Circuit Description
The DTC P0121 Throttle Position (TP) Sensor Performance diagnostic monitors the sensor activity. 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
| • | The engine speed is increasing. |
| • | The engine is operating in Closed Loop. |
| • | DTC P0121 runs continuously once the above condition has been met. |
Conditions for Setting the DTC
The vehicle speed has exceeded 30 km/h (19 mph), and the output value of the throttle position sensor is out of the applicable range.
Action Taken When the DTC Sets
| • | The PCM illuminates the malfunction indicator lamp (MIL) during the second key cycle in which the diagnostic trouble code (DTC) sets. |
| • | The PCM stores the conditions which were present when the DTC set as Freeze Frame data. |
Conditions for Clearing the MIL/DTC
| • | The PCM turns OFF the MIL on the third consecutive trip cycle during which the diagnostic has been run and the fault condition is no longer present. |
| • | A DTC clears after 40 consecutive warm-up cycles have occurred without a fault. |
| • | A DTC can be cleared by using the scan tool Clear DTC Information function. |
Diagnostic Aids
Inspect the throttle valve linkage and lever assembly for free and normal movement. Binding or misalignment of the throttle valve can affect the TP sensor input. Check that the throttle lever has not been bent or tampered with. Check that the throttle valve opens fully, and the lever contacts the throttle stop.
On a good TP sensor the scan tool will typically display a TP angle of 8-10 percent at idle, and 76-77 percent at wide open throttle (WOT).
An intermittent malfunction may be caused by fault in the TP sensor electrical circuit. Inspect the wiring harness and the components for an intermittent condition. Refer to Intermittent Conditions .
Repair any electrical circuit faults that were found. Refer to Wiring Repairs in Wiring Systems.
The information included in the Freeze Frame data can be useful in determining the vehicle operating conditions when the DTC first set.
Test Description
The numbers below refer to the step numbers on 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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The throttle angle percentage should steadily increase without binding or a large percentage jump.
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This step determines if DTC P0121 is the result of a hard failure or an intermittent condition.
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This step inspects for an open or a short in the FTP sensor 5-volt reference.
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This step inspects for excessive resistance in the reference voltage circuit. If the test lamp does not illuminate at all, there is resistance in the circuit.
-
When the 5-volt reference circuit is jumpered to the TP sensor input circuit, the scan tool parameter should read near 100 percent.
Step | Action | Values | Yes | No | ||||
|---|---|---|---|---|---|---|---|---|
Schematic Reference: Engine Controls Schematics | ||||||||
Did you perform the Diagnostic System Check-Engine Controls? | -- | Go to Step 2 | ||||||
Does the TP angle value increase steadily when the accelerator pedal is depressed and decrease steadily when the pedal is released, within the specified range? | 8-80% | Go to Step 3 | Go to Step 4 | |||||
Does DTC P0121 set? | -- | Go to Step 4 | Go to Diagnostic Aids | |||||
Is the TP sensor voltage at the specified value? | 5 V | Go to Step 5 | Go to Step 6 | |||||
Does the test lamp illuminate at least dimly? | -- | Go to Step 7 | Go to Step 9 | |||||
6 |
Did you complete the repair? | -- | Go to Step 12 | -- | ||||
Is the TP sensor percentage more than the specified value? | 90% | Go to Step 10 | Go to Step 8 | |||||
8 |
Was a repair necessary? | -- | Go to Step 12 | Go to Step 11 | ||||
9 | Locate and repair the high resistance in the reference voltage circuit of the TP sensor. Refer to Testing for Intermittent Conditions and Poor Connections and Wiring Repairs in Wiring Systems. Did you complete the repair? | -- | Go to Step 12 | -- | ||||
10 | Replace the TP sensor. Refer to Throttle Position Sensor Replacement . Is the action complete? | -- | Go to Step 12 | -- | ||||
11 | Replace the PCM. Refer to Powertrain Control Module Replacement . Is the action complete? | -- | Go to Step 12 | -- | ||||
12 |
Does the DTC run and pass? | -- | Go to Step 13 | Go to Step 2 | ||||
13 | With a scan tool, observe the stored information, Capture Info. Does the scan tool display any DTCs that you have not diagnosed? | -- | System OK | |||||
