2009 Chrysler Town & Country 4.0L: P0731 and P1790 Fault Codes - Car Won't Move, Stuck in Safe Mode
My 2009 Chrysler Town & Country with a 4.0L engine is displaying both P0731 and P1790 diagnostic trouble codes. The vehicle starts successfully but will not move — no bucking or slipping, and transmission fluid appears to be in good condition. I've attempted a relearn procedure by placing the transmission on the ground with wheels off, then engaging it while on level ground; however, there is still no movement. I suspect an input or output speed sensor issue, as the vehicle remains in safe mode. The P073-1 code indicates an input or output speed sensor error, or potential issues such as low line pressure, failed clutch components (UD or LR), or a stuck main regulator valve. The P1790 code is triggered when a gear ratio fault is detected immediately after a shift and points to a mechanical or hydraulic problem — not electrical — though not all mechanical failures will trigger this code. I'm looking for guidance on diagnosing these issues, especially whether sensor checks (input/output) are necessary. A scan tool would be ideal to confirm the exact status of the transmission system.
As a DIYer, I recommend checking the input and output speed sensors first — ensure they're properly plugged in, with no corrosion or damaged wiring. Were there any prior issues with the speedometer or shifting behavior before this problem occurred? That could point toward an output sensor fault. A scan tool is essential for accurate diagnosis. P0731 Possible Causes: - Input or Output Speed Sensor Errors (DIY check) - Low Line Pressure - Failed UD or LR Clutch - Cut or Torn UD or LR Clutch Piston Seal - Cut UD or LR Accumulator Piston Seal - Stuck Main Regulator Valve - Solenoid Module Failure - Broken Input Shaft - Failed UD Shaft - Failed Rear Sun Gear, Carrier, or Annulus Gear P1790: This code is stored when a gear ratio fault occurs immediately after a shift. It indicates a mechanical or hydraulic issue (not electrical). However, not all mechanical problems trigger this DTC. When present, diagnosis should focus exclusively on mechanical components.