What does it mean?
P1678 is a Powertrain (engine, transmission, fuel system) fault code. It indicates: A/T (TCM) FPTDR Signal Line Circuit Malfunction .
Symptoms
- MIL on together with a transmission warning lamp
- Harsh, jolting shifts because engine torque is no longer trimmed during the shift
- Engine flare or a bump felt through the driveline on upshifts
- Transmission may enter failsafe and hold a single gear
- Additional transmission or torque management codes stored at the same time
Likely causes
- Open, shorted or high-resistance signal wire between the two modules
- Corroded, bent or backed-out pins at the TCM or engine module connector
- Harness chafed where it passes a bracket, grommet or the bulkhead
- Poor shared ground between the transmission and engine modules
- Failed output driver in one module or a failed input stage in the other
- Water ingress into a floor or kick panel connector
- A module fitted without the required programming or adaptation
How to fix it
- Read the codes and freeze frame data in both the transmission and engine modules and compare them
- Unplug both connectors and inspect for corrosion, moisture, bent pins and terminals that have lost tension
- Test the signal wire end to end for continuity and for shorts to ground and to battery voltage
- Voltage-drop test the power feeds and shared grounds of both modules
- Back-probe the signal line during a road test or while moving the harness to catch an intermittent break
- Repair the damaged harness, clear both memories and road test through a full shift range
- Replace and program a module only when the wiring, grounds and supply are proven good