OBD-II Generic Error Code P0840: How to Fix It

Medium 30 minutes to 1 hour Medium Severity Verified June 2026
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Error Code
P0840
Brand
OBD-II Generic
Product Type
automotive
Severity
Medium
DIY Difficulty
Medium
Estimated Fix Time
30 minutes to 1 hour
Error P0840 indicates a problem with the transmission fluid pressure sensor/switch 'A' circuit. This sensor monitors hydraulic pressure inside the transmission; when the signal is out of range, the check engine light comes on. Common issues include a faulty sensor, wiring damage, or low transmission fluid.
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Tools You'll Need

How to Fix Error Code P0840

  1. Check Transmission Fluid Level and Condition

    Transmission fluid can be hot; avoid burns. Use a rag when handling.

  2. Inspect Wiring and Connectors

    Disconnect battery negative terminal before working on electrical components.

  3. Test the Sensor with a Multimeter

    Use proper backprobing tools to avoid damaging connectors.

  4. Replace Transmission Fluid Pressure Sensor

    Use a lint-free rag to catch any fluid drips.

  5. Clear Error Codes and Test Drive

Parts You May Need

Transmission fluid pressure sensor
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Transmission fluid (ATF)
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Transmission fluid pressure sensor
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Transmission fluid (ATF)
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Transmission fluid pressure sensor
Check Price on Amazon
Transmission fluid (ATF)
Check Price on Amazon
Transmission fluid pressure sensor
Check Price on Amazon
Transmission fluid (ATF)
Check Price on Amazon
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When to Call a Professional

If the code returns after replacing the sensor and checking wiring, there may be an internal transmission problem (e.g., clogged circuit, failed solenoid) requiring professional diagnosis.

Frequently Asked Questions

What does P0840 mean?
It means the transmission control module detected a malfunction in the 'A' circuit of the transmission fluid pressure sensor. The sensor may be faulty, the wiring damaged, or fluid level incorrect.
Can I drive with P0840?
Short trips may be possible, but it can lead to harsh shifting or transmission damage. It's best to diagnose promptly.
How much does it cost to fix P0840?
DIY sensor replacement costs $30-$100 for the part. Professional repair ranges from $150-$400 including labor.
Will P0840 clear itself?
Only if the underlying issue is resolved (e.g., fluid topped off). Otherwise, the code will persist until repaired.

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