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For 1990-2009 cars only
Makes 🢒 Pontiac 🢒 2001 🢒 Aztek - FWD 🢒 Body and Accessories 🢒 Body Control System 🢒 DTC P0140

Circuit Description

The powertrain control module (PCM) supplies a bias voltage of about 450 mV between the heated oxygen sensor (HO2S) high signal circuit and the HO2S low signal circuit. When measured with a 10 megaohm DMM, this voltage may display as low as 350 mV. The oxygen sensor signal varies from about 800 mV when the exhaust is rich, to about 50 mV when the exhaust is lean. The PCM constantly monitors the HO2S signal during Closed Loop operation. The PCM then compensates for a rich or lean condition by decreasing or increasing the injector pulse width as necessary. If the HO2S 2 voltage remains at or near the 450 mV bias for an extended period of time, DTC P0140 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 at least 4 minutes.

Conditions for Setting the DTC
    • The HO2S 2 signal voltage remains between 412-500 mV for up to 90 seconds.
    • The ignition voltage should be more than 9 volts and less than 18 volts.
    • The test only runs once per trip.
    • A throttle position (TP) sensor change of more than 8 percent at least twice during the test.

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.

Test Description

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

  1. This step checks for the correct sensor activity. When in Closed Loop fuel control, the HO2S voltage should rapidly swing above and below the bias voltage.

    2.

  2. This step checks the PCM and the HO2S high and low signal circuits between the PCM and the HO2S connector for correct operation.

    3.

  3. This step checks for correct HO2S heater circuit operation up to the HO2S connector.

    4.

  4. This step checks for the correct circuit resistance between the HO2S low signal circuit and the PCM ground.

    5.

  5. This step checks for proper circuit resistance between the HO2S low signal circuit and the PCM ground with a COLD sensor. A loose HO2S or poor thread to exhaust electrical contact will cause higher resistance when the HO2S is cold. Although 500 ohms is allowed, typical resistance should be less than 50 ohms.

Step

Action

Values

Yes

No

Schematic Reference: Engine Controls Schematics

1

Did you perform the Diagnostic System Check-Engine Controls?

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Go to Step 2

Go to Diagnostic System Check - Engine Controls

2

  1. Clear the DTCs using a scan tool.
  2. Reset the fuel trim values.
  3. Start the engine.
  4. With a scan tool, observe the HO2S voltage for the sensor that applies to this DTC.

Is the HO2S voltage fixed within the voltage range specified?

412-500 mV

Go to Step 4

Go to Step 3

3
  1. The condition that set this DTC is not present. This DTC may have been set by one of the following conditions:
    2. • HO2S connector water intrusion
    • An intermittently open HO2S high signal circuit
    • An intermittently open HO2S low signal circuit
    • A HO2S low signal circuit with high resistance to ground
    • PCM ground circuits with high resistance to ground
    • An inoperative HO2S heater
    • Poor HO2S or PCM connector terminal contact
    • An exhaust system leak--A leak is typically within 30 cm (12 inches) up-stream of the HO2S.
    • A defective HO2S
  2. Repair any of the above conditions as necessary.

Did you find and correct the condition?

--

Go to Step 17

Go to Intermittent Conditions

4

  1. Turn OFF the ignition.
  2. Disconnect the HO2S connector for the sensor that applies to this DTC.
  3. Jumper the HO2S low signal circuit to a known good ground.
  4. Jumper the HO2S high signal circuit to a known good ground.
  5. Turn ON the ignition, with the engine OFF.
  6. With a scan tool, observe the HO2S voltage for the sensor that applies to this DTC.

Is the voltage less than the specified value?

20 mV

Go to Step 7

Go to Step 5

5
  1. Turn OFF the ignition.
  2. Remove the jumpers from the HO2S terminals.
  3. Disconnect the PCM connector containing the HO2S high and low circuits.
  4. Use a DMM in order to first measure the continuity of the HO2S high signal circuit. Then measure the continuity of the HO2S low signal circuit between the PCM connector terminal and the engine harness side of the HO2S connector terminal.

