GM Service Manual Online
For 1990-2009 cars only
Makes 🢒 Buick 🢒 2009 🢒 Lucerne 🢒 Driver Information and Entertainment 🢒 Cellular, Entertainment, and Navigation 🢒 DTC P0068 or P0121

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 Descriptors

DTC P0068: Throttle Body Airflow Performance

DTC P0121: Throttle Position (TP) Sensor Performance

Circuit/System Description

The actual throttle position and airflow rate is compared to an expected throttle position and airflow rate based on engine load. The engine control module (ECM) determines engine load based on inputs from the mass air flow (MAF) sensor and the manifold absolute pressure (MAP) sensor. Further comparisons are done to distinguish if a sensor is at fault, and the respective DTC will set.

The ECM uses the following information to calculate an expected airflow rate:

    • The throttle position (TP)
    • The barometric pressure (BARO)
    • The MAP
    • The intake air temperature (IAT)
    • The engine RPM

Conditions for Running the DTC

P0068

    • DTCs P0122, P0123, P0220, P0221, P0222, P0223, P0601, P0602, P0603, P0604, P060D, P062F, P0606, P0652, P0653, P1516, P2101, P2119, P2135, P2176 are not set.
    • The engine is running.
    • DTC P0068 runs continuously when the above conditions are met.

P0121

    • DTCs P0102, P0103, P0107, P0108, P0112, P0113, P0117, P0118, P0116, P0128, P0122, P0123, P0220, P0221, P0222, P0223, P0335, P0336, P0401, P0405, P0601, P0602, P0603, P0604, P060D, P062F, P0606, P0652, P0653, P1404, P1516, P2101, P2119, P2135, P2176 are not set.
    • The engine coolant temperature (ECT) is between 70-125°C (158-257°F).
    • The engine IAT is between -7 and +100°C (+19 and +212°F).
    • The engine speed is between 550-5,000 RPM.
    • DTC P0121 runs continuously when the above conditions are met.

Conditions for Setting the DTC

P0068

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

P0121

the ECM determines that the TP sensor is stuck within the normal range for more than 0.2 second.

Action Taken When the DTC Sets

P0068

    • DTC P0068 is a Type A DTC.
    • DTC P0121 is a Type B DTC.
    • The ECM 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
    • DTC P0068 is a Type A DTC.
    • DTC P0121 is a Type B DTC.

Reference Information

Schematic Reference

Engine Controls Schematics

Connector End View Reference

Component Connector End Views

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. Verify that DTCs P0101, P0106, or P0120 are not set.
    2. ⇒If any of the DTCs are set, refer to Diagnostic Trouble Code (DTC) List - Vehicle .
  2. Ignition ON, observe the scan tool MAP Sensor parameter. The MAP Sensor parameter should be within the range specified for your altitude. Determine the altitude at your current location. Refer to Altitude Versus Barometric Pressure .
  3. 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
  1. Inspect for the following conditions:
    2. • Vacuum hoses for splits, kinks, and proper connections as shown on Vehicle Emission Control Label
    • Inspect thoroughly for any type of leak or restriction.
    • Air leaks at throttle body mounting area and intake manifold sealing surfaces
    • Inspect throttle body for dirt, debris, and coking. Refer to Throttle Body Inspection and Cleaning .
  2. Allow the engine to reach operating temperature. Observe the Manifold Absolute Pressure (MAP) Sensor Voltage parameter with a scan tool. The voltage should be between 0.8-4 volts.
    3. ⇒If greater than 4 volts or less than 0.8 volt, refer to DTC P0106 .
  3. Idle the engine. Observe the MAP Sensor kPa parameter with a scan tool. Increase the engine speed slowly to 3,000 RPM, then return the engine speed to a stabilized idle. The MAP sensor kPa parameter should change smoothly as the engine speed is increased and returned to idle.
    4. ⇒If the MAP sensor kPa parameter does not change, refer to DTC P0106 .
  4. Take a snapshot of the engine data list while performing the actions listed below. The MAF sensor g/s parameter should change smoothly as the engine speed is increased and returned to an idle.
    5. 4.1. Idle the engine.
    4.2. Increase the engine speed slowly to 3,000 RPM, then return to an idle.
    4.3. Observe the Mass Air Flow (MAF) Sensor parameter frame by frame with a scan tool. The MAF sensor parameter should change smoothly and gradually as the engine speed is increased and is returned to idle .
    ⇒If the MAF Sensor parameter does not change smoothly and gradually as the engine speed is increased and is returned to idle, refer to DTC P0101 or P1101 .
  5. Inspect the throttle body for the following conditions:
    6. • Loose or damaged throttle blade
    • Broken throttle shaft
    • Any throttle body damage
  6. If all conditions test normally, test or replace the throttle body assembly.

Repair Instructions

Perform the Diagnostic Repair Verification after completing the diagnostic procedure.

    •  Throttle Body Assembly Replacement
    •  Control Module References for ECM replacement, setup, and programming