installed.
Inspect cable: Look for damage along the sensor cable route. Repair or replace if cut or crushed.
Technician tasks: Measure sensor resistance versus outdoor temperature and replace if out of range. Reconfigure weather compensation settings on the controller.

Medium 3/5

Outdoor Temperature Sensor

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