Transmission Fluid Pressure Sensor/Switch "A" Circuit Low
P0842 indicates the transmission fluid pressure sensor/switch A circuit is detecting a voltage lower than expected, meaning the PCM cannot properly read transmission fluid pressure. This electrical fault prevents the transmission control module from accurately monitoring hydraulic pressure, which can affect shift timing and transmission performance.
What You Might Notice
- Transmission shifting erratically or at wrong RPMs
- Check Engine Light illuminated
- Transmission may default to limp mode or safe mode
- Delayed or harsh gear engagement
- Possible loss of transmission responsiveness
Most Common Causes
- 1
Transmission Fluid Pressure Sensor failure
The TFPS sensor itself has failed or developed an internal short to ground, preventing accurate pressure signal transmission to the PCM. This is the most common cause of P0842.
- 2
Wiring short to ground in sensor circuit
Damaged, pinched, or corroded wiring between the TFPS sensor and PCM has created an unwanted ground connection, causing the signal voltage to drop below normal operating range.
- 3
Loose or corroded sensor connector
Poor electrical connection at the transmission fluid pressure sensor connector or PCM connection can reduce signal voltage and trigger the low pressure code.
- 4
PCM/TCM programming issue or failure
Rarely, a software glitch in the powertrain control module or a completely failed PCM can cause false low pressure readings; this is less common than sensor failure.
How to Diagnose It
- 1
Check Transmission Fluid Pressure Sensor failure — The TFPS sensor itself has failed or developed an internal short to ground, preventing accurate pressure signal transmission to the PCM. This is the most common cause of P0842.
- 2
Check Wiring short to ground in sensor circuit — Damaged, pinched, or corroded wiring between the TFPS sensor and PCM has created an unwanted ground connection, causing the signal voltage to drop below normal operating range.
- 3
Check Loose or corroded sensor connector — Poor electrical connection at the transmission fluid pressure sensor connector or PCM connection can reduce signal voltage and trigger the low pressure code.
- 4
Check PCM/TCM programming issue or failure — Rarely, a software glitch in the powertrain control module or a completely failed PCM can cause false low pressure readings; this is less common than sensor failure.
How to Fix It
- 1
Replace transmission fluid pressure sensor
Locate the TFPS sensor on the transmission, disconnect the electrical connector, unscrew the sensor, and install a new OEM or quality replacement. Refill transmission fluid if needed and clear the code to verify the fix.
- 2
Inspect and repair wiring harness
Visually inspect the entire wiring path from the TFPS sensor to the PCM for cuts, corrosion, or damage. Repair any damaged insulation, replace corroded sections, or reposition pinched wires away from heat sources and moving parts.
- 3
Clean or replace sensor connector
Disconnect the TFPS sensor connector and inspect both the male and female sides for corrosion or bent pins. Clean with electrical contact cleaner and dielectric grease, or replace the connector if severely damaged. Reconnect firmly and test.
- 4
Reprogram or replace PCM/TCM
If sensor and wiring are confirmed good, the PCM/TCM may require reprogramming with updated software or complete replacement. This requires specialized diagnostic equipment and is typically a last resort after confirming all sensor circuits are functional.
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