What does it mean?
P1820 is a Powertrain (engine, transmission, fuel system) fault code. It indicates: Internal Mode Switch 'A' Circuit Low Input (4T80-E) .
Symptoms
- Check engine lamp on and the gear indicator shows the wrong range or nothing at all
- Transmission starts off in the wrong gear and engages with a bump
- No crank in Park, or cranking possible in a range where it should be blocked
- Reverse unavailable or engaging late
- Harsh 1-2 shift and limp mode with markedly reduced drivability
Likely causes
- Mode switch wire shorted to ground inside the unit, often where the internal harness rubs the valve body or case
- Worn or pitted contacts on the mode switch track, so the A position never opens correctly
- Internal harness connector loose or leaking, letting fluid track between pins
- Mode switch misaligned after manual shaft, valve body or switch service
- Corroded terminals in the external case connector
- Failed mode switch assembly
- Module input circuit fault after a persistent short to ground
How to fix it
- Read the codes and watch the mode switch state parameters as the selector is moved slowly through every position
- Check the external case connector for fluid intrusion, corrosion and terminal tension
- Test circuit A between the module and the case connector for a short to ground with the switch disconnected
- If the circuit is clean externally, the fault is internal: drop the pan and inspect the internal harness and the switch connector
- Verify mode switch alignment to the manual shaft before condemning it
- Replace the internal mode switch and, where the internal harness shows fluid tracking or chafe, renew that as well
- Clear the codes, cycle the selector through all ranges and confirm the switch states now match