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DTC codes / P08xx · Transmission / P0845

P0845Transmission Fluid Pressure Sensor/Switch 'B' Circuit

P0845 indicates that the transmission control module (TCM) has detected an abnormal condition in the electrical circuit for transmission fluid pressure sensor or switch 'B'. The TCM monitors the signal voltage or switch state and sets this code when the reading falls outside the expected range for the current operating conditions, often indicating an open circuit, short to ground, or short to power.

SystemPowertrain
Code typeGeneric (all makes)
SeverityMedium
Fix difficultyIntermediate
AreaTransmission

Symptoms you may notice

  • Illumination of the check engine light or transmission warning light
  • Transmission may enter limp mode or fail to shift properly
  • Harsh or delayed gear engagements
  • Erratic shifting or slipping sensations
  • Often no symptoms besides the warning light, especially if the fault is intermittent

Common causes, most likely first

  1. Faulty transmission fluid pressure sensor or switch 'B'
  2. Damaged or corroded wiring harness or connector at the sensor
  3. Open or shorted circuit in the signal or ground wire
  4. Poor electrical connection at the TCM
  5. Internal transmission mechanical issue causing abnormal pressure readings
  6. Contaminated transmission fluid affecting sensor operation

How to diagnose and fix it

  1. Perform a visual inspection of the sensor and its wiring for obvious damage or corrosion
  2. Check the transmission fluid level and condition; service if contaminated
  3. Use a scan tool to read live data and verify the sensor 'B' signal rationality
  4. Test the sensor circuit with a multimeter for continuity, shorts, and reference voltage
  5. Inspect and clean the TCM connectors and grounds
  6. Replace the transmission fluid pressure sensor or switch 'B' if testing indicates failure
  7. Clear the code and perform a road test to verify the repair

What P0845 really tells you about sensor B

  • This code often sets due to wiring problems rather than a failed sensor, so always inspect the harness before replacing parts.
  • A low fluid level or contaminated fluid can mimic a sensor failure by causing pressure readings outside the expected range.
  • Intermittent faults may not trigger a warning light until the circuit fails consistently, so live data monitoring is crucial.
  • The TCM may use sensor 'B' for adaptive shift strategies, so ignoring the code can lead to poor shift quality over time.
  • Some vehicles use a combined pressure switch assembly where only one circuit fails, requiring replacement of the entire unit.

Can I keep driving with P0845?

If the transmission is shifting normally and the light is steady, you can drive cautiously to a repair shop, but avoid heavy loads or aggressive driving. If the transmission is in limp mode or shifting harshly, stop driving and seek professional diagnosis to prevent further damage.

Tools that help

Tools we recommend are in the Auto Veteran shop on Amazon. As an Amazon Associate we earn from qualifying purchases.

Guidance is general and written for all OBD-II vehicles. Exact tests and values vary by model, so confirm the fault before replacing parts.

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