What does it mean?
P1789 is a Powertrain (engine, transmission, fuel system) fault code. It indicates: A/T Overdrive/2/4 Pressure Switch Circuit Malfunction .
Symptoms
- MIL (check engine lamp) is on and second or fourth gear is missing
- The transmission holds a single gear in limp mode, typically third
- Slipping or flaring when the missing gear should engage
- Harsh, abrupt shifts as the module compensates
- High engine speed on the motorway and rising fuel consumption
- The fault tends to appear once the fluid is fully warm
Likely causes
- Pressure switch stuck open or closed inside the pressure switch assembly on the valve body
- Low pressure in the 2-4 clutch circuit, so the switch never sees the applied pressure
- Open or shorted wiring between the switch assembly and the control module
- Corroded case connector, with fluid working past the seal into the terminals
- Worn 2-4 clutch pack or a leaking seal ring bleeding away applied pressure
- Contaminated fluid, a clogged filter or a worn pump reducing available pressure
- Module input circuit fault
How to fix it
- Check fluid level, colour and smell, and inspect the pan and magnet for clutch debris
- Watch all pressure switch states in live data while shifting through every gear on a road test
- Measure voltage and resistance on the switch circuit at the case connector
- With the connector unplugged, test the switch wiring for continuity and shorts to ground
- Fit a pressure gauge and run a line pressure test, checking that the 2-4 circuit builds and holds pressure
- Replace the pressure switch assembly once wiring is verified; if pressure is genuinely low, the 2-4 clutch or its seals need internal repair