378F

What Code 378F Means

Code 378F indicates a loss of communication over the BSD (Bit Serial Data) interface with the electric coolant pump. The engine control unit (DME) is not receiving diagnostic data from the pump, which points either to faulty wiring and connectors or to a failed pump.

The cooling system on modern BMW engines with an electric coolant pump uses the BSD bus to exchange information between the DME and the pump. Control commands and diagnostic parameters (speed, temperature, fault codes) travel over this line. If the BSD line is open or shorted, or the pump has an internal fault, communication drops and the DME sets code 378F.

Components involved: electric coolant pump, wiring harness and connectors, engine control unit (DME).

Symptoms

• Engine overheating.
• Coolant temperature warning light comes on.
• Possible emergency engine shutdown.

Diagnosis and Repair

1. Visual inspection of the wiring and connectors at the coolant pump: check insulation integrity and look for corrosion or damaged contacts.
2. Check the BSD line (multimeter or oscilloscope): confirm there is no open or short circuit.
3. Pump function test: apply power and the control signal (with ISTA/INPA diagnostic equipment) to verify rotation and current draw.
4. Replace the pump once the fault is confirmed (wiring is good but the pump does not respond to commands).

Secondary Damage

Ignoring code 378F and continuing to drive with a dead electric coolant pump leads to engine overheating. Overheating can warp the cylinder head, damage the head gasket, cause cylinder bore scoring, and in severe cases destroy the engine entirely. The repair in that case will be far more expensive and involved than replacing the pump or fixing the wiring on time.