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Roy
Roy, import/domestic specialest
Category: Chevy
Satisfied Customers: 5298
Experience:  ASE master tech for 40 years
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Re-occurring code po121 on a chevy pick up 5.3 engine Not

Customer Question

re-occurring code po121 on a chevy pick up 5.3 engine Not sure if I should change the throttle body or the electronic pedal
JA: Thanks. Can you give me any more details about your issue?
Customer: truck wont pass emisssions test in Pa. due to check eng light coming on and drive monitors difficult to reset pa. only allows 1 monitor not reset on this yr truck to be able run emission test Truck runs fine Has 170 k on it can clear code but returns on 2nd key on cycle throttle body has been cleaned but code keeps coming back and starts process forstate req d test not sure how to diagnose throttle body with tps or pedal w switch on it
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Submitted: 1 year ago.
Category: Chevy
Expert:  Roy replied 1 year ago.

god evening my name is ***** ***** I will assist

what is the year, make, model??

Customer: replied 1 year ago.
2007 chevy Silverado 4wd 5.3 eng
Expert:  Roy replied 1 year ago.

ok, thanks. I attached the description for you to view. in most cases, it is a failed throttle body as you have no other codes

Roy

Diagnostic Instructions

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

Conditions for Running the DTC

P0068

  • DTCs P0641, P0651, P1516, P2101, P2119, P2176 are not set.
  • The engine is running.
  • DTC P0068 run continuously when the above conditions are met.

P0121

  • DTCs P0102, P0103, P0107, P0108, P0112, P0113, P0116, P0117, P0118, P0128, P0315, P0335, P0336 are not set.
  • The engine speed is more than 450 RPM .
  • DTC P0121 run continuously when the above conditions are met.

Conditions for Setting the DTC

P0068

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

P0121

The predicted air flow and the predicted MAP combined are outside a calibrated range for more than 3 seconds .

Action Taken When the DTC Sets

P0068

  • DTC P0068 is a Type A DTC.
  • The control module commands the throttle actuator control (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.

P0121

DTC P0121 is a Type B DTC.

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 Control Schematics, See: Powertrain Management\Diagrams\Electrical

Connector End View Reference

Engine Control Module Connector End Views, See: Powertrain Management\Computers and Control Systems\Engine Control Module\Diagrams\Connector Views

Electrical Information Reference

DTC Type Reference

Powertrain Diagnostic Trouble Code (DTC) Type Definitions, See: Diagnostic Trouble Code Descriptions\Diagnostic Trouble Code (DTC) Type Definitions

Scan Tool Reference

Scan Tool Data List, See: Powertrain Management\Computers and Control Systems\Testing and Inspection\Scan Tool Testing and Procedures\Scan Tool Data List

Circuit/System Testing

  1. 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
    • Throttle body for dirt, debris, and coking-Refer to Throttle Body Cleaning.
  1. Allow the engine to reach operating temperature. Observe the MAP Sensor Voltage parameter with a scan tool. Voltage should be more than 0.8 volt and less than 4 volts .

¤If more than 4 volts or less than 0.8 volt , refer to DTC P0106. See: P0106

  1. 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.

¤If the MAP sensor kPa does not change, refer to DTC P0106. See: P0106

  1. 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.
    • Idle the engine.
    • Increase the engine speed slowly to 3,000 RPM , then back to idle.
    • Exit from the snapshot and review the data.
    • 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. See: P0101

  1. Inspect the throttle body for the following conditions:
    • 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. See: Verification Tests

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