What does it mean?
P1685 is a Powertrain (engine, transmission, fuel system) fault code. It indicates: Transmission Control Module (TCM) Power Up Temperature Sensor Circuit Low Voltage .
Symptoms
- MIL on right after key-on, with the code present before the vehicle is even driven
- Fluid temperature in live data pinned at its maximum value
- Torque converter lock-up refused or applied at the wrong time
- Harsh shifting, especially while the transmission is still cold
- Limp mode with a fixed gear
- Cooling fan running more than usual on designs that use this signal
Likely causes
- Signal wire shorted to ground between the sensor and the module
- Sensor failed internally with a short across its element
- Transmission fluid wicking into the case connector and bridging the signal and ground pins
- Chafed harness grounding against the transmission case or a bracket
- Corroded terminals lowering the circuit resistance inside the connector
- Internal transmission harness damaged by heat or contact with a rotating part
- Module input circuit failure
How to fix it
- Read the fluid temperature at key-on after the vehicle has sat overnight; an impossibly hot value confirms a low signal voltage
- Unplug the sensor and watch the reading swing to the opposite extreme, which clears the wiring between connector and module
- Measure the sensor resistance and compare it against the temperature/resistance table
- Test the signal wire for a short to ground with the module disconnected
- Inspect the case connector for fluid intrusion and corrosion bridging adjacent pins
- Repair the chafed harness section and protect it away from sharp edges before refitting
- Replace the sensor or the internal harness assembly if the short is inside the transmission