Symptoms of P0840
- Harsh, delayed, or slipping shifts, or being stuck in one gear.
- Check-engine light, sometimes with limp-home mode.
- Flaring RPM or a shudder under load.
Common causes of P0840
- Low or degraded transmission fluid.
- A failing speed sensor, shift solenoid, or pressure-control circuit.
- Wiring, connector, or internal hydraulic/mechanical faults.
Useful checks before replacing parts
- Compare pressure sensor A data with commanded pressure, gear state, line pressure if available, and shift symptoms.
- Inspect sensor wiring, connector condition, reference voltage, ground, internal harness, and transmission case connector sealing.
- Review fluid level and condition plus pressure-control or shift solenoid codes that may affect pressure readings.
Related OBD-II codes
These nearby transmission entries can help compare wording and pin down the fault area.