P0845: Transmission Fluid Pressure Sensor/Switch B Circuit
What to do
Expect similar symptoms to the sensor A codes. The useful thing about having two pressure sensors is that a workshop can compare them and narrow down where pressure is being lost, so ask whether both readings were looked at together rather than one in isolation.
What this code means
A fault in a second pressure monitoring circuit, watching a different point in the hydraulic system from sensor A.
The value of a second measurement is the same as it was for the intermediate shaft speed sensor: it turns a single number into a comparison. Pressure is not one quantity — there is line pressure from the pump, and there is the pressure actually arriving at a particular clutch after passing through the valve body. Two sensors at different points let the module see the difference between them, and a difference is what a leak looks like. Correct pressure at A with low pressure at B locates the loss between the two, which is a far more useful statement than knowing pressure is low somewhere.
On a transmission with only one sensor, that localisation is unavailable and the search is correspondingly broader.
Likely causes
Start here
- Low or degraded transmission fluid · easy
- Faulty transmission fluid pressure sensor · moderate
- Worn valve body · specialist
Less common
- Damaged transmission wiring harness · moderate
- Restricted transmission fluid filter · moderate
Symptoms
Components involved
Information, not instruction
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Have anything you intend to act on checked by a trained person against the manufacturer's own procedure for your vehicle. Nothing here replaces that procedure.
Community experience
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What makes one worth reading:
- The car — make, model, year and engine. The same fault behaves differently on each.
- What you measured, not only what you replaced.
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