What does it mean?
P1840 is a Powertrain (engine, transmission, fuel system) fault code. It indicates: Transmission Fluid Pressure Sensor Switch A Circuit .
Symptoms
- Check engine lamp on with a transmission warning
- Harsh, erratic or delayed shifts that vary from one drive to the next
- Slipping under load and rpm flare between gears
- Torque converter clutch behaving unpredictably
- Limp mode holding a single gear and noticeably reduced drivability
Likely causes
- Pressure switch contaminated or stuck by varnish and debris in the fluid
- Low or degraded fluid so the circuit never reaches the pressure it should
- Wire to the switch shorted to ground or open, most often at the case connector
- Fluid intrusion into the case connector bridging or corroding pins
- Valve body bore worn or a valve sticking, so pressure never reaches the switch
- Switch assembly failed internally
- Module input circuit damaged after long-term wiring trouble
How to fix it
- Read the codes and freeze frame, then inspect fluid level, colour and smell
- Compare the switch or sensor state on the scan tool with the gear actually engaged during a road test
- Inspect the case connector and external harness for fluid, corrosion and chafe
- Test the switch circuit for continuity, shorts to ground and shorts to voltage with the connector unplugged
- Fit a pressure gauge and compare measured line pressure with the value the module reports
- Drop the pan, inspect for debris and check the internal harness and the switch or sensor assembly
- Replace the pressure switch assembly, and address valve body wear where the gauge shows the pressure itself is wrong