P0840 moderate

Transmission Fluid Pressure Sensor/Switch A Circuit

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.

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Keep driving?: Short distances only
DIY difficulty: hard
Estimated cost: $150 - $600 (parts and labor)

Check Your Fluid First

ANCEL AD310 OBD-II Scanner

Read and clear transmission codes. Many trans codes are caused by low or burnt fluid -- check level and color before assuming the worst.

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Read Transmission Codes

LAUNCH CRP123X (Reads All Systems)

Basic scanners only read engine codes. This reads transmission, ABS, and airbag too.

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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

Check transmission fluid level and condition
Monitor transmission fluid pressure sensor A readings on a scan tool
Inspect wiring and connector at pressure sensor A
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.