GM Service Manual Online
For 1990-2009 cars only
Makes 🢒 Pontiac 🢒 2001 🢒 Firebird 🢒 Body and Accessories 🢒 Body Control System 🢒 DTC P0140

Circuit Description

In order to control emissions of hydrocarbons (HC), carbon monoxide (CO), and oxides of nitrogen (NOx), a 3-way catalytic converter is used. The catalyst within the converter promotes a chemical reaction which oxidizes the HC and the CO present in the exhaust gas, converting these chemicals into harmless water vapor and carbon dioxide. The catalyst also reduces NOx, converting the NOx to nitrogen. The powertrain control module (PCM) has the ability to monitor this process using the post-catalyst heated oxygen sensor (HO2S). This sensor produces an output signal which indicates the oxygen storage capacity of the catalyst; this in turn indicates the catalyst's ability to convert exhaust gases efficiently. If the post HO2S signal voltage remains at or near the 450 mV bias for an extended period of time, this diagnostic trouble code (DTC) will set.

Conditions for Running the DTC
    • DTCs P0101, P0102, P0103, P0107, P0108, P0112, P0113, P0117, P0118, P0121, P0122, P0123, P0125, P0128, P0201-P0206, P0410, P0440, P0442, P0443, P0446, P0449, or P1441 are not set.
    • The engine has been running for more than 4 minutes.

Conditions for Setting the DTC
    • The post-HO2S signal voltage remains between 412-490 mV during normal Closed Loop operation.
    • The APP indicated angle change is more than 1.5 percent at least 6 times.

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 malfunction indicator lamp (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.

Diagnostic Aids

If the condition is intermittent, refer to Intermittent Conditions .

Test Description

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

  1. When the system is operating correctly, the HO2S voltage should toggle above and below the specified values.

    2.

  2. If the HO2S low signal circuit is shorted to ground the HO2S voltage will be less than 400 mV when the HO2S high signal circuit is jumped to ground.

    3.

  3. The specified value is what is measured on a correctly operating system.

    4.

  4. The specified value is what is measured on a correctly operating system.

    5.

  5. The new PCM must be programmed.

Step

Action

Values

Yes

No

Schematic Reference: Engine Controls Schematics

1

Did you perform the Diagnostic System Check-Engine Controls?

--

Go to Step 2

Go to Diagnostic System Check - Engine Controls

2

Important: If any other DTCs are set, except HO2S DTCs, refer to the other DTCs first before proceeding.

  1. Engine at operating temperature.
  2. Operate engine above 1,200 RPM for 2 minutes.
  3. With a scan tool, observe the HO2S voltage parameter.

Does scan tool indicate HO2S voltage varying outside the specified values?

412-490 mV

Go to Step 3

Go to Step 4

3

Operate the vehicle within Failure Records conditions.

Does the scan tool indicate this DTC failed this ignition?

--

Go to Step 4

Go to Intermittent Conditions

4

Inspect and test for the following:

    • An exhaust leak
    • An incorrectly installed HO2S
    • Damaged wiring--Refer to Wiring Repairs in Wiring Systems.

Did you find and correct the condition?

--

Go to Step 15

Go to Step 5

5

  1. Turn ON the ignition, with the engine OFF.
  2. Disconnect the HO2S.
  3. Jumper the high signal circuit, located on the PCM side, to a good ground.
  4. With a scan tool, observe HO2S voltage parameter.

Does the scan tool indicate HO2S voltage more than the specified value?

400 mV

Go to Step 6

Go to Step 10

6

  1. Remove the jumper wire.
  2. Measure the voltage on the HO2S high signal circuit, located on the PCM side, using a DMM that is connected to a good ground.

Does the voltage measure near the specified value?

4.5 V

Go to Step 7

Go to Step 9

7

Measure the voltage on the HO2S low signal circuit, located on the PCM side, using a DMM that is connected to a good ground.

Does the voltage measure near the specified value?

5 V

Go to Step 11

Go to Step 8

8

Test the HO2S low signal circuit for an open or high resistance. Refer to Circuit Testing and Wiring Repairs in Wiring Systems.

Did you find and correct the condition?

--

Go to Step 15

Go to Step 13

9

Test the HO2S high signal circuit for an open or high resistance. Refer to Circuit Testing and Wiring Repairs in Wiring Systems.

Did you find and correct the condition?

--

Go to Step 15

Go to Step 13

10

Test the HO2S low signal circuit for a short to ground. Refer to Circuit Testing and Wiring Repairs in Wiring Systems.

Did you find and correct the condition?

--

Go to Step 15

Go to Step 14

11

Test for poor connections at the HO2S. 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 15

Go to Step 12

12

Important: Determine the cause of contamination before replacing the sensor.

  1. Inspect and test for the following:
    2. • Fuel contamination--Refer to Alcohol/Contaminants-in-Fuel Diagnosis .
    • Engine oil or coolant consumption--Refer to Oil Consumption Diagnosis in Engine Mechanical.
    • Use of improper RTV sealant--Refer to

    Notice: Contamination of the oxygen sensor can result from the use of an inappropriate RTV sealant (not oxygen sensor safe) or excessive engine coolant or oil consumption. Remove the HO2S and visually inspect the portion of the sensor exposed to the exhaust stream in order to check for contamination. If contaminated, the portion of the sensor exposed to the exhaust stream will have a white powdery coating. Silicon contamination causes a high but false HO2S signal voltage (rich exhaust indication). The control module will then reduce the amount of fuel delivered to the engine, causing a severe driveability problem. Eliminate the source of contamination before replacing the oxygen sensor.

    in Cautions and Notices.
  2. Replace the affected HO2S sensor--Refer to Heated Oxygen Sensor Replacement - Bank 1 Sensor 2 , h Heated Oxygen Sensor Replacement - Bank 1 Sensor 1 , or Heated Oxygen Sensor Replacement - Bank 2 Sensor 1 .

Did you complete the replacement?

--

Go to Step 15

--

13

Inspect for poor connections 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 15

Go to Step 14

14

Replace the PCM. Refer to Powertrain Control Module Replacement .

Did you complete the replacement?

--

Go to Step 15

--

15
  1. Use the scan tool in order to clear the DTCs.
  2. Turn OFF the ignition for 30  seconds.
  3. Operate the vehicle within the Conditions for Running the DTC, as specified in the supporting text.

Does the DTC run and pass?

--

Go to Step 16

Go to Step 2

16

With a scan tool, observe the Stored information, Capture Info.

Does the scan tool indicate any DTCs that you have not diagnosed?

--

Go to Diagnostic Trouble Code (DTC) List

System OK