Your boiler is currently running a background check on its internal temperature readings to ensure they are accurate before it starts up.

Return temperature sensor self-test. The self-test for the return temperature sensor is activated.

What causes this fault?

This code appears when the boiler's internal computer notices a potential discrepancy between its temperature sensors and triggers a verification check to ensure the readings are safe. It usually happens because the return sensor is beginning to fail or there is a minor electrical communication issue within the internal wiring. If the test fails repeatedly, it prevents the boiler from firing up to protect the system from overheating.