P0842: Transmission Fluid Pressure Sensor/Switch A Circuit Low
What to do
Ask for the pressure to be checked with a mechanical gauge, not just read off a scan tool — the scan tool reads the sensor that may itself be the fault. Until that has been done, treat the possibility of genuinely lost pressure seriously: if the transmission is also slipping or refusing to engage, have the vehicle recovered rather than driven.
What this code means
The pressure sensor circuit is reading below its valid electrical range. This is not the same thing as low transmission pressure, and confusing the two is the mistake this code invites.
A circuit shorted to ground sits at the bottom of its voltage range, and the module has no way to distinguish that from a sensor faithfully reporting almost no pressure at all. One is a wiring fault on a healthy transmission; the other is a transmission that has lost its hydraulics. They demand completely different responses and the code cannot tell them apart.
What settles it takes minutes: fit a mechanical gauge to the transmission's test port and read the actual pressure. If the gauge shows normal pressure while the sensor reports none, the fault is electrical and the gearbox is fine. If the gauge agrees with the sensor, the hydraulics genuinely have failed and the vehicle should not be driven. No amount of scan tool data substitutes for that gauge, because every electronic reading in the vehicle comes through the very sensor under suspicion.
Likely causes
Start here
- Wiring shorted to ground · moderate
- Faulty transmission fluid pressure sensor · moderate
- Low or degraded transmission fluid · easy
Less common
- Fluid tracking into the transmission connector · moderate
- Worn transmission fluid pump · specialist
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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