What does it mean?
P1302 is a Powertrain (engine, transmission, fuel system) fault code. It indicates: Transmission Self-Learning Signal Not Received .
Symptoms
- MIL (check engine lamp) on, usually together with a transmission warning lamp
- Harsh, delayed or flaring gear changes
- Transmission locked in one gear (limp mode)
- Clutch or shift adaptation values missing or showing as zero on the scan tool
- On some vehicles the selector will not release from park, or gear display goes blank
Likely causes
- Fault on the data bus between the TCM (transmission control module) and the engine control module: open, short or a corrupted line
- TCM without power or ground because of a blown fuse or a corroded ground point
- Adaptation never carried out after a transmission, TCM or engine control module replacement
- Software or calibration mismatch between the two modules after a repair or update
- Corroded, wet or backed-out pins in the TCM or engine control module connectors
- Failed TCM that no longer transmits on the bus
How to fix it
- Scan every module and record all communication faults before clearing anything
- Check TCM power and ground fuses, then measure supply and ground at the module connector
- Test the data bus lines for correct resistance and for shorts to power or ground
- Confirm both modules carry compatible software versions and the correct calibration for the vehicle
- Inspect the module connectors for corrosion, moisture and pin tension
- Repair the wiring or communication fault first, then run the manufacturer's transmission adaptation procedure
- Replace the TCM only if it stays silent with confirmed power, ground and an intact bus