Code Definition

P1909 — Trans Temp Sensor Circuit Open or Shorted to Pwr or Gnd

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

About P1909

P1909 is a manufacturer-specific powertrain diagnostic trouble code. It belongs to the P-prefix family of codes, which covers engine, transmission, emissions, or fuel-system faults.

When P1909 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 sensor-related fault — the reported value from the named sensor is out of spec.

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.

P1909 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

The P1909 fault code indicates that the Transmission Control Module (TCM) has detected an open circuit or a short to power/ground in the Transmission Temperature Sensor circuit. This sensor monitors the temperature of the transmission fluid and provides critical data for shift strategy and thermal protection. If the circuit is open or shorted, the TCM may receive incorrect or no temperature readings, potentially affecting transmission operation.

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