GM Service Manual Online
For 1990-2009 cars only

Circuit Description

The throttle body assembly contains 2 throttle position (TP) sensors. The TP sensors are mounted to the throttle body assembly and are not serviceable. The TP sensors provide a signal voltage that changes relative to throttle blade angle. The engine control module (ECM) supplies the TP sensors with a common 5-volt reference circuit, a common low reference circuit, and 2 independent signal circuits.

The TP sensors have opposite functionality. TP sensor 1 signal voltage decreases from above 4 volts at idle to below 1 volt at wide open throttle (WOT). TP sensor 2 signal voltage increases from below 1 volt at idle to above 4 volts at WOT.

If the ECM detects TP sensor 1 signal voltage is too low or too high, this DTC sets.

DTC Descriptor

This diagnostic procedure supports the following DTC:

DTC P0120 Throttle Position (TP) Sensor 1 Circuit

Conditions for Running the DTC

    • DTCs P0341, P0351, P0601, P0602, P0603, P0604, P0606, P0607 are not set.
    • The ignition is ON, with the engine OFF, or the engine is operating.
    • The ignition 1 voltage is more than 5.23 volts.
    • DTC P0120 runs continuously once the above conditions are met.

Conditions for Setting the DTC

The ECM detects the TP sensor 1 signal voltage is less than 0.275 volts for less than 1 second.

The ECM detects the TP sensor 1 signal voltage is more than 4.725 volts for less than 1 second.

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

    • Use the J 35616 Connector Test Adapter Kit for any test that requires probing the ECM harness connector or a component harness connector.
    • If there is a condition with the TP sensors, the ECM defaults to reduced power mode for the entire ignition cycle, even if the condition is corrected.
    • For an intermittent condition, refer to Intermittent Conditions .

Test Description

The number below refers to the step number on the diagnostic table.

  1. This step verifies that other DTCs are set.

Step

Action

Yes

No

Schematic Reference: Engine Controls Schematics

Connector End View Reference: Engine 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 in Vehicle DTC Information

2

Are any other DTCs set?

Go to Diagnostic Trouble Code (DTC) List - Vehicle in Vehicle DTC Information

Go to DTC P0121