GM Service Manual Online
For 1990-2009 cars only
Makes 🢒 Cadillac 🢒 2008 🢒 SLS 🢒 DTC 🢒 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 Description

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

    • Throttle position (TP)
    • Barometric pressure (BARO)
    • Manifold absolute pressure (MAP)
    • Intake air temperature (IAT)
    • Engine speed
    • Mass air flow (MAF)

Conditions for Running the DTC

P0068

    • The engine is running and the engine speed is more than 800 RPM.
    • DTC P0068 runs 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 70-121°C (158-249°F).
    • The engine IAT is at 0.7 to +60°C (+19 to +140°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

The ECM detects that the throttle position and indicated engine load do not correspond with the expected load and throttle position for less than 1 second.

Conditions for Setting the DTC
    • The ECM detects that the difference between the MAP sensor pressure value and expected value is greater than the specified value.
    • The ECM detects that the difference between the MAF value and the desired value is greater than the specified value.

Action Taken When the DTC Sets
    • DTC P0068 is a Type A DTC.
    • DTC P0121 is a Type B DTC.
    • The control module commands the TAC system to operate in Reduced Engine Power mode.
    • The message center or the indicator displays Reduced Engine Power.

Conditions for Clearing the 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

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

Electrical Information Reference

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

DTC Type Reference

Diagnostic Trouble Code (DTC) Type Definitions

Scan Tool Reference

    •  Scan Tool Data List
    •  Scan Tool Data Definitions
    •  Engine Control Module Scan Tool Information

Circuit/System Verification
  1. If DTC P0101, P0106 or P0120 is set, for further diagnosis, refer to Diagnostic Trouble Code (DTC) List - Vehicle .
  2. Ignition ON, observe the scan tool MAP sensor parameters. The MAP sensor parameter should be within the range specified for your altitude. Determine the altitude for your current area and refer to the Altitude Versus Barometric Pressure .

Circuit/System Testing
  1. Ignition OFF, inspect the throttle body for the following conditions:
    2. • A loose or damaged throttle blade
    • A broken throttle shaft
    • Any throttle body damage
         ⇒If any of these conditions exists, replace the throttle body assembly.
  2. With the engine running, inspect the whole air intake system for the following conditions:
    3. • The vacuum hoses for splits, leaks and proper connection
    • Air leaks at throttle body mounting area and intake manifold sealing surfaces
         ⇒If a condition is found, repair as necessary.
  3. 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.
    4. ⇒If more than 4 volts or less than 0.8 volt, refer to DTC P0106 .
  4. Operate the engine at idle. Observe the MAP Sensor kPa parameter with a scan tool. Increase the engine speed slowly to 3,000 RPM, and then allow the engine speed to return to idle speed. The MAF sensor kPa should change smoothly and gradually as the engine speed is increased and is returned to idle.
    5. ⇒If the MAP sensor kPa does not change, refer to DTC P0106 .
  5. Take a snapshot of the engine data list while performing the actions listed below:
    6. 5.1. Run the engine at idle speed.
    5.2. Increase the engine speed slowly to 3,000 RPM, then return to idle.
    5.3. The MAF sensor parameter should change smoothly and gradually as the engine speed is increased and returned to idle. The MAF sensor parameter should change smoothly and gradually as the engine speed is increased and returned to idle.
    ⇒If the MAF sensor parameter doesn't change smoothly and gradually as the engine speed increases and returns to idle, refer to DTC P0101 or P1101 .

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