2004 Buick Century CustomFACTORY OEM SERVICE PROCEDURE

Test Description

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

  1. 11:  This step calculates the resistance in the 5-volt reference circuit.
  2. 12:  This step calculates the resistance in the low reference circuit.
DTC P0121

Step Action Values Yes No
Schematic Reference:  Engine Controls Schematics 
Connector End View Reference:  Engine Controls Connector End Views  or  Powertrain Control Module (PCM) Connector End Views 
1 Did you perform the Diagnostic System Check-Engine Controls? - Go to Step 2  Go to Diagnostic System Check - Engine Controls
2 Inspect for the following conditions:
  • Any vacuum hoses disconnected, damaged, or incorrectly routed
  • The manifold absolute pressure (MAP) sensor disconnected from the vacuum source
  • Restrictions in the MAP sensor vacuum source
  • Any intake manifold vacuum leaks

Did you find and correct the condition?		 - Go to Step 18  Go to Step 3 
3
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 4  Go to Step 5 
4
  1. Turn ON the ignition, with the engine OFF.
  2. Observe the MAP sensor pressure with a scan tool.
  3. Observe the MAP sensor pressure in the known good vehicle with a scan tool.
  4. Compare the values.
Is the difference between the values less than the specified value?		 3 kPa Go to Step 6  Go to Manifold Absolute Pressure (MAP) Sensor Diagnosis
5
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.
  1. Turn ON the ignition, with the engine OFF.
  2. Observe the MAP sensor pressure with a scan tool. Refer to Altitude vs Barometric Pressure .
  3. 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 6  Go to Manifold Absolute Pressure (MAP) Sensor Diagnosis
6
  1. Observe the MAP sensor pressure with a scan tool.
  2. Start the engine.
Does the MAP sensor pressure change?		 - Go to Step 7  Go to Manifold Absolute Pressure (MAP) Sensor Diagnosis
7
  1. Turn OFF the ignition.
  2. Turn ON the ignition, with the engine OFF.
  3. Observe the throttle position (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 8  Go to Step 9 
8
  1. Observe the TP sensor parameter with the scan tool.
  2. Depress the accelerator pedal slowly until the throttle is in the wide open position.
  3. Release the accelerator pedal slowly until the throttle is returned to the closed position.
  4. 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%
1%		 Go to Intermittent Conditions Go to Step 10 
9 Inspect the throttle body for the following conditions:
  • A binding or damaged throttle blade
  • A binding or damaged throttle cable

Did you find and correct the condition?		 - Go to Step 18  Go to Step 10 
10 Test for an intermittent and for a poor connection at the 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 18  Go to Step 11 
11 
  1. Disconnect the TP sensor harness connector.
  2. 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".
  3. 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.
  4. Measure the amperage with the DMM. Note the measurement as "Amperage".
  5. Remove the DMM from the circuit.
  6. Connect the test lamp between the 5-volt reference circuit and the low reference circuit of the TP sensor, at the harness connector.
  7. 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".
    8. IMPORTANT: Before any calculations are performed, ensure that all measurements are converted to like units. For example, volts/amps or millivolts/milliamps.
  8. Subtract the "Load voltage drop" from the "Supply voltage". Note the result as "Supply voltage drop".
  9. Divide the "Supply voltage drop" by the "Amperage".
Is the result more than the specified value?		 5 ohm Go to Step 13  Go to Step 12 
12 
  1. Measure the voltage from the low reference circuit of the TP sensor at the test lamp to a good ground, with the DMM. Note the result as "Low reference voltage drop".
    2. IMPORTANT: Before any calculations are performed, ensure that all measurements are converted to like units. For example, volts/amps or millivolts/milliamps.
  2. Divide the "Low reference voltage drop" by the "Amperage".
Is the result more than the specified value?		 5 ohm Go to Step 14  Go to Step 16 
13 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 18  Go to Step 15 
14 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 18  Go to Step 15 
15 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 18  Go to Step 17 
16 Replace the TP sensor. Refer to Throttle Position (TP) Sensor Replacement .
Did you complete the replacement?		 - Go to Step 18  -
17 Replace the PCM. Refer to Powertrain Control Module (PCM) Replacement .
Did you complete the replacement?		 - Go to Step 18  -
18
  1. Clear the DTCs with a scan tool.
  2. Turn OFF the ignition for 30 seconds.
  3. Start the engine.
  4. 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 
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 System OK
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When to See a Mechanic

Stop DIY work and contact a certified mechanic immediately if any of the following apply:

  • • You smell fuel, burning insulation, or see smoke.
  • • Brakes feel soft, pull hard to one side, or make grinding noises.
  • • The engine overheats, stalls repeatedly, or misfires under load.
  • • You are missing required tools, torque specs, or safe lifting equipment.
  • • You are not confident in the next step or safety outcome.