Your boiler's internal control module is failing to communicate with the fan, causing the system to shut down immediately after trying to start.

BIM is defective or not properly connected, when started, the fan will rotate for a moment and then stops.

What causes this fault?

This fault occurs when the boiler's internal computer loses its connection with the fan, which is responsible for pushing exhaust gases safely out of the flue. Because the system cannot verify that the fan is spinning at the correct speed, it shuts down immediately to prevent an unsafe build-up of heat or gases. The most common cause is a failure in the electronic interface chip or a loose wire that has vibrated out of place over time.