What does it mean?
P1677 is a Powertrain (engine, transmission, fuel system) fault code. It indicates: A/T (TCM) FI Data Line Circuit Malfunction .
Symptoms
- MIL and transmission warning lamp on
- Shift quality deteriorates: flaring, shift shock or gears held far too long
- Intermittent failsafe operation that clears after switching the ignition off and on
- Lock-up clutch engagement not smooth, sometimes a shudder at steady speed
- Reduced fuel economy and, in some cases, no engagement of the higher gears
Likely causes
- Open or intermittent data wire between the transmission and engine control modules
- Damaged or oxidised terminals at either module connector
- Insulation rubbed through where the harness crosses a bracket or passes a sharp edge
- Shared ground point loose, painted over or corroded
- Voltage drop during cranking from a weak battery or bad terminal clamps
- Splice or repair joint in the data line that has corroded internally
- Internal communication fault in one of the two modules
How to fix it
- Start with battery, terminal and charging checks; a weak supply produces intermittent data faults
- Read both modules and note the freeze frame conditions when the fault occurred: speed, load and temperature give useful clues
- Inspect the whole route of the data wire, focusing on brackets, sharp edges and any previous repair splices
- Measure continuity end to end and test for shorts to ground and to battery voltage
- Clean and retighten the shared ground points and confirm with voltage-drop tests
- Move the harness while monitoring live data or module communication to reproduce the dropout
- Repair the wiring, clear both memories and road test; replace and program a module only if the fault persists