What does it mean?
P1823 is a Powertrain (engine, transmission, fuel system) fault code. It indicates: Internal Mode Switch 'P' Circuit Low Input (4T65-E) .
Symptoms
- Check engine lamp on with the gear indicator stuck on P or showing dashes
- Engine cranks in ranges other than Park and Neutral, or will not crank at all
- The shift interlock stays engaged so the selector will not leave Park
- Wrong-gear starts and harsh engagement
- Limp mode with the transmission holding a single gear
Likely causes
- Park circuit shorted to ground through a chafed internal harness inside the transmission
- Worn Park contact in the mode switch staying grounded across several positions
- Fluid or debris bridging pins in the internal switch connector
- Manual shaft or shift cable adjustment off, so the switch sits partly in Park in another range
- Corrosion in the external case connector on the Park pin
- Mode switch assembly worn out after high mileage
- Module input fault after a persistent ground short
How to fix it
- Read the codes and watch the Park state parameter while shifting slowly through every range
- Verify shift cable and manual shaft adjustment before any electrical work
- Inspect the external case connector for fluid, corrosion and spread terminals
- Disconnect the mode switch and check the Park circuit for a short to ground back to the module
- If the external side is clean, remove the pan and check the internal harness and connector
- Realign the mode switch to specification, and replace it if the Park track is worn
- Clear codes and confirm both the display and the start interlock behave correctly in every range