What does it mean?
P1781 is a Powertrain (engine, transmission, fuel system) fault code. It indicates: A/T Overdrive Pressure Switch Circuit Malfunction .
Symptoms
- MIL (check engine lamp) is on and the transmission refuses to shift into top gear
- Engine speed stays high on the motorway and fuel consumption increases
- Harsh or delayed engagement of the highest gear
- The transmission hunts between two gears on gentle inclines
- Transmission may lock into limp mode
- The fault often appears when the fluid is hot and disappears when cold
Likely causes
- Overdrive pressure switch stuck open or stuck closed inside the pressure switch assembly
- Low line pressure in the overdrive circuit, so the switch never sees enough pressure to close
- Open or shorted wiring between the pressure switch assembly and the control module
- Corroded transmission case connector, with fluid working into the terminals
- Worn overdrive clutch pack or leaking seal rings bleeding off the applied pressure
- Contaminated fluid or a clogged filter reducing pump output
- Control module input circuit fault
How to fix it
- Verify fluid level and condition before any electrical testing
- Watch the pressure switch states in live data while shifting through every range and gear
- Measure resistance or voltage at the switch circuit with the ignition on and the engine off
- Check continuity of the switch wiring with the case connector unplugged, and test for shorts to ground
- Fit a gauge and run a line pressure test to prove the overdrive circuit builds the correct pressure
- Replace the pressure switch assembly once wiring is verified; if pressure is low with a good switch, the overdrive clutch or seals need internal repair