Safety first: Turn off power before working on low-voltage wiring.

Step 1 – Locate outdoor sensor: Find the outdoor temperature sensor mounted on an exterior wall, typically on the north or shaded side.
Step 2 – Inspect wiring: Check the cable from the sensor back to the boiler for cuts, staples through the wire, or UV damage. Ensure connections at both ends are tight.
Step 3 – Measure resistance: With power OFF, disconnect the sensor and measure its resistance. Compare to the manufacturer’s temperature/resistance chart. Replace the sensor if readings are open, shorted, or far off expected values.
Step 4 – Verify placement: Ensure the sensor is not in direct sunlight, near dryer vents, or other heat sources that can skew readings.
Step 5 – Control board check: If the sensor and wiring are good, the outdoor sensor input on the control board may be defective and require replacement.

Tip: If outdoor reset is not required, some controls allow disabling the feature; consult the installation manual.

Boiler displays E12 and may shut down intermittently; often related to outdoor reset sensor issues on models equipped with outdoor reset.

Possible Causes