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For 1990-2009 cars only
Makes 🢒 Cadillac 🢒 2005 🢒 CTS 🢒 Accessories 🢒 Navigation Systems 🢒 DTC P1551

Circuit 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 (TP) sensors 1 and 2 to determine the actual throttle valve position. If the ECM detects that the throttle position is NOT within a predetermined range when commanded to the rest position, this DTC sets.

DTC Descriptor

This diagnostic procedure supports the following DTC:

DTC P1551 Throttle Valve Rest Position Not Reached During Learn

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 5-85°C (41-185°F).
    • The intake air temperature (IAT) is 5-60°C (41-140°F).
    • The ignition 1 voltage is more than 10 volts.
    • The accelerator pedal position (APP) is less than 15 percent.
    • DTC P1551 runs when the TAC motor is deactivated.

Conditions for Setting the DTC

The ECM detects the TP sensor angle is less than 1.8 percent or more than 13.1 percent for more than 4 seconds, when the throttle actuator control motor is deactivated.

Action Taken When the DTC Sets
    • The control module illuminates the malfunction indicator lamp (MIL) when the diagnostic runs and fails.
    • The control module records the operating conditions at the time the diagnostic fails. The control module stores this information in the Freeze Frame/Failure Records.

Conditions for Clearing the MIL/DTC
    • The control module turns OFF the malfunction indicator lamp (MIL) after 4 consecutive ignition cycles that the diagnostic runs and does not fail.
    • A current DTC, Last Test Failed, clears when the diagnostic runs and passes.
    • A history DTC clears after 40 consecutive warm-up cycles, if no failures are reported by this or any other emission related diagnostic.
    • Clear the MIL and the DTC with a scan tool.

Diagnostic Aids
    • Use the J 35616-A/BT-8637 Connector Test Adapter Kit for any test that requires probing the ECM harness connector or a component harness connector.
    • Operating the throttle valve with the Throttle Position control function of the scan tool may cause additional DTCs to set. DO NOT attempt to diagnose DTCs set during this function.
    • 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.
    • For an intermittent condition, refer to Intermittent Conditions .

Test Description

The number below refers to the step number on the diagnostic table.

  1. The throttle valve is spring loaded to a slightly open position. The throttle valve should be open approximately 3-5 percent. This is referred to as the rest position. The throttle valve should not be completely closed, nor should they be open any more than the specified 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. Replace the throttle body if any of these conditions are found.

    The throttle valve is spring loaded in a slightly open position and should move in either direction without binding. The throttle valve should always be under spring pressure.

DTC P1551 Throttle Valve Rest Position Not Reached During Learn

Step

Action

Yes

No

Schematic Reference: Engine Controls Schematics

Connector End View Reference: Engine Control Module Connector End Views or Engine Controls Connector End Views

1

Did you perform the Diagnostic System Check - Vehicle?

Go to Step 2

Go to Diagnostic System Check - Vehicle

2
  1. Turn ON the ignition, with the engine OFF for 30 seconds.
  2. Observe the DTC information with a scan tool.

Does the scan tool display DTC P1551 failed this ignition cycle?

Go to Step 3

Go to Step 4

3

Observe the DTC information.

Are DTCs P0121, P0122, P0123, P0221, P0222, P0223, P2100, P2101, or P2119 also set?

Go to Diagnostic Trouble Code (DTC) List - Vehicle

Go to Step 5

4

  1. Observe the Freeze Frame/Failure Records for this DTC.
  2. Turn OFF the ignition for 30 seconds.
  3. Start the engine.
  4. Operate the vehicle within the Conditions for Running the DTC. You may also operate the vehicle within the conditions that you observed from the Freeze Frame/Failure Records.

Did the DTC fail this ignition?

Go to Step 5

Go to Diagnostic Aids

5

  1. Turn OFF the ignition.
  2. Disconnect the air intake duct at the throttle body. Refer to Air Cleaner Assembly Replacement .
  3. Inspect the throttle body for the following conditions:
    4. • 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

Did you find any of the above conditions with the throttle body?

Go to Step 6

Go to Diagnostic Aids

6

Replace the throttle body assembly. Refer to Throttle Body Assembly Replacement .

Did you complete the replacement?

Go to Step 7

--

7

  1. Clear the DTCs with a scan tool.
  2. Turn OFF the ignition for 30 seconds.
  3. Start the engine.
  4. Operate the vehicle within the Conditions for Running the DTC. You may also operate the vehicle within the conditions that you observed from the Freeze Frame/Failure Records.

Did the DTC fail this ignition?

Go to Step 2

Go to Step 8

8

Observe the Capture Info with a scan tool.

Are there any DTCs that have not been diagnosed?

Go to Diagnostic Trouble Code (DTC) List - Vehicle

System OK