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DTC P1551

DTC P1551

Diagnostic Instructions


• Perform the Diagnostic System Check - Vehicle prior to using this diagnostic procedure.

• Review Strategy Based Diagnosis for an overview of the diagnostic approach.

• Diagnostic Procedure Instructions provides an overview of each diagnostic category.


DTC Descriptor

DTC P1551

Throttle Valve Rest Position Not Reached During Learn

Circuit/System Description

The engine control module (ECM) controls the throttle valve by applying a varying voltage to the control circuits of the throttle actuator control (TAC) motor. The ECM monitors the duty cycle that is required to actuate the throttle valve. The ECM monitors the throttle position sensors 1 and 2 to determine the actual throttle valve position.

Conditions for Running the DTC


• The vehicle speed is 0 km/h (0 mph).

• The engine speed is less than 40 RPM.

• The engine coolant temperature (ECT) is between 5-100°C (41-212°F).

• The intake air temperature (IAT) is between 5-143°C (41-290°F).

• The ignition voltage is greater than 10 volts.

• The accelerator pedal position (APP) is less than 15 percent.

• DTC P1551 runs when the conditions above have been met for greater than 5 seconds.


Conditions for Setting the DTC


• The ECM detects the TP sensor angle is less than 2 percent or greater than 13 percent when the throttle actuator control motor is deactivated.

• The condition exists for greater than 1 second or for a cumulative of 10 seconds.


Action Taken When the DTC Sets

DTC P1551 is a type A DTC.

Conditions for Clearing the DTC

DTC P1551 is a type A DTC.

Diagnostic Aids


• The scan tool has the ability to operate the throttle actuator control system using Special Functions. This function operates the throttle through the entire range in order to determine if the throttle body and system operate correctly.

• Inspect for a condition in which the throttle valve may have been held open.

• Inspect for conditions in which ice may have formed in the throttle bore.

• The throttle valve is spring loaded to a slightly open position. This is referred to as the rest position. The throttle valve should not be completely closed, nor should it be open any greater than predetermined amount. The throttle valve should move open and to the closed position without binding under the normal spring pressure. The throttle should NOT be free to move open or closed WITHOUT spring pressure.


Reference Information

Schematic Reference

Engine Controls Schematics (3.0L (LF1)) Engine Controls Schematics (2.8L (LAU))

Connector End View Reference

Component Connector End Views

Description and Operation

Throttle Actuator Control (TAC) System Description

Electrical Information Reference


• Circuit Testing

• Connector Repairs

• Testing for Intermittent Conditions and Poor Connections

• Wiring Repairs


DTC Type Reference

Powertrain Diagnostic Trouble Code (DTC) Type Definitions

Scan Tool Reference

Control Module References for scan tool information

Circuit/System Verification


1. Ignition ON, observe the DTC information with a scan tool. Verify that DTC P0121, P0122, P0123, P0221, P0222, P0223, P2100, P2101, or P2119 is not set.

2. If a DTC is set, refer to Diagnostic Trouble Code (DTC) List - Vehicle for further diagnosis.

3. Start the engine. Check the DTC information with a scan tool for 30 seconds. DTC P1551 should not be set.

4. Operate the vehicle within the Conditions for Running the DTC to verify the DTC does not reset. You may also operate the vehicle within the conditions that you observed from the Freeze Frame/Failure Records data.


Circuit/System Testing

Ignition OFF, disconnect the air intake duct at the Q38 throttle body and inspect for the following:


• Throttle valve that is NOT in the rest position

• Throttle valve that is binding open or closed

• Throttle valve that is free to move open or closed WITHOUT spring pressure


Repair Instructions

Perform the Diagnostic Repair Verification after completing the diagnostic procedure.

Throttle Body Assembly Replacement (LAU) Throttle Body Assembly Replacement (LF1)