What it means
The control module detects an electrical fault in the transmission fluid pressure sensor/switch A circuit. The code does not specify whether the cause is the sensor, wiring, connector, or another circuit fault.
At a glance
- Usually lights the check-engine light
How it works
Transmission fluid pressure sensor or switch A provides the control module with an electrical signal used to monitor pressure in the transmission’s hydraulic system. P0840 indicates an electrical fault in the A circuit; by itself, it does not identify whether the sensor, connector, wiring, or another part of the circuit is responsible.
Common causes
Most likely first.
- Pressure sensor drift
- Internal hydraulic leak
- Sensor element aged beyond calibration window
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
How to diagnose it
- Record freeze-frame data and review pressure sensor A live data and related transmission codes to see when the circuit fault was detected.
- Check the transmission fluid level and condition using the vehicle’s specified procedure, since a hydraulic issue may affect pressure readings even though this is a circuit code.
- Inspect sensor A’s connector and nearby harness for loose connections, fluid intrusion, corrosion, or damaged insulation.
- Using the wiring diagram, check the sensor A supply, ground, and signal circuit for opens, shorts, or poor connections.
- Compare sensor A’s signal with scan-tool data and the manufacturer’s specification under the applicable test conditions before deciding whether the sensor or another circuit component has failed.
Common fixes
- Reconnect and secure a loose sensor A connector.
- Repair damaged wiring or corroded terminals in the sensor A circuit.
- Correct the transmission fluid level if it is low, following the specified procedure and using the required fluid.
- Replace sensor A if its electrical output is outside the manufacturer’s specification and the wiring checks out.
Can I keep driving?
A warning light with normal, predictable shifting may allow a short, cautious trip, but avoid towing, hard acceleration, and long journeys until the fault is diagnosed. Stop driving if the transmission slips, loses drive, or shifts harshly, as continued use can worsen transmission damage.
A code shows where the control module detected a fault, not always which part failed. Check the causes in order and confirm with a test before replacing parts. Have several codes? Check them together
This is a generic code: it means the same on every make. The car’s maker may add more detail in its own service information.