Nissan Altima P0840 Transmission Fluid Pressure Sensor A Sec Pressure Sensor

ommon Symptoms

The owner describes erratic RPM behavior, acceleration problems, and an illuminated check engine light. The Diagnostic Trouble Code (DTC) P0840 indicates an issue with the transmission fluid pressure sensor A (secondary pressure sensor).

Owner’s Perspective

Paul, a Nissan mechanic, suggests several steps for diagnosis:

1. Check Transmission Fluid Pressure Sensor A Voltage

  • Scan data for transmission fluid pressure sensor A voltage should be approximately 1.0 volt at idle in neutral.

2. Check Secondary Pressure

  • Verify secondary pressure at 1.3 MPa at idle in neutral using scan data.

3. Backprobe Transmission Fluid Pressure Sensor A Voltage

  • Check voltage on the Violet/White wire in terminal 15 of the Transmission Control Module (TCM).

4. Check 5.0 Volt Reference

  • Confirm 5.0 volt reference to the transmission fluid pressure sensor A and B on the Blue/Orange wire in terminal 26 of the TCM.

5. Check Ground

  • Examine the ground for transmission fluid pressure sensor A and B on the White/Red wire in terminal 25 of the TCM, aiming for 0.05 volts or less.

6. Transmission Fluid Pressure Sensor B

  • Verify scan data for transmission fluid pressure sensor B voltage (0.7 – 3.5 volts at idle in neutral).

7. Backprobe Transmission Fluid Pressure Sensor B Voltage

  • Check the voltage on the Light Green wire in terminal 14 of the TCM.

8. TCM Reprogramming

  • Consider having the TCM reprogrammed per Nissan TSB #NTB09-044a.

This diagnostic process helps determine if the issue lies with the pressure itself or if a faulty sensor is the culprit.

Service Manual Guidelines

The Nissan Altima 2007-2012 Service Manual provides a systematic approach for addressing DTC P0840:

DTC Logic

  • Describes how the transmission fluid pressure sensor A (secondary pressure sensor) detects secondary pressure of the CVT and sends a signal to the TCM.

DTC Confirmation Procedure

  1. Confirm the DTC by checking data monitor and following a specific procedure with CONSULT-III.

Diagnosis Procedure

  1. Check Input Signal:
    • Verify voltage between TCM harness connector F16 terminal 15 and ground.
  2. Check Power and Sensor Ground:
    • Confirm voltage between TCM harness connector F16 terminal 25 and 26.
  3. Check Harness Between TCM and CVT Unit (Part 1 and Part 2):
    • Ensure continuity between TCM and CVT unit harness connectors for various terminals.
  4. Check Harness Between TCM and CVT Unit (Sensor Power and Sensor Ground) (Part 1 and Part 2):
    • Verify continuity between TCM harness connector F16 terminal 25, 26 and CVT unit harness connector F46 terminal 19, 20.
  5. Check TCM:
    • If issues persist, replace the TCM following specific steps.
  6. Detect Malfunctioning Items:
    • Examine TCM connector pin terminals for damage or loose connections.

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