WOMS
P code powertrain Medium severity

P0840 Transmission Fluid Pressure Sensor/Switch "A" Circuit

The OBD2 code P0840 indicates a problem with the transmission fluid pressure sensor or switch "A" circuit, which may involve issues such as a malfunctioning sensor, wiring problems, or poor connections affecting the...

Definition

The OBD2 code P0840 indicates a problem with the transmission fluid pressure sensor or switch "A" circuit, which may involve issues such as a malfunctioning sensor, wiring problems, or poor connections affecting the transmission's ability to monitor fluid pressure accurately

Common causes

  • Faulty transmission fluid pressure sensor
  • Wiring issues (shorts, opens, or corrosion)
  • Poor electrical connections
  • Low transmission fluid level
  • Contaminated transmission fluid
  • Faulty transmission control module (TCM)
  • Internal transmission problems

Common misdiagnoses

  • Faulty transmission fluid pressure sensor
  • Wiring issues or short circuits in the sensor circuit
  • Transmission control module (TCM) failure
  • Low transmission fluid level or contamination
  • Faulty transmission solenoids
  • Internal transmission problems
  • Engine control module (ECM) issues
  • Incorrectly diagnosed transmission problems

Troubleshooting steps

  1. 1. Check for Additional Codes

    Use an OBD-II scanner to check for any other trouble codes that may be present. This can provide more context and help identify related issues

  2. 2. Inspect the Transmission Fluid

    Fluid Level: Ensure that the transmission fluid is at the correct level. Low fluid can cause pressure issues

  3. 3. Fluid Condition

    Check the condition of the fluid. If it is dirty, burnt, or has a strange odor, it may need to be changed

  4. 4. Visual Inspection

    Wiring and Connectors: Inspect the wiring and connectors related to the transmission fluid pressure sensor. Look for signs of damage, corrosion, or loose connections

  5. 5. Sensor Condition

    Check the physical condition of the transmission fluid pressure sensor. If it appears damaged or worn, it may need to be replaced

  6. 6. Test the Sensor

    Resistance Check: Use a multimeter to check the resistance of the transmission fluid pressure sensor. Compare the readings to the specifications provided in the vehicle’s service manual

  7. 7. Voltage Check

    With the ignition on (engine off), check the voltage at the sensor connector. Ensure it is within the expected range

  8. 8. Check for Short Circuits

    Inspect the wiring harness for any signs of short circuits or grounding issues. A short can cause incorrect readings and trigger the P0840 code

  9. 9. Inspect the Transmission Control Module (TCM)

    Ensure that the TCM is functioning properly. A malfunctioning TCM can misinterpret sensor signals and cause erroneous codes

  10. 10. Perform a Pressure Test

    If applicable, perform a transmission fluid pressure test to verify that the transmission is operating within the expected pressure range

  11. 11. Clear Codes and Test Drive

    After performing repairs or adjustments, clear the trouble codes using the OBD-II scanner and take the vehicle for a test drive to see if the code returns

  12. 12. Professional Help

    If the issue persists after following these steps, consider seeking help from a professional mechanic or transmission specialist, as they may have access to advanced diagnostic tools and expertise. By following these troubleshooting steps, you can systematically identify and address the underlying issue causing the P0840 code