Boiler displays E12 and may not respond to thermostat calls correctly

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

• Step 1 – Locate outdoor sensor: Find the Rheem outdoor temperature sensor on an exterior wall, typically on the north or shaded side.
• Step 2 – Inspect sensor and cable: Check for physical damage, UV deterioration, or rodent-chewed cable. Ensure the sensor housing is intact and sealed.
• Step 3 – Measure resistance: Disconnect the sensor at the boiler control and measure resistance. Compare to Rheem’s outdoor sensor chart. Replace if out of range.
• Step 4 – Check wiring run: If the sensor is good, test continuity of the cable from boiler to sensor. Repair or replace damaged cable.
• Step 5 – Configuration: Verify in the boiler’s installer menu that outdoor reset is enabled only if an outdoor sensor is installed. Disable the feature if no sensor is present.

Warning: Do not substitute non-approved sensors; incorrect resistance curves will cause improper boiler operation.