The throttle actuator control (TAC) module contains a direct current actuator motor controlled by the engine control module (ECM) used to move the throttle plate, in either direction and 2 throttle position (TP) sensors to determine throttle plate position. The throttle plate is held at a 7 percent rest position to a mechanical stop by a constant force return spring. This spring will hold the throttle plate to the stop when there is no current flowing to the actuator motor. There is another return spring, which creates constant force on the throttle plate only when the throttle plate is moved towards the full closed position. The ECM will monitor both the actuator motor and TP sensors for circuit faults, as well as check to make sure both springs can return the throttle to the 7 percent position no matter where the throttle plate remains.

The ECM will go through a throttle relearn procedure 29 seconds after the ignition is turned ON with the engine OFF as long as certain conditions have been met. During this learn procedure, the ECM will move the throttle from the rest to full closed position, then to around 14 percent. During this period, the TP sensors lower limits, rest position of the TP sensors, the actuator motor force to overcome both spring pressures as well as the return rates of both springs are learned. If any of these parameters are out of range, a DTC will be set. DTC P1523 will set if the throttle plate does not return within a certain amount of time to the 7 percent idle air position during a throttle relearn procedure or during the key-up closed throttle test indicating the throttle plate is sticking or the throttle plate return springs have weakened.

DTC P1523 will set if the throttle plate is 3 percent above the throttle rest position for longer than 0.56 seconds when:

DTC  P1523 diagnostic runs during a throttle relearn procedure or key up closed throttle test when the above conditions have been met.

DTC P1523 is a type A DTC.

The malfunction indicator lamp (MIL) will illuminate after 5 seconds.

Important: If a TP sensor 1 or 2 DTC is set, diagnose that DTC first.

If DTC P1523 is set, the ECM will default to 1 of the 2 limp home modes, random injector disable with engine speed limited to 1,800 RPM. The REDUCED POWER telltale will be illuminated and the ECM will remain in this condition for the entire ignition cycle even if the fault is corrected.

Possible causes of DTC P1523: