What it means
Electrical fault on the transmission fluid pressure sensor C circuit. Sensor C typically monitors a third hydraulic clutch or band line.
At a glance
- Usually lights the check-engine light
How it works
Transmission pressure sensor or switch C monitors a hydraulic pressure circuit that the TCM uses to assess transmission operation; on some designs it relates to a third clutch or band circuit. P0870 sets when the electrical circuit for sensor C has a fault, so the TCM cannot trust that pressure signal.
Common causes
Most likely first.
- Sensor failed
- Wiring fault
Symptoms
- Check engine or transmission warning light on
- Harsh, delayed, or missing shifts
- Possible limp mode (locked in one gear)
- Slipping or shuddering during gear changes
How to diagnose it
- Save freeze-frame data and check live pressure sensor C status or readings to see whether the signal is missing or implausible.
- Locate sensor C using the vehicle’s wiring or service information and inspect its connector and nearby harness for damage, corrosion, or fluid intrusion.
- Check the sensor C circuit wiring, power, ground, and signal connections using the wiring diagram and manufacturer’s procedure.
- Test circuit continuity and check for shorts or poor terminal contact before condemning the sensor.
- If the circuit is sound, test transmission pressure sensor or switch C against the manufacturer’s specification.
Common fixes
- Repair damaged wiring or connector faults in the sensor C circuit.
- Clean or replace corroded or poorly seated sensor C terminals.
- Restore the specified power or ground connection if testing identifies a fault.
- Replace transmission pressure sensor or switch C if it fails testing.
Can I keep driving?
The transmission may shift poorly or enter a protective mode if the TCM loses sensor C information. Limit driving if shifting becomes abnormal, and stop if the transmission slips or loses drive.
A code shows where the control module detected a fault, not always which part failed. Check the causes in order and confirm with a test before replacing parts. Have several codes? Check them together
This is a generic code: it means the same on every make. The car’s maker may add more detail in its own service information.