P0791 means the powertrain control module has detected an electrical or rationality fault in the intermediate shaft speed sensor circuit. This sensor reports the rotational speed of a specific automatic transmission shaft, and the PCM uses that signal for shift timing, clutch control, and torque-converter lockup. When the signal is missing, erratic, or out of an expected range, the code is stored.
Internal coil, Hall-element, or magnetic-pickup failure in the sensor is the most common cause, often from age, heat, or exposure to transmission fluid.
Chafed, pinched, corroded, or loose terminals in the harness between the sensor and PCM can create an open, short, or intermittent signal.
A cracked, debris-covered, misaligned, or worn tone wheel changes the signal pattern and can set the circuit code even if the sensor is good.
Low or contaminated fluid can cause internal slippage and shaft-speed patterns that the PCM sees as implausible, producing a rationality fault.
Worn clutches, bands, drums, or gearset problems can make the intermediate shaft speed differ from the commanded speed and trigger the code.
A failed input circuit inside the PCM is possible but uncommon; it should only be considered after the sensor, wiring, and mechanical causes have been ruled out.
Short trips may be possible, but erratic or limp-mode shifting reduces drivability and can increase transmission wear; have it diagnosed as soon as practical.
No. Wiring, connector, tone ring, fluid, and internal transmission faults are common causes, so proper diagnosis is important before replacing parts.
No. PCM failure is uncommon for this code; most cases are resolved by replacing the sensor, repairing wiring, or correcting transmission hardware or fluid issues.
A generic tool can read the code and freeze frame, but a professional-level scan tool or oscilloscope is often needed to view live shaft-speed signals and compare them with other transmission speed sensors.