Circuit Description
The throttle position (TP) sensor contains a potentiometer whose resistance value changes as the throttle valve position changes. 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 converts the voltage into a throttle angle. The TP sensor resistance decreases as the throttle valve opening increases, and increases as the throttle valve opening decreases. The TP sensor voltage increases as the throttle valve opening increases.
Conditions for Running the DTC
| • | The engine speed is between 1,600-3,000 RPM. |
| • | The engine coolant temperature is between 70°C (158°F) and 110°C (230°F). |
| • | The intake air temperature is between -10° and +70°C (14°-158°F). |
| • | The fuel level is above 25%. |
| • | The arometric pressure is more than 75 kPa. |
| • | The change in MAP is less than 13 kPa per 16 firing events. |
Conditions for Setting the DTC
| • | The difference between the expected TP sensor reading and the actual input is more than 20 degrees. |
| • | The above condition is present 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 is stored in the Freeze Frame buffer. |
Conditions for Clearing the MIL/DTC
| • | The MIL turns OFF after 3 consecutive trips during which the diagnostic runs and no fault is detected. |
| • | A History DTC clears after 40 consecutive warm-up cycles without a fault. |
| • | Use the scan tool Clear DTC Information function or disconnect the PCM battery feed in order to clear the DTC. |
Diagnostic Aids
Check for any of the following conditions:
| • | Check the resistance of the TP sensor. TP sensor resistance between terminals 1 and 2 should be 2.5k ohms to 6.0k ohms. TP sensor resistance between terminals 1 and 3 should be 100 ohms to 20k ohms. Replace the TP sensor if the measured resistance is not within the specified values. Refer to Throttle Position Sensor Replacement . |
| • | A faulty electrical connection to the PCM. |
An intermittent malfunction may be caused by a fault in the TP sensor electrical circuit. Inspect the wiring harness and components for any of the following conditions:
| • | Backed out terminals. |
| • | Improper mating of terminals. |
| • | Broken electrical connector locks. |
| • | Improperly formed or damaged terminals. |
| • | Faulty terminal to wire connections. |
| • | Physical damage to the wiring harness. |
| • | A broken wire inside the insulation. |
| • | Corrosion of electrical connections, splices, or terminals. |
If a 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 Powertrain OBD system check prompts the technician to complete some basic checks, and to save the freeze frame data on the scan tool, if applicable. This creates an electronic copy of the data that was taken when the fault occurred. The scan tool saves the information for later reference.
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Perform this check with the vehicle at operating temperature.
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This step checks the TP sensor signal circuit for a short or for an open. This step also checks the TP sensor ground circuit for a open.
Step | Action | Value(s) | Yes | No | ||||||
|---|---|---|---|---|---|---|---|---|---|---|
Did you perform the Powertrain On-Board Diagnostic (OBD) System Check? | -- | |||||||||
Is the TP sensor voltage within both specified values? | 0.2-1.0 volts at idle 3.0-4.8 volts at WOT | Go to Diagnostic Aids | ||||||||
3 |
Is the TP sensor voltage between the specified value? | 4-5 V | ||||||||
4 | Measure the voltage from the harness side of the TP sensor signal circuit to ground with the DMM. Is the voltage on the TP sensor signal circuit within the specified value? | 4-5 V | ||||||||
Was a repair necessary? | -- | |||||||||
6 |
Was a repair necessary? | -- | ||||||||
7 | Replace the TP sensor. Refer to Throttle Position Sensor Replacement . Is the action complete? | -- | -- | |||||||
8 | Replace the PCM. Is the action complete? | -- | -- | |||||||
9 |
Are any DTCs displayed on the scan tool? | -- | Go to the applicable DTC table | System OK |
