What does it mean?
P1686 is a Powertrain (engine, transmission, fuel system) fault code. It indicates: Transmission Control Module (TCM) Power Up Temperature Sensor Circuit High Voltage .
Symptoms
- MIL on shortly after key-on
- Fluid temperature in live data stuck at the coldest possible value no matter how long the vehicle runs
- Shifts stay in the cold pattern: firm, late and without torque converter lock-up
- Higher engine speed at cruise and noticeably worse fuel consumption
- Limp mode with a fixed gear on some vehicles
- The transmission can overheat unnoticed because the module cannot see the real temperature
Likely causes
- Open circuit in the sensor signal wire or its ground return
- Sensor unplugged or a terminal pushed back out of the connector
- Sensor failed with an open internal element
- Corrosion adding high resistance inside the transmission case connector
- Broken internal transmission harness
- Loss of the sensor ground reference at the module
- Module input circuit failure
How to fix it
- Watch the fluid temperature after a long drive; a value locked at the low limit points straight at an open circuit
- Briefly bridge the signal and ground pins at the sensor connector; if the reading swings hot, the wiring to the module is intact
- Measure the sensor resistance and compare it with the specification
- Check continuity of both the signal wire and the ground return all the way to the module
- Inspect the case connector for backed-out pins, corrosion and spread terminals
- Repair the broken wire with a properly sealed joint
- Replace the sensor or internal harness assembly if the element is electrically open