Transmission Fluid Pressure Sensor/Switch B Circuit
The P0845 code indicates that the Powertrain Control Module (PCM) or Transmission Control Module (TCM) has detected a malfunction in the transmission fluid pressure sensor/switch 'B' circuit. This code is set when the voltage signal from the sensor falls outside the manufacturer's expected parameters for the current operating conditions.
This code should be addressed immediately. Driving with erratic transmission fluid pressure readings can lead to severe internal transmission damage or sudden loss of motive power.
Common symptoms
- Illuminated Check Engine Light
- Harsh or erratic transmission shifting
- Transmission slipping between gears
- Vehicle entering 'limp mode' (restricted to a single gear)
- Failure of the transmission to engage or shift
Likely causes
- Defective transmission fluid pressure sensor/switch 'B'
- Low or contaminated transmission fluid
- Damaged, corroded, or loose wiring and connectors in the sensor circuit
- Faulty transmission valve body
- Internal transmission mechanical failure
- Defective PCM or TCM (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 indicates a leak or internal wear that must be addressed first.
Inspect the wiring harness and connectors for the transmission fluid pressure sensor 'B'.
Look for: Look for frayed wires, melted insulation, or corroded pins that could cause an open or short circuit.
Test the transmission fluid pressure sensor 'B' circuit for reference voltage and ground.
Look for: Using a multimeter, verify that the sensor is receiving the correct reference voltage (usually 5V) and has a solid ground; lack of either indicates a wiring or module issue.
Test the transmission fluid pressure sensor 'B' signal output.
Look for: Monitor the sensor's signal voltage using a scan tool to see if it changes smoothly with pressure; a stuck or erratic voltage indicates a faulty sensor.
Perform a mechanical transmission fluid pressure test.
Look for: Connect a mechanical pressure gauge to the transmission test port and compare the actual pressure to the sensor's reading on a scan tool; a mismatch confirms a bad sensor, while low mechanical pressure indicates internal transmission failure.
This page explains the generic P0845 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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