Bosch Stuck Fan/Pump Relay or Overheat Recovery Mode

Furnace blower or circulation pump runs continuously even when there is no call for heat

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

Stuck relay on control board, High-limit switch recently tripped, Incorrect thermostat wiring (fan call always on), Faulty pump overrun setting

Safety: Turn off power before inspecting wiring.

Thermostat wiring: Verify that the fan or pump call terminals are wired according to the Bosch wiring diagram. Remove the thermostat wires temporarily; if the fan stops, the issue is in the thermostat or wiring.
Overheat recovery: After an overheat, some Bosch units run the fan or pump to cool down. Allow 10–15 minutes to see if it stops.
Control board: If the fan/pump runs with thermostat disconnected and no overheat condition, the relay on the PCB may be stuck, requiring board replacement.

Medium 3/5

Main Control Board

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