GM Service Manual Online
For 1990-2009 cars only
Makes 🢒 Buick 🢒 2006 🢒 Terraza Ext. 🢒 Engine 🢒 Engine Controls - 3.9L 🢒 DTC P0068 or P0121

DTC Descriptors

DTC P0068: Throttle Body Airflow Performance

DTC P0121: Throttle Position (TP) Sensor Performance

Diagnostic Fault Information

Always perform the Diagnostic System Check - Vehicle prior to using this diagnostic procedure.

Circuit/System Description

The engine control module (ECM) compares actual throttle position (TP) to an expected throttle position based on engine load, air flow and speed. The ECM determines engine load and air flow based on inputs from the mass air flow (MAF) and the manifold absolute pressure (MAP) sensors. The ECM monitors the engine air flow, and throttle position for errors.

Conditions for Running the DTC

P0068

    • The engine is running.
    • DTC P0120, P0220, P0601, P0602, P0603, P0604, P0606, P0607, P060D, P1516, P2101, P2119, or P2176 is not set.
    • The DTC runs continuously when the above conditions are met.

P0121

    • The engine is running.
    • DTC P0102, P0103, P0107, P0108, P0112, P0113, P0116, P0117, P0118, P0128, P0335, or P0336 is not set.
    • The engine coolant temperature (ECT) is between 70-125°C (158-257°F)
    • The intake air temperature (IAT) is between -7 and +125°C (+19 and +257°F)
    • The DTC runs continuously when the above conditions are met.

Conditions for Setting the DTC

The ECM detects that the airflow rate is more than the calculated airflow for more than 0.2 second.

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 and/or the Failure Records.
    • The control module commands the TAC system to operate in the Reduced Engine Power mode.
    • A message center or an indicator displays Reduced Engine Power.
    • Under certain conditions the control module commands the engine OFF.

Conditions for Clearing the MIL/DTC
    • The control module turns OFF the malfunction indicator lamp (MIL) after 3 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.

Reference Information

Schematic Reference

Engine Controls Schematics

Connector End View Reference

    •  Engine Control Module Connector End Views
    •  Engine Controls Connector End Views

Electrical Information Reference

    •  Circuit Testing
    •  Connector Repairs
    •  Testing for Intermittent Conditions and Poor Connections
    •  Wiring Repairs

Scan Tool Reference

    •  Scan Tool Data List
    •  Scan Tool Data Definitions

Circuit/System Verification

With the ignition ON, observe the DTCs with the scan tool.

⇒If other DTCs are set, diagnose them first. Refer to Diagnostic Trouble Code (DTC) List - Vehicle .

Circuit/System Testing
  1. With the ignition OFF, inspect the air cleaner outlet duct for loose connecting clamps, cracks, or other damage.
    2. ⇒Tighten any loose duct clamps, if necessary.
    ⇒If the air cleaner outlet duct is damaged, replace it.
  2. Remove and inspect the air cleaner element. The element should be clean, dry, and free from debris.
    3. ⇒If the air cleaner element is saturated with water or contains excessive dirt or debris, replace the element.
  3. Remove the MAF/intake air temperature (IAT) sensor. Inspect the housing for cracks or other damage. Inspect the sensor elements and diffuser for contamination, debris, or other damage.
    4. ⇒If the MAF/IAT sensor is contaminated or damaged, replace it.
  4. Remove the air duct attached to the throttle body. Inspect the throttle body for the following conditions:
    5. • Loose or damaged throttle blade
    • Broken throttle shaft
    • Any throttle body damage
    ⇒If any damage to the throttle body is found, replace the throttle body assembly.
  5. Inspect all vacuum lines for the following conditions:
    6. • Cracks
    • Contact with other components
    • Deterioration
    • Proper routing
    ⇒If any vacuum lines are damaged, replace them.
    ⇒If any vacuum lines are found to be incorrectly routed, correct the routing.
  6. Inspect the manifold absolute pressure (MAP) sensor and the sensor seal for damage.
    7. ⇒If the MAP sensor is damaged, replace it.
  7. Start the engine and inspect the following components for vacuum leaks:
    8. • Upper intake manifold and gaskets
    • Intake manifold tuning valve (IMTV) seal
    • Throttle body seal
    • Lower intake manifold and gaskets
    ⇒If any leaks are found, replace any parts as necessary.
  8. While observing the MAP Sensor parameter on the scan tool, start the engine. The MAP should decrease from the barometric pressure (BARO) to between 19-42 kPa at idle on a normally running engine, at operating temperature, with all accessories turned off.
    9. ⇒If the MAP does not respond accordingly, refer to DTC P0106, P0107, or P0108 for MAP sensor diagnosis.
  9. Turn OFF the ignition. Determine the altitude at your current location and refer to Altitude Versus Barometric Pressure .
  10. With the ignition ON and the engine OFF, observe the MAP Sensor parameter with the scan tool. The MAP should be within the range specified for your altitude.
    11. ⇒If the MAP is not within the range specified, refer to DTC P0106, P0107, or P0108 for MAP sensor diagnosis.
  11. Test the MAF sensor and related wiring for proper operation. Refer to DTC P0101 or P1101 .
    12. ⇒If problems are found with the MAF sensor wiring, MAF sensor, or ECM, repair as necessary.

Repair Instructions

Important: Always perform the Diagnostic Repair Verification after completing the diagnostic procedure.

    •  Air Cleaner Element Replacement for air cleaner element replacement
    •  Mass Airflow Sensor/Intake Air Temperature Sensor Replacement for MAF/IAT sensor replacement
    •  Air Cleaner Outlet Duct Replacement for air cleaner outlet duct replacement
    •  Lower Intake Manifold Replacement for lower intake manifold and gasket replacement
    •  Upper Intake Manifold Replacement for upper intake manifold, manifold gasket, and IMTV seal replacement
    •  Throttle Body Assembly Replacement for throttle body and seal replacement
    •  Manifold Absolute Pressure Sensor Replacement for MAP sensor replacement