GM Service Manual Online
For 1990-2009 cars only
Makes 🢒 Cadillac 🢒 2008 🢒 XLR 🢒 Diagnostic Navigation 🢒 Vehicle Diagnostic Information 🢒 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 1 Performance

Circuit/System Description

The engine control module (ECM) uses the following information to calculate an expected airflow rate:

    • The throttle position (TP)
    • The barometric pressure (BARO)
    • The manifold absolute pressure (MAP)
    • The intake air temperature (IAT)
    • The engine RPM
    • The mass air flow (MAF)

Conditions for Running the DTC

P0068

    • DTCs P0122, P0123, P0220, P0221, P0222, P0223, P0601, P0602, P0603, P0604, P0606, P060D, P062F, P0641, P0651, P0652, P0653, P1516, P2101, P2119, P2135, P2176 are not set.
    • The engine is running.
    • The DTCs run continuously when the above conditions are met.

P0121

    • DTCs P0102, P0103, P0107, P0108, P0112, P0113, P0116, P0117, P0118, P0122, P0123, P0128, P0220, P0221, P0222, P0223, P0335, P0336, P0401, P0405, P0601, P0602, P0603, P0604, P0606, P060D, P062F, 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 intake air temperature (IAT) is between -7 and +60°C (+19 and +140°F).
    • The engine is running.
    • The DTC runs continuously when the above conditions are met.

Conditions for Setting the DTC

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

Action Taken When the DTC Sets
    • DTC P0068 is a Type A DTC.
    • DTC P0121 is a Type B DTC.
    • The ECM commands the TAC system to operate in 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. If DTCs P0101, P0106, or P0120 are set, refer to Diagnostic Trouble Code (DTC) List - Vehicle for further diagnosis.
  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 and refer to Altitude Versus Barometric Pressure.
  3. Engine running, observe the DTC information with a scan tool. DTCs P0068 or P0121 should not set.
  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 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 Information 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 MAP Sensor Voltage parameter with a scan tool. Voltage should be greater than 0.8 volt and less than 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 and then back to idle. The MAP sensor kPa should change smoothly and gradually as engine speed is increased and returned to idle.
    4. ⇒If the MAP sensor kPa does not change refer to DTC P0106.
  4. Take a snapshot of the engine data list while performing the actions listed below. The mass air flow (MAF) sensor g/s should change smoothly and gradually as the engine speed is increased and is returned to idle.
    5. 4.1. Idle the engine.
    4.2. Increase the engine speed slowly to 3,000 RPM, then back to idle.
    4.3. Exit from the snapshot and review the data.
    4.4. Observe the Mass Air Flow (MAF) Sensor parameter frame by frame with a scan tool. The MAF sensor g/s should change smoothly and gradually as the engine speed is increased and is returned to idle.
    ⇒If the MAF sensor g/s 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
         ⇒If any of these conditions exist, replace the throttle body assembly.

Repair Instructions

Perform the Diagnostic Repair Verification after completing the diagnostic procedure.

    •  Air Cleaner Element Replacement
    •  Manifold Absolute Pressure Sensor Replacement
    •  Mass Airflow Sensor/Intake Air Temperature Sensor Replacement
    •  Throttle Body Assembly Replacement
    •  Intake Manifold Replacement