What does it mean?
DTC P0791 sets when the PCM/TCM has received an input signal from the transmission intermediate shaft speed sensor circuit that is either irrational, erratic, or in some way incorrect.
Without a usable output speed signal the transmission cannot provide a proper shift strategy NOTE: Whenever diagnosing a 700 series DTC (Diagnostic Trouble Code) with a transmission, transfer case or other driveline components, check for a DTC under chassis and body systems, not just under “P” or powertrain/engine or emission control type codes. Module communications DTCs are “U” codes. These are most often found when looking for DTCs under chassis or body systems.
Chassis related codes are labeled “C” and body system related codes are labeled “B” codes and these can cause drivetrain issues if they affect a sensor that is also used by the heating and air conditioning system.