Transmission Fluid Pressure Sensor/Switch A Circuit
What it means
P0840 means the transmission module has detected a fault in the circuit for fluid pressure sensor/switch A, which reports hydraulic pressure the module relies on to time shifts. The engine still starts and runs, but without a trustworthy pressure signal the transmission may shift harshly, slip, or drop into a protective limp mode.
Can I keep driving? Often driveable, but the transmission may shift harshly or slip and can enter a limp mode; drive gently and fix it soon to avoid clutch wear.
Real-world scan data
Figures reflect only diagnostic scans recorded through our own apps and tools. Cars are typically scanned because a symptom or warning light prompted it, so these counts describe our scan records — not the incidence of faults in any model, and not the quality, reliability, or safety of any vehicle. No comparison between manufacturers or models is implied. All trademarks belong to their respective owners; no affiliation or endorsement is implied.
Common symptoms
What you'll notice
- Check-engine light on
- Harsh, delayed, or slipping shifts
- Transmission stuck in a limp / fail-safe gear
- Reduced fuel economy from poor shift control
Common causes
Most likely first
- Faulty transmission fluid pressure sensor/switch ALikely
- Damaged wiring or connector in the sensor circuitCommon
- Loose or corroded electrical connectionsCommon
- Low or degraded transmission fluid affecting pressureLess often
- Failed transmission control module (least likely)Less often
How to diagnose & fix
Work through it in order
- Read freeze-frame data and check transmission fluid level and condition first.
- Inspect the pressure sensor connector and wiring for corrosion, damage, or looseness.
- Compare the sensor's signal against spec with a scan tool to confirm it reads out of range.
- Repair wiring faults, then replace the pressure sensor/switch A if it's the cause.
- Clear the code and road-test; diagnose the TCM only if the fault persists.
FAQ
Quick answers
Is it safe to drive with P0840?+
Can I just clear P0840?+
Related codes
Read & clear P0840 yourself
Scan your car, confirm the code, and clear it once it's fixed — with OBD DashX on your phone.