The pressure control solenoid is in your automatic transmission.  It's a
solenoid-operated valve that regulates line pressure.  It causes the
transmission to shift hard and fast, soft and slow, or somewhere in
between.  Most pressure control solenoids use a pulse-width modulated
signal with a variable duty cycle.

The solenoid may fail mechanically (stuck open or stuck closed) or
electrically (shorted to power, shorted to ground, or open).  When the
relevant OBD module detects a problem with the pressure control solenoid,
the "Check Engine" light illuminates, and other transmission default
operating conditions are usually enabled.  Depending on the transmission,
the torque converter lock-up clutch may be disabled, certain gear ranges
may be disabled, etc.

Unfortunately, P0745 is the catch-all code for a general pressure control
solenoid failure.  Other DTC numbers, such as DTC P0748 will tell you
up front that it's an electrical failure mode.

The pressure control solenoid is diagnosed with a breakout box attached to
whatever module (TCM, PCM, VCM, or ECM) controls the transmission.  First,
check for shorts, opens, and specified resistance (don't forget to correct
for temperature) with a DVOM. If nothing is found, use the breakout box to
disable and enable the solenoid while driving the car. The transmission
shifting behavior should alternate between shifting hard and fast to soft
and slow. (Don't let the transmission slip excessively.)  If the
transmission shifting behavior doesn't change, the valve is faulty (stuck
open or closed).  If the shifting behvior of the transmission does change,
the module that controls the transmission is faulty.