GM Service Manual Online
For 1990-2009 cars only
Makes 🢒 Chevrolet 🢒 2004 🢒 Tracker - 4WD 🢒 DTC 🢒 DTC P0121

Circuit Description

The DTC P0121 Throttle Position (TP) Sensor Performance diagnostic checks the accuracy of the TP sensor signal. The TP sensor is a potentiometer whose resistance value changes along with the throttle valve position. The powertrain control module (PCM) provides a 5-volt reference voltage to the TP sensor. The PCM reads the voltage across the TP sensor and uses the voltage to calculate the percentage of throttle valve opening. The TP sensor resistance decreases as the throttle valve opening increases. When the TP sensor resistance decreases the voltage being monitored by the PCM increases. The TP sensor resistance increases when the throttle valve opening decreases, therefore the voltage being monitored by the PCM will decrease.

Throttle Opening

TP Resistance

TP Percentage

TP Signal Voltage

Large

High

High

High

Small

Low

Low

Low

Conditions for Running the DTC
    • DTC P0101, DTC P0102, DTC P0103, DTC P0116, DTC P0117, DTC P0118, DTC P0122, DTC P0123, DTC P0335, and DTC P1451 are not set.
    • The engine speed is less than 3,500 RPM.
    • The mass air flow (MAF) sensor is indicating that the maximum flow rate--minimum flow rate is more than or equal to 25.5 g/s.
    • The manifold absolute pressure (MAP) sensor is less than 26 kPa and the throttle valve opening is less than 35 degrees.
    • The MAP sensor is more than 67 kPa and the throttle valve opening is more than 35 degrees.

Conditions for Setting the DTC
    • The difference between expected TP sensor voltage and actual input is more than a specified value.
    • The above conditions are met for at least 3 seconds.

Action Taken When the DTC Sets
    • The PCM illuminates the malfunction indicator lamp (MIL) after two consecutive ignition cycles in which the diagnostic runs with the fault active.
    • The PCM records the operating conditions at the time the diagnostic fails. This information will be stored in the Freeze Frame buffer.

Conditions for Clearing the MIL/DTC
    • The MIL turns OFF after three consecutively passing cycles without a fault present.
    • A history DTC clears after 40 consecutive warm-up cycles without a fault.
    • Perform the scan tool clear DTC Information function in order to clear the DTC.

Diagnostic Aids

Check for any of the following conditions:

    • If a DTC P0122 or P0123 are also set, diagnose those DTCs first.
    • A damaged wiring harness--Inspect the wiring harness for damage. If the harness appears to be OK, observe the TP display on the scan tool while moving connectors and wiring harnesses related to the TP sensor. A change in the TP display will indicate the location of the fault.
    • A binding or sticking throttle plate may cause a DTC P0121 to set.
    • On a cold engine the TP sensor may indicate 3 percent or more with the throttle closed at idle. After the engine warms up and the plunger of the fast idle-up valve has retracted, the TP sensor will indicate 0 percent at idle.

If a DTC P0121 is intermittent, driving the vehicle under the following conditions can verify whether the fault is present. Perform the scan tool clear DTC information function. Road test the vehicle while monitoring the DTC P0121 diagnostic on the scan tool under the Not Ran Since Code Cleared selection in the DTC Information menu. If a DTC P0121 appears in the Not Ran Since Code Cleared list, the DTC P0121 diagnostic has not yet run. When the DTC P0121 does not appear in the Not Ran Since Code Cleared list, the DTC P0121 diagnostic has run. If the MIL is NOT ILLUMINATED, and there is no PENDING DTC Status in DTC Information, the DTC P0121 diagnostic has passed. DTCs MUST BE CLEARED in order to view the CURRENT STATUS of the Not Ran Since Code Cleared list. DO NOT FORGET that the Not Ran Since Code Cleared list only indicates that the test has run, not whether the test passed or failed. The DTC Information screen must be checked for CURRENT or PENDING status, in order to determine the outcome of the diagnostic test involved.

An intermittent malfunction may be caused by a fault in the TP sensor electrical circuit. Inspect the wiring harness and components for an intermittent condition. Refer to Intermittent Conditions .

If the DTC P0121 cannot be duplicated, the information included in the Freeze Frame data can be useful in determining vehicle operating conditions when the DTC was first set.

Test Description

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

  1. The Diagnostic System Check-Engine Controls prompts the technician to complete some basic checks and store the Freeze Frame data on the scan tool if applicable. This creates an electronic copy of the data taken when the fault occurred. The information is then stored in the scan tool for later reference.

    2.

