Transmission Fluid Pressure Sensor/Switch A Circuit
What does P0840 mean?
The P0840 code means the vehicle's onboard diagnostic system has detected: Transmission Fluid Pressure Sensor/Switch A Circuit. This is a moderate severity code.
Common Symptoms
- Check Engine Light illuminated
- Harsh or erratic shifting
- Transmission slipping or delayed engagement
- Vehicle may enter limp mode
Probable Causes (Ranked by Likelihood)
- Faulty transmission fluid pressure sensor A Very High
- Wiring issue in pressure sensor A circuit High
- Low transmission fluid level Moderate
- Clogged transmission filter affecting pressure readings Low
Step-by-Step Diagnostic Procedure
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Check transmission fluid level and condition
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Monitor transmission fluid pressure sensor A readings on a scan tool
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Inspect wiring and connector at pressure sensor A
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Compare sensor reading to actual line pressure using a mechanical gauge
Frequently Asked Questions
What does the transmission fluid pressure sensor do?
The transmission fluid pressure sensor monitors the hydraulic pressure inside the transmission. The TCM uses this data to control shift timing, quality, and torque converter clutch engagement.
Can low fluid cause P0840?
Yes, low transmission fluid reduces hydraulic pressure, which can cause abnormal readings from the pressure sensor and trigger P0840.
Is the pressure sensor inside the transmission?
On most vehicles, the pressure sensor is mounted on the outside of the transmission or on the valve body and is accessible without full disassembly. Some designs require pan removal.