What does it mean?
In the modern automatic transmission, hydraulic pressure is generated in the pump, usually located at the front of the transmission, driven by the engine via the torque converter housing. Channels direct automatic transmission fluid to the valve body, which contains the shift solenoid (SS) valves and pressure control solenoid (PCS) valves. The transmission control module (TCM) controls the activation and modulation of the various solenoid valves, depending on driver demand and driving conditions.
The TCM may or may not be integrated into the ECM (engine control module). Electronically-controlled valves are used to shift and maintain gears. The SS valves control the activation and deactivation of clutches, brakes, and bands inside the transmission, effecting the engagement and disengagement of whatever gear is desired or required by the driver and operating conditions.
One or more PCS valves control hydraulic pressure inside the transmission, which can affect how shift engagements occur, whether slow and soft or hard and fast. If the TCM detects a problem with one of the valves, such as a short circuit, it commands the malfunction indicator lamp (MIL) “On” and stores a diagnostic trouble code (DTC) in system memory. DTC P0971 is defined as “Pressure Control Solenoid ‘C’ Control Circuit High,” though different automakers may have slightly different definitions.
Note: Circuit High Input codes are mostly caused by defective alternators that develop excessive voltages, although there may be other causes as well, such as short circuits that “leak” current into a particular system from other, unrelated systems that work on higher voltages. Note that in the case of short circuits that leak power into a control circuit, there is likely to be other, seemingly unrelated trouble codes present along with the code(s) being investigated.
Diagnosing a “high input” code will always involve a thorough testing of the charging system as a first step, followed by measures to isolate the system from all other possible sources of power during resistance, continuity, and reference voltage tests.