Transmission Fluid Pressure Sensor/Switch A Circuit Low
The Powertrain Control Module (PCM) or Transmission Control Module (TCM) detected that the voltage signal from the Transmission Fluid Pressure (TFP) Sensor/Switch 'A' is lower than the manufacturer's specified range. This typically indicates a short to ground in the sensor circuit, a faulty sensor, or an actual low internal hydraulic pressure condition.
This code should be addressed immediately to prevent safety hazards and costly repairs. Driving with incorrect transmission fluid pressure or a failing sensor can lead to harsh shifting, slipping, and severe internal transmission damage.
Common symptoms
- Illuminated Check Engine Light
- Harsh or erratic transmission shifting
- Transmission slipping between gears
- Vehicle entering 'limp mode' (stuck in a single gear)
- Delayed engagement when shifting into Drive or Reverse
Likely causes
- Low transmission fluid level or dirty/contaminated fluid
- Faulty Transmission Fluid Pressure (TFP) Sensor/Switch 'A'
- Damaged, corroded, or loose wiring and connectors in the sensor circuit (short to ground)
- Clogged transmission fluid filter or internal hydraulic blockages
- Defective valve body or internal transmission mechanical failure
- 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, which can cause pressure drops and trigger the code.
Inspect the wiring harness and connector for TFP Sensor 'A'.
Look for: Frayed wires, corrosion, bent pins, or signs of a short to ground in the circuit harness.
Scan for related transmission codes and monitor live data with a scan tool.
Look for: The TFP sensor voltage or pressure reading remaining stuck low or at zero regardless of engine RPM or gear selection.
Test the TFP Sensor 'A' circuit with a digital multimeter.
Look for: Less than the specified reference voltage (usually 5V) at the connector, or continuity to ground on the signal wire indicating a short.
Perform a manual transmission fluid pressure test using a mechanical gauge.
Look for: Actual hydraulic pressure matching the manufacturer's specifications, which confirms the sensor is faulty rather than an actual internal low-pressure mechanical failure.
This page explains the generic P0842 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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