What does it mean?
P1785 is a Powertrain (engine, transmission, fuel system) fault code. It indicates: TCM A/T System Malfunction . This is a standardised OBD-II code. The technical definition is the same regardless of the make or model of vehicle, although specific causes and symptoms can vary between vehicles.
Symptoms
- MIL (check engine lamp) is on and the transmission runs in limp mode with one fixed gear
- No automatic shifting at all, or shifts only work in some ranges
- Several transmission codes stored together rather than a single circuit code
- Gear position display on the dashboard is wrong, flashing or blank
- Harsh engagement into drive and reverse
- On some vehicles the engine will not crank in park
Likely causes
- Poor power supply or ground to the module: corroded ground stud, loose battery terminal, weak battery or a failing alternator
- Water intrusion into the module or its connector, common where the module sits low in the vehicle or inside the transmission
- One or more solenoid, sensor or switch circuits faulty at the same time, which the module reports as a system fault
- Corroded, bent or backed-out pins in the module connector
- Outdated or incorrect module calibration after a repair or a replacement part was fitted
- Loss of communication with the engine control module
- Internal module failure, including a failed driver stage or memory fault
How to fix it
- Scan every module, record all codes and freeze frames, and repair supporting circuit faults first
- Load-test the battery and measure charging voltage at idle and at raised engine speed
- Perform voltage drop tests on the module's power feeds and grounds while the transmission is being operated
- Unplug and inspect the module connector for water, corroded pins, bent terminals and fluid intrusion
- Check whether an updated calibration is available and reprogram if one exists
- Replace and program the module only after wiring, supply, grounds and all input circuits have been proven good