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
| • | 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. |
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 P0121 diagnostic has not yet run. When the DTC P0121 does not appear in the Not Ran Since Code Cleared list, the P0121 diagnostic has run. If the MIL is NOT illuminated, and there is no pending DTC Status in DTC Information, the 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.
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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.
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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 P0121 diagnostic has not yet run. When the DTC P0121 does not appear in the Not Ran Since Code Cleared list, the P0121 diagnostic has run. If the MIL is NOT illuminated, and there is no Pending DTC Status in DTC Information, the 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.
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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.
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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 | ||||||||||||||
Did you perform the Diagnostic System Check-Engine Controls? | -- | Go to Step 2 | ||||||||||||
2 | Is DTC P0122 or DTC P0123 set? | -- | Go to Step 3 | |||||||||||
3 |
Is the TP sensor voltage within both specified values? | 0.5-1.2 V at idle 3.4-4.7 V at WOT | Go to Step 4 | Go to Step 5 | ||||||||||
Is DTC P0121 set? | -- | Go to Step 5 | Go to Diagnostic Aids | |||||||||||
5 |
Is the resistance within the specified range? | 4-6k ohms | Go to Step 6 | Go to Step 8 | ||||||||||
6 |
Is the resistance within the specified range? | 0.5-1.0k ohms | Go to Step 7 | Go to Step 8 | ||||||||||
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 |
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 | -- | ||||||||||
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? | -- | System OK | |||||||||||