Is the individual resistance of both circuits less than the specified value?

5

Go to Step 6

Go to Step 9

6
  1. Check for correct PCM connector terminal contact.
  2. Repair the PCM terminals as necessary. Refer to Repairing Connector Terminals in Wiring Systems.

Was a PCM terminal repair made?

--

Go to Step 17

Go to Step 16

7

  1. Remove the jumpers from the HO2S terminals.
  2. Connect a test lamp between the HO2S ignition 1 voltage circuit and the engine harness side of the heater ground circuit terminals.
  3. Start the engine.

Does the test lamp illuminate?

--

Go to Step 8

Go to Step 10

8

  1. Turn OFF the ignition.
  2. Reconnect the HO2S connector.
  3. Disconnect the PCM connector containing the HO2S low signal circuit and the PCM connector containing the PCM ground circuits.
  4. Use the DMM in order to measure the resistance between the HO2S low signal circuit terminal and one of the PCM ground circuit terminals.

Is the resistance measured less than the specified value?

500 ohms

Go to Step 12

Go to Step 11

9

Repair the circuit that measured high resistance. Refer to Wiring Repairs in Wiring Systems.

Did you complete the repair?

--

Go to Step 17

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10

Repair the HO2S ignition 1 voltage circuit or HO2S heater low control circuit. Refer to Wiring Repairs in Wiring Systems. Probable causes of the condition include the following conditions:

    • An open circuit
    • High circuit resistance
    • A poor ground connection
    • An open heater fuse--If this is the cause, other DTCs should also be set.

Did you complete the action?

--

Go to Step 17

--

11

Repair the high resistance between the HO2S low signal circuit and the PCM ground circuit. Refer to Wiring Repairs in Wiring Systems. Probable causes of the condition include the following items:

    • Poor HO2S connector terminal contact
    • An open sensor harness low signal circuit which requires HO2S replacement
    • A sensor harness low signal circuit with high resistance--This requires HO2S replacement. Refer to Heated Oxygen Sensor 2 Replacement .
    • A poor PCM ground connection
    • High PCM ground circuit resistance
    • An ungrounded exhaust system

Did you complete the action?

--

Go to Step 17

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12
  1. Check for exhaust system leaks up-stream of the suspect HO2S. The leak may be very small and will typically be within 30 cm (12 inches) of the suspect HO2S.
  2. Repair the exhaust as necessary.

Was an exhaust system repair made?

--

Go to Step 17

Go to Step 13

13

  1. Allow the engine to cool to ambient temperature.
  2. Use the DMM in order to measure the resistance between the HO2S low signal circuit terminal and a PCM ground circuit terminal.

Is the resistance as specified?

13 ohms

Go to Step 15

Go to Step 14

14
  1. Remove the HO2S from the exhaust.
  2. Clean the HO2S mounting threads.
  3. Apply anti-seize compound P/N 5613695 or an equivalent to the sensor threads.
  4. Tighten the HO2S to the proper torque. Refer to Heated Oxygen Sensor 2 Replacement .
  5. Use the DMM in order to again measure the resistance between the HO2S low signal circuit terminals and a PCM ground circuit terminal.

Is the resistance less than the specified value?

500

Go to Step 17

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15

Replace the HO2S. Refer to Heated Oxygen Sensor 2 Replacement .

Did you complete the replacement?

--

Go to Step 17

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16

Replace the PCM. Refer to Powertrain Control Module Replacement .

Did you complete the replacement?

--

Go to Step 17

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17
  1. Use a scan tool in order to clear the DTCs.
  2. Turn OFF the ignition for 30 seconds.
  3. Start the engine.
  4. 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 18

Go to Step 2

18

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

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

--

Go to Diagnostic Trouble Code (DTC) List

System OK