DTC P0121. Buick LeSabre 2005

For 1990-2009 cars only

Makes 🢒 Buick 🢒 2005 🢒 LeSabre 🢒 Engine 🢒 Engine Electrical 🢒 DTC P0121

Circuit Description

The throttle position (TP) sensor is used by the powertrain control module (PCM) to determine the throttle plate angle for various engine management systems. The TP sensor is a potentiometer type sensor with the following three circuits:

•	A 5-volt reference circuit

•	A low reference circuit

•	A signal circuit

The PCM provides the TP sensor with 5 volts on the 5-volt reference circuit and a ground on the low reference circuit. Rotation of the TP sensor rotor from the closed throttle position to the wide open throttle (WOT) position provides the PCM with a signal voltage from less than 1 volt to more than 4 volts through the TP sensor signal circuit. When the conditions for running this DTC are met, the PCM will use the manifold absolute pressure (MAP) sensor to determine if the predicted operating range of the TP sensor is correct. If the PCM detects that the TP sensor voltage is out of the predicted range DTC P0121 sets.

DTC Descriptor

This diagnostic procedure supports the following DTC:

DTC P0121 Throttle Position (TP) Sensor Replacement

Conditions for Running the DTC

•	DTCs P0107, P0108, P0122, P0123, P0506, P0507 are not set.

•	The engine is running for more than 2 minutes.

•	The ECT Sensor parameter is more than 75°C (167°F).

•	The MAP Sensor parameter is less than 50 kPa for a TP sensor skewed high test.

•	The MAP Sensor parameter is more than 70 kPa for a TP sensor skewed low test.

•	The MAP Sensor parameter does not change more than 5 kPa for more than 5 seconds.

•	The IAC Position parameter is between 0-130 counts.

•	DTC P0121 runs continuously once the above conditions are met.

Conditions for Setting the DTC

The TP sensor voltage is more or less than a predicted value for more than 10 seconds.

Action Taken When the DTC Sets

•	The control module illuminates the malfunction indicator lamp (MIL) on the second consecutive ignition cycle that the diagnostic runs and fails.

•

The control module records the operating conditions at the time the diagnostic fails. The first time the diagnostic fails, the control module stores this information in the Failure Records. If the diagnostic reports a failure on the second consecutive ignition cycle, the control module records the operating conditions at the time of the failure. The control module writes the operating conditions to the Freeze Frame and updates the Failure Records.

Conditions for Clearing the MIL/DTC

•	The control module turns OFF the 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.

Test Description

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

This step calculates the resistance in the 5-volt reference circuit.

This step calculates the resistance in the low reference circuit.

Step

Action

Values

Yes

Schematic Reference: Engine Controls Schematics

Connector End View Reference: Engine Controls Connector End Views or Powertrain Control Module Connector End Views

Did you perform the Diagnostic System Check - Vehicle?

Go to Step 2

Go to Diagnostic System Check - Vehicle in Vehicle DTC Information

Turn ON the ignition, with the engine OFF.
Monitor the DTC Information with a scan tool.

Did DTC P0641 also set?

Go to DTC P0641

Go to Step 3

Inspect for the following conditions:

•	Vacuum hoses disconnected, damaged, or incorrectly routed

•	Restrictions or leaks in the manifold absolute pressure (MAP) sensor vacuum source

•	Intake manifold vacuum leaks

Did you find and correct the condition?

Go to Step 21

Go to Step 4

Important: The vehicle used for the comparison is not limited to the same type of vehicle as is being serviced. A vehicle known to provide an accurate reading is acceptable.

Do you have access to another vehicle in which the MAP sensor pressure can be observed with a scan tool?

Go to Step 5

Go to Step 6

Turn ON the ignition, with the engine OFF.
Observe the MAP sensor pressure with a scan tool.
Observe the MAP sensor pressure in the known good vehicle with a scan tool.
Compare the values.

Is the difference between the values less than the specified value?

3 kPa

Go to Step 7

Go to Manifold Absolute Pressure Sensor Diagnosis

Important: The Altitude vs. Barometric Pressure table indicates a pressure range for a given altitude under normal weather conditions. Weather conditions consisting of very low or very high pressure and/or very low or very high temperature may cause a reading to be slightly out of range.

Turn ON the ignition, with the engine OFF.
Observe the MAP sensor pressure with a scan tool. Refer to Altitude Versus Barometric Pressure .
The MAP sensor pressure should be within the range specified for your altitude.

