What does it mean?
P1688 is a Powertrain (engine, transmission, fuel system) fault code. It indicates: Unmanaged Engine Torque Delivered to Transmission Control Module (TCM) Signal .
Symptoms
- Harsh, banging upshifts under acceleration
- MIL on together with a transmission warning
- Transmission falling into limp mode and holding one gear
- Driveline shock felt through the whole vehicle on gear changes
- Reduced engine power in some operating ranges
- Repeated shift flare followed by a hard engagement
Likely causes
- The engine module did not carry out the torque reduction request during the shift
- Communication fault on the link between the engine and transmission modules from corroded pins, a broken wire or high resistance
- Throttle actuator or throttle position sensor fault preventing accurate torque control
- Calibration mismatch between the two modules after a replacement or reflash
- Faulty input to the engine torque model such as the mass air flow, manifold pressure or crankshaft position sensor
- Internal engine module fault in the torque management routine
- Aftermarket engine tuning that alters or removes the torque reduction strategy
How to fix it
- Read stored codes from both the engine and transmission modules; engine-side faults are usually the root cause and must be fixed first
- Watch live data during a shift and compare the requested torque reduction with the actual engine torque value
- Inspect the communication harness and connectors between the two modules for corrosion and damaged pins
- Verify both modules are running current, matching calibration software
- Test the throttle position signal and the throttle actuator response with the scan tool
- Repair any engine-side sensor fault, clear the codes and confirm with a road test under load
- Consider an internal engine module fault only after wiring, calibrations and engine sensors have all checked out