Transmission

P0845 — Transmission Fluid Pressure Sensor/Switch B Circuit Malfunction

Severity: medium Drive with care, to a shop

What P0845 means

The transmission control unit monitors hydraulic pressure in the transmission using pressure sensors or switches. P0845 sets when the signal from sensor B is out of range, missing, or does not match what the unit expects. The cause can be the sensor itself, its wiring or connector, or the valve body.

Symptoms

  • Check Engine light on
  • Harsh or delayed shifts
  • Transmission stuck in one gear (limp mode)
  • Slipping under acceleration
  • Jerking during gear changes
  • Worse fuel economy

Possible causes

  1. Failed pressure sensor/switch B
  2. Damaged internal transmission wiring harness
  3. Open, short or corroded wiring or connector
  4. Valve body problem (sticking valves, clogged passages)
  5. Low or contaminated transmission fluid
  6. Rarely, a faulty PCM/TCM

How to diagnose

  1. Scan all codes and freeze-frame data, clear the code and see if it returns
  2. Check transmission fluid level and condition: color, smell, debris
  3. Inspect the transmission connector and harness for oil, corrosion and chafed wires
  4. Test sensor power, ground and reference voltage with a multimeter
  5. Compare the live pressure sensor reading on a scan tool across operating conditions
  6. Check wiring to the control unit for opens and shorts
  7. Drop the pan and inspect the internal harness, sensor and valve body

Common fixes

  • Replace pressure sensor/switch B
  • Repair wiring or the connector and clean the terminals
  • Replace the internal transmission harness
  • Change fluid and filter and set the correct level
  • Repair or replace the valve body
  • Reflash or replace the control unit only after confirming it is faulty

Questions and answers

Can I drive with a P0845 code?

Only briefly and carefully, for example to a shop. The transmission may go into limp mode, and continued driving raises the risk of wear and costly repairs.

What does the "B" in P0845 mean?

It labels one of several pressure sensors or circuits in the transmission. Which one it is depends on the manufacturer, so check the service information for your model.

Is replacing the pressure sensor enough?

Not always. Check the fluid, wiring and connector first, since the harness or valve body is often the real cause and a new sensor will not fix it.

This is general information about the code. The meaning can differ between makes and models: a scan tool and the repair manual for your car will show the exact cause.