GM Service Manual Online
For 1990-2009 cars only
Makes 🢒 Isuzu 🢒 2007 🢒 Isuzu - N Series 🢒 Engine 🢒 Engine Controls and Fuel - 4.8L, 5.3L, and 6.0L 🢒 DTC P0068

Circuit Description

The powertrain control module (PCM) uses the following readings to calculate the predicted mass air flow (MAF) rate:

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

The PCM compares the predicted MAF value to the actual MAF value, and to the speed density calculation, in order to verify the proper throttle operation.

DTC Descriptor

This diagnostic procedure supports the following DTC:

DTC P0068 Throttle Body Airflow Performance

Conditions for Running the DTC
    • DTCs P0601, P0602, P0604, P0606, P1516, P2101, P2108, U0107 are not set.
    • DTCs P0120 and P0220 are not active at the same time.
        
    • The engine operates longer than 1 second.
    • The engine speed is more than 500 RPM.
    • DTC P0068 runs continuously once the above conditions are met.

Conditions for Setting the DTC

The PCM detects that the difference between the actual airflow and the speed density calculated air flow is greater than expected for more than 1.88 seconds.

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.

Diagnostic Aids
    • Inspect the throttle blade for being broken, bent, or missing.
    • Inspect the TP sensor for proper installation. A sensor that is misaligned could set this DTC.
    • Inspect the throttle actuator control (TAC) module connectors for signs of water intrusion. If water intrusion occurs, multiple DTCs may set without any circuit or component conditions found during diagnostic testing.
    • Physically and visually inspect the throttle body assembly and correct any problems that you observe. Manually move the throttle blade from closed to wide open throttle (WOT). You should not need to use excess force. The throttle blade should move smoothly through the full range, then should independently return to a slightly open position.
    • When the TAC module detects a condition within the TAC system, more than one TAC system related DTC may set. This is due to the many redundant tests run continuously on this system. Locating and repairing one individual condition may correct more than one DTC. Disconnecting components during testing may set additional DTCs. Remember this if you review the stored information in Capture Info.
    • For an intermittent condition, refer to Testing for Intermittent Conditions and Poor Connections .

Test Description

The numbers below refer to the step numbers on the diagnostic table.

  1. This step will determine if the manifold absolute pressure (MAP) sensor voltage is within the proper range at idle.

    2.

  2. This step will determine if the MAP sensor responds properly to the change in manifold pressure.

    3.

  3. A throttle blade that sticks or binds may set this code. Opening the throttle through the entire range will indicate problems such as these.

    4.

  4. When the PCM detects a condition within the electronic temperature control (ETC) system, other DTCs may set due to the many redundant tests run continuously on this system. Locating and repairing one individual condition may correct more than one DTC. Keep this in mind when reviewing captured DTC info.

Step

Action

Values

Yes

No

Schematic Reference: Engine Controls Schematics

Connector End View Reference: Powertrain 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

Are any other DTCs set?

--

Go to Diagnostic Trouble Code (DTC) List - Vehicle

Go to Step 3

3
  1. Observe the Freeze Frame/Failure Records for this DTC.
  2. Turn OFF the ignition.
  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 4

Go to Diagnostic Aids

4

Inspect for the following conditions:

    • 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

Did you find and correct the condition?

--

Go to Step 8

Go to Step 5

5

  1. Allow the engine to reach operating temperature.
  2. Observe the MAP sensor voltage parameter with a scan tool.

Is the manifold absolute pressure (MAP) sensor voltage within the specified range?

0.8-2 V

Go to Step 6

Go to DTC P0106

6

  1. Idle the engine.
  2. Observe the MAP sensor kPa parameter with a scan tool.
  3. Increase the engine speed slowly and then back to idle.

Does the MAP sensor kPa change smoothly and gradually as engine speed is increased and returned to idle?

--

Go to Step 7

Go to DTC P0106

7

    Caution: Turn OFF the ignition before inserting fingers into the throttle bore. Unexpected movement of the throttle blade could cause personal injury.

  1. Inspect the throttle body for the following conditions while modulating the throttle through the entire range using the scan tool:
    2. • Loose or damaged throttle blade
    • Broken throttle shaft
    • Drive mechanism damage
  2. If any of the above conditions exist, replace the throttle body assembly. Refer to Throttle Body Assembly Replacement .

Did you find and correct the condition?

--

Go to Step 6

Go to Diagnostic Aids

8
  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 7

9

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