Code Definition

P1904 — Engine coolant hydraulic blower motor solenoid open or shorted to ground

System: Powertrain  |  Type: Manufacturer-Specific  |  Make: Audi

About P1904

P1904 is a manufacturer-specific powertrain diagnostic trouble code recorded for Audi vehicles. It belongs to the P-prefix family of codes, which covers engine, transmission, emissions, or fuel-system faults.

When P1904 is stored, the module has detected an open (broken or disconnected) electrical path in the monitored circuit; a short-circuit (voltage pulled to ground or to supply) in the monitored circuit; and a signal or supply voltage lower than the expected operating range.

Scanning & clearing: A generic OBD-II scan tool will usually read and clear powertrain (P) codes. For live data, freeze-frame capture, and bi-directional tests you may need a more capable scan tool that supports the vehicle's ECM protocol.

P1904 is a manufacturer-specific code. The same code string on a different make may have a different meaning. Always confirm the code's definition against the vehicle's service information for the specific make and model under test.

Code Information

DTC P1904 sets when the engine control module (ECM) detects an open circuit or a short to ground in the engine coolant hydraulic blower motor solenoid circuit. On the Audi models that use it, the radiator fan is driven hydraulically rather than by an electric motor, and this solenoid valve meters the pump flow that spins the fan, setting its speed. When the control circuit is open or shorted to ground, the ECM cannot energise the solenoid correctly, so the fan may fail to run or fail to reach the commanded speed. This can allow coolant temperature to rise, particularly at idle, in slow traffic...

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