P0840
Transmission Fluid Pressure Sensor/Switch A Circuit Range/Performance
The transmission fluid pressure sensor A reading is implausible compared to commanded line pressure. The TCM detects sensor drift or hydraulic leakage.
- Check engine light: Yes
- Emissions related: No
- Repair difficulty: hard
Symptoms
- Check engine or transmission warning light on
- Harsh, delayed, or missing shifts
- Possible limp mode (locked in one gear)
- Slipping or shuddering during gear changes
Common causes
Ordered by how often each turns out to be the cause.
- Pressure sensor driftCommon
- Internal hydraulic leakPossible
- Sensor element aged beyond calibration windowLess likely
Parts involved
- Transmission Fluid Pressure Sensor
- Valve Body
- Transmission Pump
P0840 is a generic OBD-II code defined by SAE J2012. Exact wording and diagnostic procedure vary by manufacturer and model. Code definitions are drawn from the public-domain OBDex dataset; the workshop notes are our own.