What Does P0840 Mean?
The TCM has detected a general malfunction in the transmission fluid pressure sensor/switch "A" circuit. This sensor is typically the primary line pressure sensor that monitors main hydraulic line pressure within the transmission. Without accurate readings, the TCM cannot properly regulate shift pressure, potentially causing harsh shifts or slipping.
Common Causes
35%
Faulty transmission fluid pressure sensor/switch A
25%
Damaged or corroded wiring in sensor A circuit
20%
Poor electrical connection at the sensor
15%
Low or contaminated transmission fluid
5%
TCM internal circuit fault
Diagnostic Steps
1
Check the transmission fluid level and condition — correct any low-fluid or contamination issues before further diagnosis.
2
Monitor the transmission pressure sensor A PID in live data across different gears and loads; compare readings to factory specifications.
3
Inspect the sensor connector for corrosion, damage, or improper seating (may require dropping the transmission pan).
4
Measure the sensor resistance and reference voltage at the connector and compare to specifications.
5
Check continuity and insulation of all wires in the sensor A circuit from the sensor to the TCM.
6
Replace the sensor if electrical tests confirm it is out of specification.
Estimated Repair Cost
$200 - $800
Parts + labor, varies by vehicle and location
The TCM has detected a general malfunction in the transmission fluid pressure sensor/switch "A" circuit. This sensor is typically the primary line pressure sensor that monitors main hydraulic line pressure within the transmission. Without accurate readings, the TCM cannot properly regulate shift pre...
The most common cause of P0840 (Transmission Fluid Pressure Sensor/Switch "A" Circuit) is: Faulty transmission fluid pressure sensor/switch A
Typical repair costs for P0840 range from $200 to $800, depending on the vehicle, location, and whether you do it yourself or go to a shop.
The transmission may shift harshly, slip, or enter limp mode. The vehicle can be driven at reduced capability for a short distance but should be repaired promptly to prevent internal transmission damage.
Start by connecting an OBD2 scanner to read the code and any freeze frame data. Then follow the diagnostic steps specific to P0840 to identify the root cause.
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Quick Info
Category
Powertrain
System
Automatic Transmission
Difficulty
Type
Generic (SAE)
Recommended Tools
OBD2 Scanner
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