Transmission Fluid Pressure Sensor/Switch B 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 'B' circuit is lower than the manufacturer's specified range. This indicates a potential electrical issue within the sensor circuit or an actual low transmission fluid pressure condition.
Transmission pressure issues can lead to severe internal transmission damage or leave the vehicle undrivable. It should be diagnosed and repaired immediately to prevent costly transmission failure.
Common symptoms
- Illuminated Check Engine Light
- Harsh, delayed, or erratic transmission shifting
- Transmission slipping between gears
- Vehicle entering limp mode (failsafe mode) with limited gears
- Inability to shift into certain gears
Likely causes
- Faulty Transmission Fluid Pressure (TFP) Sensor/Switch 'B'
- Low transmission fluid level or dirty/contaminated fluid
- Damaged, shorted, or corroded wiring and connectors in the TFP sensor circuit
- Mechanical failure inside the transmission causing actual low fluid pressure
- Faulty Powertrain Control Module (PCM) or Transmission Control Module (TCM)
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 issues and trigger the code.
Visually inspect the wiring harness and connectors for TFP Sensor 'B'.
Look for: Frayed wires, melted insulation, or corroded pins that could cause a short to ground or low voltage reading.
Scan for other related transmission codes.
Look for: Additional codes related to fluid temperature, shift solenoids, or gear ratios that might indicate a broader mechanical or electrical issue.
Test the TFP Sensor 'B' circuit with a digital multimeter.
Look for: A reference voltage (typically 5V) at the connector; if voltage is present, check the sensor's resistance against factory specifications to see if it is internally shorted.
Perform a manual transmission fluid pressure test using a mechanical gauge.
Look for: Actual line pressure matching the manufacturer's specifications; if mechanical pressure is normal, the issue is strictly electrical or sensor-related.
This page explains the generic P0847 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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