Transmission Fluid Pressure Sensor/Switch A Circuit
The P0840 code indicates that the Powertrain Control Module (PCM) or Transmission Control Module (TCM) has detected an abnormal voltage or resistance in the transmission fluid pressure (TFP) sensor 'A' circuit. This means the sensor's signal does not match the expected manufacturer parameters for the current operating conditions, pointing to an electrical circuit malfunction or an actual pressure issue.
This code is considered high severity because incorrect transmission pressure readings can lead to erratic shifting, transmission slipping, and potentially severe internal transmission damage. The vehicle should be diagnosed and repaired as soon as possible to prevent costly repairs.
Common symptoms
- Illuminated Check Engine Light
- Harsh, delayed, or erratic transmission shifting
- Transmission slipping between gears
- Vehicle entering a fail-safe or 'limp' mode (stuck in a single gear)
- Inability to shift into certain gears
Likely causes
- Low, dirty, or degraded transmission fluid
- Faulty transmission fluid pressure sensor/switch 'A'
- Damaged, corroded, or loose wiring and connectors in the sensor circuit
- Internal transmission mechanical failure causing incorrect fluid pressure
- Faulty Transmission Control Module (TCM) or PCM (rare)
What to check before replacing parts
Check the transmission fluid level and condition.
Look for: Fluid that is low, burnt, or heavily contaminated with metal shavings, which can cause pressure issues and trigger the sensor code.
Visually inspect the wiring harness and connector for TFP sensor 'A'.
Look for: Frayed wires, melted insulation, broken clips, or corroded pins that would cause high resistance, a short, or an open circuit.
Scan for other related transmission diagnostic trouble codes.
Look for: Additional codes related to gear ratios, shift solenoids, or temperature that might indicate a broader internal transmission failure rather than an isolated sensor issue.
Test the TFP sensor 'A' circuit for reference voltage and ground using a digital multimeter.
Look for: A steady 5-volt reference signal (or manufacturer-specific voltage) and a solid ground at the connector; absence indicates a wiring harness or module issue.
Test the TFP sensor's internal resistance or output voltage while applying pressure.
Look for: Resistance or voltage values that fall outside the manufacturer's specified range for the given pressure, indicating the sensor itself has failed internally and requires replacement.
This page explains the generic P0840 definition and is general reference information, not a diagnosis for your specific vehicle. Actual causes, severity, and repairs vary by make, model, engine, and mileage — always confirm with a scan tool and a qualified technician before replacing parts.
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