Does the MAP sensor indicate the correct barometric pressure?

Go to Step 7

Go to Manifold Absolute Pressure Sensor Diagnosis

Observe the MAP sensor pressure with a scan tool.
Start the engine.

Does the MAP sensor pressure change?

Go to Step 8

Go to Manifold Absolute Pressure Sensor Diagnosis

Turn OFF the ignition.
Turn ON the ignition, with the engine OFF.
Observe the TP sensor parameter with the scan tool.

Is the voltage less than the specified value with the throttle in the closed position?

0.85 V

Go to Step 9

Go to Step 10

Observe the TP sensor parameter with the scan tool.
Depress the accelerator pedal slowly until the throttle is in the wide open position.
Release the accelerator pedal slowly until the throttle is returned to the closed position.
Repeat this procedure several times.

Does the TP sensor parameter increase steadily to more than the first specified value, and then decrease steadily returning to less than the second specified value?

98%

Go to Intermittent Conditions

Go to Step 11

Inspect the throttle body for the following conditions:

•	Throttle blade binding or damaged

•	Throttle cable binding or damaged

Did you find and correct the condition?

Go to Step 21

Go to Step 11

Test for an intermittent and for a poor connection at the throttle position (TP) sensor. Refer to Testing for Intermittent Conditions and Poor Connections and Connector Repairs in Wiring Systems.

Did you find and correct the condition?

Go to Step 21

Go to Step 12

Disconnect the TP sensor harness connector.
Turn ON the ignition, with the engine OFF.
Measure the voltage from the 5-volt reference circuit of the TP sensor to a good ground, with a DMM. Note the measurement as Supply voltage.
Connect a test lamp and a DMM in series between the 5-volt reference circuit and the low reference circuit of the TP sensor at the harness connector.
Measure the amperage with the DMM. Note the measurement as Amperage.

Is the amperage equal to the specified value?

0 mA

Go to Step 16

Go to Step 13

Remove the DMM from the circuit.
Connect the test lamp between the 5-volt reference circuit and the low reference circuit of the TP sensor, at the harness connector.
Measure the voltage from the 5-volt reference circuit at the test lamp to a good ground, with the DMM. Note the measurement as Load voltage drop.

Important: Before any calculations are performed, ensure that all measurements are converted to like units, for example, volts/amps or millivolts/milliamps.

Subtract the Load voltage drop from the Supply voltage. Note the result as Supply voltage drop.
Divide the Supply voltage drop by the Amperage.

Is the result more than the specified value?

5 ohms

Go to Step 15

Go to Step 14

Measure the voltage from the low reference circuit of the TP sensor at the test lamp to a good ground, with a DMM. Note the result as Low reference voltage drop.

Important: Before any calculations are performed, ensure that all measurements are converted to like units, for example, volts/amps or millivolts/milliamps.

Divide the Low reference voltage drop by the Amperage.

Is the result more than the specified value?

Go to Step 17

Go to Step 19

Test the 5-volt reference circuit between the powertrain control module (PCM) and the TP sensor for high resistance. Refer to Circuit Testing and Wiring Repairs in Wiring Systems.

Did you find and correct the condition?

Go to Step 21

Go to Step 18

Test the low reference circuit between the PCM and the TP sensor for an open. Refer to Circuit Testing and Wiring Repairs in Wiring Systems.

Did you find and correct the condition?

Go to Step 21

Go to Step 18

Test the low reference circuit between the PCM and the TP sensor for high resistance. Refer to Circuit Testing and Wiring Repairs in Wiring Systems.

Did you find and correct the condition?

Go to Step 21

Go to Step 18

Test for an intermittent and for a poor connection at the PCM. Refer to Testing for Intermittent Conditions and Poor Connections and Connector Repairs in Wiring Systems.

Did you find and correct the condition?

Go to Step 21

Go to Step 18

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

Did you complete the replacement?

Go to Step 18

Replace the PCM. Refer to Control Module References in Computer/Integrating Systems for replacement, setup, and programming.

Did you complete the replacement?

Go to Step 21

Clear the DTCs with a scan tool.
Turn OFF the ignition for 30 seconds.
Start the engine.
Operate the vehicle within the Conditions for Running the DTC. You may also operate the vehicle within the conditions that you observed from the Freeze Frame/Failure Records.

Did the DTC fail this ignition?

Go to Step 2

Go to Step 19

Observe the Capture Info with a scan tool.

Are there any DTCs that have not been diagnosed?

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

System OK