Transmission Fluid Pressure Sensor/Switch Circuit Malfunction
The P0840 code indicates a problem in the electrical circuit of the transmission fluid pressure sensor or switch. This sensor monitors hydraulic pressure inside the transmission and sends a signal to the powertrain control module (PCM) to adjust shift timing and line pressure. A circuit malfunction can be caused by a faulty sensor, damaged wiring, poor electrical connections, or low transmission fluid. This can lead to improper transmission operation, harsh shifting, or limp mode.
01. Most Common Causes
- Faulty transmission fluid pressure sensor
- Damaged or corroded wiring/connectors in the sensor circuit
- Low or contaminated transmission fluid
- Internal transmission mechanical failure
02. Symptoms You Will Notice
- Check Engine Light illuminated
- Harsh or erratic shifting
- Transmission slipping or stuck in gear
- Reduced fuel economy
03. Step-by-Step Diagnosis
Step 1: Scan for codes and record freeze frame data.
Step 2: Inspect transmission fluid level and condition; top off or replace if necessary.
Step 3: Visually check wiring and connectors for damage, corrosion, or loose connections.
Step 4: Test the pressure sensor's resistance and voltage with a multimeter per manufacturer specs.
Step 5: If sensor and wiring are okay, check for internal transmission issues (requires professional scan tool or mechanic).
04. Commonly Affected Makes
05. Frequently Asked Questions
Is it safe to drive with this code?▼
It is not recommended to drive long distances. The transmission may shift poorly or go into limp mode, which can cause further damage. Have it diagnosed promptly.
How much does it cost to fix?▼
The cost typically ranges from $50 to $150 for diagnosis and sensor replacement. If wiring repair is needed, it may be higher. Internal transmission repairs can cost significantly more.