P0840
Transmission Fluid Pressure Sensor/Switch A Circuit
Transmission / fluid pressure
Generic OBD-II
Medium
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.
Medium
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.
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?+
Short trips are usually okay, but harsh or slipping shifts can wear the transmission. Get it looked at promptly.
Can I just clear P0840?+
It returns until the sensor or wiring is repaired, because the module keeps seeing the faulty pressure signal.
Related codes
Often seen with P0840
Read & clear P0840 yourself
Scan your car, confirm the code, and clear it once it's fixed — with OBD DashX on your phone.
✓ Verified name — SAE J2012 / ISO 15031-6Guidance reviewed by OBD DashX