  2. This step checks whether the malfunction that caused the DTC P0121 is still present. Driving the vehicle under these conditions will verify whether the fault is present. After performing the scan tool clear DTC information function, the DTC P0121 diagnostic can be monitored on the scan tool under the Not Ran Since Code Cleared selection in the DTC Information menu. If a DTC P0121 appears in the Not Ran Since Code Cleared list, the DTC P0121 diagnostic has not yet run. When the DTC P0121 does not appear in the Not Ran Since Code Cleared list, the DTC P0121 diagnostic has run. If the MIL is NOT illuminated, and there is no Pending DTC Status in DTC Information, the DTC P0121 diagnostic has passed. The DTCs MUST be cleared in order to view the Current Status of the Not Ran Since Code Cleared list. DO NOT forget that the Not Ran Since Code Cleared list only indicates that the test has run, not whether the test passed or failed. The DTC Information screen must be checked for Current or Pending status, in order to determine the outcome of the diagnostic test involved.

    3.

  3. This step inspects the TP sensor performance. If there are any breaks, dips, or fixed values observed in the resistance readings while opening and closing the throttle, the TP sensor is faulty.

    4.

  4. Clearing the DTCs by removing the DOME fuse also clears the PCM long term memory. Using the scan tool to clear DTCs leaves the long term memory active. DTC emitting diagnostics may not run to completion as expected unless the long term memory is cleared. Removal of the DOME fuse also clears the time display of the radio. The DTC P0121 diagnostic can be monitored on the scan tool under the Not Ran Since Code Cleared selection in the DTC Information menu. If a DTC P0121 appears in the Not Ran Since Code Cleared list, the DTC P0121 diagnostic has not yet run. When the DTC P0121 does not appear in the Not Ran Since Code Cleared list, the DTC P0121 diagnostic has run. If the MIL is NOT illuminated, and there is no Pending DTC Status in DTC Information, the DTC P0121 diagnostic has passed. The DTCs MUST be cleared in order to view the Current Status of the Not Ran Since Code Cleared list. DO NOT forget that the Not Ran Since Code Cleared list only indicates that the test has run, not whether the test passed or failed. The DTC Information screen must be checked for Current or Pending status, in order to determine the outcome of the diagnostic test involved.

Step

Action

Value(s)

Yes

No

Schematic Reference: Engine Controls Schematics

1

Did you perform the Diagnostic System Check-Engine Controls?

--

Go to Step 2

Go to Diagnostic System Check - Engine Controls

2

Is DTC P0122 or DTC P0123 set?

--

Go to DTC P0122 or DTC P0123

Go to Step 3

3
  1. Operate the engine until a normal operating temperature is reached.
  2. Turn ON the ignition, leaving the engine OFF.
  3. Install a scan tool.
  4. Observe the throttle position (TP) sensor voltage parameter on the scan tool.

Is the TP sensor voltage within both specified values?

0.35-1.2 V at idle

3.4-4.7 V at WOT

Go to Step 4

Go to Step 5

4

  1. Perform the scan tool Clear DTC Information function.
  2. Operate the vehicle within the Freeze Frame conditions as specified, or until the DTC P0121 diagnostic has run. Refer to the Test Description.

Is DTC P0121 set?

--

Go to Step 5

Go to Diagnostic Aids

5
  1. Turn OFF the ignition.
  2. Disconnect the TP sensor electrical connector.
  3. Connect a DMM between terminal 1 and terminal 3.
  4. Measure the resistance between the TP sensor terminal 1 and terminal 3 with a DMM.

Is the resistance within the specified range?

4-6k ohms

Go to Step 6

Go to Step 8

6
  1. Check that the throttle plate is closed.
  2. Connect a DMM between terminal 2 and terminal 3.
  3. Measure the resistance between the TP sensor terminal 2 and terminal 3 with the DMM.

Is the resistance within the specified range?

0.5-1.0k ohms

Go to Step 7

Go to Step 8

7

  1. Leave the DMM connected between terminal 2 and terminal 3.
  2. Slowly move the throttle from closed to open while observing the DMM.

Does the resistance value change smoothly from low to high, while staying within the specified range?

0.5-6.0k ohms

Go to Step 9

Go to Step 8

8

Replace the TP sensor. Refer to Throttle Position Sensor Replacement .

Is the action complete?

--

Go to Step 11

--

9
  1. Inspect the TP sensor electrical circuits for any of the following conditions:
    2. • A short to ground
    • A short to voltage
    • A high electrical resistance
    • An intermittent or faulty connection
  2. Repair as necessary. Refer to Wiring Repairs in Wiring Systems.

Was a repair necessary?

--

Go to Step 11

Go to Step 10

10

Replace the PCM. Refer to Powertrain Control Module Replacement .

Is the action complete?

--

Go to Step 11

--

11

  1. Turn OFF the ignition.
  2. Remove the DOME fuse for 30 seconds.
  3. Install the DOME fuse.
  4. Reset the radio clock time. Refer to Radio/Audio System Description and Operation in Entertainment.
  5. Start the engine.
  6. Operate the vehicle within the Conditions for Running the DTC as specified in the supporting text or until the DTC P0121 diagnostic test has run.

Does the DTC run and pass?

--

Go to Step 12

Go to Step 2

12

With a scan tool, observe the stored information, Capture Info.

Does the scan tool display any DTCs that you have not diagnosed?

--

Go to Diagnostic Trouble Code (DTC) List

System OK