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). When the PCM detects a voltage or resistance outside the expected range, it sets this code. Common causes include a faulty sensor, damaged wiring, poor electrical connections, or low transmission fluid level. This issue can lead to improper transmission operation, harsh shifting, or limp mode.
1. Symptoms You Will Notice
- Check Engine Light illuminated
- Harsh or erratic shifting
- Transmission stuck in limp mode
- Decreased fuel economy
2. Most Common Causes
- Faulty or erratic transmission fluid pressure (TFP) sensor 'A'
- Damaged, corroded, or high-resistance wiring or connectors in the sensor circuit
- Short to ground or short to voltage in the signal circuit
- Open circuit in the sensor signal, reference, or ground wires
- Intermittent or internal TCM/PCM failure (less common)
3. Step-by-Step Diagnosis
Step 1: Scan for codes and record freeze frame data.
Step 2: Inspect transmission fluid level and condition.
Step 3: Visually check wiring and connectors for damage or corrosion.
Step 4: Test sensor resistance and voltage with a multimeter.
Step 5: Replace sensor if faulty; clear codes and test drive.
4. Frequently Asked Questions
Is it safe to drive with this code?▼
It is not recommended to drive long distances as it can cause transmission damage or unsafe shifting. Limp mode may limit speed.
How much does it cost to fix?▼
Typically $50-$150 for parts and labor, but can be higher if internal transmission issues are found.
5. Commonly Affected Makes
This is a generic OBD2 code, potentially applicable to any manufacturer using this diagnostic standard.