What does it mean?
P1684 is a Powertrain (engine, transmission, fuel system) fault code. It indicates: Transmission Control Module (TCM) Power Up Temperature Sensor Performance .
Symptoms
- MIL on, often with a transmission warning
- Harsh or delayed shifts in the first minutes after a cold start
- Torque converter lock-up applying too early or not at all
- Transmission temperature in live data clearly wrong for the conditions
- Limp mode with the transmission held in one gear
- Slightly higher fuel consumption from an incorrect shift schedule
Likely causes
- Transmission fluid temperature sensor has drifted, reading a plausible but incorrect value
- Added resistance in the sensor circuit from corroded terminals or a partly broken wire, shifting the measured value
- Low or contaminated transmission fluid so the sensor is not properly immersed
- Damaged internal transmission harness or lead frame carrying the sensor circuit
- Coolant temperature or intake air temperature sensor out of range, so the plausibility comparison fails even though the transmission sensor is good
- Poor module ground creating a small offset on all analogue inputs
- Module reference voltage fault
How to fix it
- After an overnight soak, read transmission fluid temperature, coolant temperature and intake air temperature in live data; all three should agree within a few degrees
- Check the fluid level and condition, since low or burnt fluid alone produces implausible readings
- Measure the sensor resistance at the case connector and compare it with the temperature/resistance table
- Inspect the transmission connector and internal harness for fluid wicking, corrosion and bent terminals
- Voltage drop test the module ground and check the sensor reference voltage
- Verify the other temperature sensors used in the comparison before condemning the transmission sensor
- Replace the temperature sensor or the internal harness assembly if the resistance is out of specification