Transmission Fluid Pressure Sensor/Switch A Circuit High

P0843 means the transmission fluid pressure (TFP) sensor/switch 'A' circuit is reading HIGH voltage -- typically an open circuit, a disconnected sensor, or low/aerated fluid, not always a bad sensor. On the 6L80/6L90 the pressure switches live inside the valve body / TEHCM, so check fluid level and condition and inspect the internal wiring harness and pass-through connector first. If the sensor itself is bad it requires a pan drop, and many cases need the control-solenoid (TEHCM) assembly plus a relearn.

On the 6R80, P0843 is a HIGH-voltage fault on the transmission fluid pressure sensor circuit. Because high voltage usually means an open or out-of-range signal, inspect the internal main control (valve body) wiring harness and the case connector before condemning the sensor -- a chafed lead or a loose connector is cheaper than a teardown. Verify fluid level and condition (Mercon LV) first. The sensor is internal, so a pan drop and fluid refill are part of the job.

P0843 = Transmission Fluid Pressure Sensor/Switch 'A' Circuit HIGH. Work cheapest-to-most-expensive: (1) check fluid level and condition -- low or burnt fluid skews pressure readings; (2) inspect the sensor connector and wiring for corrosion, chafing, or an open circuit, since 'circuit high' often means an open rather than a failed sensor; (3) only then replace the fluid pressure sensor/switch, which on most units is internal and requires dropping the pan. Clear codes and road-test to confirm before assuming an internal failure.