What does it mean?
P1676 is a Powertrain (engine, transmission, fuel system) fault code. It indicates: A/T (TCM) FI Data Line Circuit Malfunction .
Symptoms
- MIL on together with a transmission warning lamp
- Shifts become harsh, delayed or completely missing
- Transmission enters failsafe and stays in one gear until the ignition is cycled
- No torque converter lock-up, so engine speed is high at cruise and fuel consumption rises
- Scan tool communication with the TCM is unstable or unavailable
Likely causes
- Broken or high-resistance data wire between the two modules
- Fretting corrosion or spread terminals in the module connectors, common after repeated disconnection
- Wire shorted to ground or to battery positive somewhere along the harness
- Poor or loose ground point shared by the transmission and engine modules
- Weak battery and poor charging voltage, which disturbs serial communication
- A module replaced without programming, so it does not answer correctly on the data line
- Corrosion at a bulkhead or floor connector after water ingress
How to fix it
- Confirm battery condition and charging voltage before chasing wiring, since low voltage causes the same symptoms
- Inspect both module connectors closely for fretting corrosion, moisture and terminals that no longer grip
- Measure the data line resistance from module to module and test it for shorts to ground and to battery positive
- Voltage-drop test the shared grounds of both modules under load
- Check whether either module was recently replaced and verify it was programmed for this vehicle
- Repair the damaged harness section and protect it from the source of the damage, then clear the codes
- Road test through a full shift range and re-scan both modules to confirm the repair