P1551

Wiring Diagram:

P1551 - THROTTLE REST POSITION NOT REACHED DURING LEARN
The throttle actuator control (TAC) module contains a direct current actuator motor controlled by the ECM used to move the throttle plate (in either direction) and two throttle position sensors to determine throttle plate position. The throttle plate is held at a 7% 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% 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%. 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 P1551 will set if the throttle position falls outside a calibrated range when no current is applied to the throttle actuator motor during a throttle relearn procedure indicating the rest position is not being reached.

DTC PARAMETERS
DTC P1551 will set if the throttle position is NOT in the range of 3.8-11.7% when commanded to the rest position (no current flowing to actuator motor) during a throttle relearn procedure when:
^ Engine speed is less than 250 rpm
^ Vehicle speed = 0 mph
^ ECT is between 5° and 100°C (41°and 212°F)
^ IAT is greater than 5°C (41°F)
^ Accelerator pedal angle is less than 15% Battery voltage is greater than 10 volts

DTC P1551 diagnostic runs continuously during a throttle relearn procedure when the above conditions have been met.

P1551 is a type A and B DTC

NOTE: The MIL will illuminate after 5 seconds.

DIAGNOSTIC AIDS

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

If DTC P1551 is set, the ECM will default to one of the two limp home modes (random injector disable with engine speed limited to 1800 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 P1551:
^ Weak throttle body return springs or throttle plate sticking
^ TP sensor 1 or 2 skewed or resistance in the wiring.

Refer to DIC P0121 Diagnostic Aids to test for a skewed TP sensor 1 or skewed TP sensor 2.