P0870: Transmission Fluid Pressure Sensor/Switch C Circuit
What to do
Expect symptoms confined to particular gears rather than across the whole gearbox. Ask whether the pressure readings were compared against each other — with this many monitoring points, they can usually identify which clutch circuit is affected before anything is dismantled.
What this code means
A fault in a third pressure monitoring circuit. By the time a transmission carries this many pressure measurements, individual clutch circuits are being watched separately rather than line pressure alone — which takes the localisation argument from P0845 to its conclusion.
With one sensor, the module knows pressure is wrong. With two, it knows roughly where. With three or four, each monitored circuit corresponds closely enough to a specific clutch that the code effectively names which one has lost its pressure. That is a considerable diagnostic advantage on a transmission whose internals are otherwise invisible without opening it, and it is worth asking a workshop to use rather than treating this as one more sensor code.
Likely causes
Start here
- Faulty transmission fluid pressure sensor · moderate
- Low or degraded transmission fluid · easy
- 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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Community experience
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