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For 1990-2009 cars only

Circuit Description

The heated oxygen sensor (HO2S) is a sensor designed to create a voltage relative to the oxygen content in the engine exhaust stream. The control module supplies the HO2S with signal high and low circuits. Ignition voltage and ground are supplied to the HO2S heater by independent circuits. The oxygen content of the exhaust indicates when the engine is operating lean or rich. When the HO2S detects that the engine is operating rich, the signal voltage is high, and decreases the signal voltage as the engine runs leaner. This oscillation above and below the bias voltage, sometimes referred to as activity or switching, can be monitored with the HO2S signal voltage.

The HO2S contains a heater that is necessary in order to quickly warm the sensor to the operating temperature. The heater also maintains the operating temperature during extended idle conditions. The HO2S needs to be at a high temperature in order to produce a voltage. When the HO2S reaches the operating temperature, the control module monitors the HO2S bias, or reference, voltage. The control module also monitors the HO2S signal voltage for Closed Loop fuel control. During normal Closed Loop fuel control operation, the control module will add fuel, or enrich the mixture, when the HO2S detects a lean exhaust content. The control module will subtract fuel, or "lean out" the mixture, when the HO2S detects a rich exhaust condition.

This diagnostic trouble code (DTC) determines if the HO2S heater circuit is functioning properly by monitoring the time required for the HO2S to reach the operating temperature. This DTC sets when the powertrain control module (PCM) fails to detect HO2S voltage transitions above and below the bias range within a specified time.

Conditions for Running the DTC

    • DTCs P0101, P0102, P0103, P0106, P0107, P0108, P0112, P0113, P0117, P0118, P0121, P0122, P0123, P0300, P0351, P0401, P0404, P0405, P0440, P0442, P0446, P0452, P0453, P1258, P1404, or P1441 are not set.
    • The HO2S voltage is between 425-475 mV at engine startup.
    • The intake air temperature (IAT) and the engine coolant temperature (ECT) are less than 50°C (122°F), and are within 8°C (14.5°F) of each other at engine start-up.
    • The ignition 1 signal is between 9-18 volts.
    • Intrusive tests are not in progress.
    • The scan tool output controls are not active.
    • The scan tool is disconnected.

Conditions for Setting the DTC

The HO2S voltage remains within 150 mV of the startup voltage for a predetermined amount of time, based on ECT and air flow.

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

Important: Remove any debris from the PCM connector surfaces before servicing the PCM. Inspect the PCM connector gaskets when diagnosing or replacing the PCM. Ensure that the gaskets are installed correctly. The gaskets prevent water intrusion into the PCM.

    • Use the Freeze Frame/Failure Records in order to help locate an intermittent condition. If you cannot duplicate the DTC, the information in the Freeze Frame/Failure Records can help to determine how many miles have occurred since the DTC set. The Fail Counter and the Pass Counter can also help determine how many ignition cycles the diagnostic reported a pass or a fail. In order to isolate when the DTC failed, operate the vehicle within the same Freeze Frame conditions such as RPM, load, vehicle speed, temperature, etc., that you observed. For an intermittent condition, refer to Intermittent Conditions . These conditions include:
       - The RPM
       - The vehicle load
       - The vehicle speed
       - The temperature
    • The heater diagnostic will only run on a cold start and run once per ignition cycle.
    • An oxygen supply inside the HO2S is necessary for proper operation. The HO2S wires provide the supply of oxygen. Inspect the HO2S wires and inspect the connections for breaks or for contamination. Refer to Heated Oxygen Sensor Wiring Repairs in Wiring Systems.

Test Description

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

  1. Allow the engine to cool before you perform this test. If the sensor is at the operating temperature, the HO2S voltage will stay high or low. If the HO2S voltage stays between 300-600 mV, the HO2S heater is inoperative.

  2. If more than one HO2S DTC is set, the HO2S fuse may be open. Test all the related circuits going to all the heated oxygen sensors for a short to ground. If you cannot locate a shorted circuit, it may be necessary to disconnect each HO2S one at a time in order to locate a shorted sensor.

  3. This step tests whether an ignition 1 positive voltage supply is available at the sensor.

  4. This step tests whether a ground is available at the sensor.

  5. This step tests whether the HO2S heater element is internally open.

  6. Inspect the ignition 1 positive voltage circuits at the underhood electrical center for poor connections.

  7. Test the ground circuits for an open if more than one heater DTC sets.

Step

Action

Value(s)

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

Important: Allow the engine to cool for one-half hour before proceeding with this table.

Important:  If DTC P0300 is set, diagnose that DTC first. Refer to Diagnostic Trouble Code (DTC) List .

  1. Turn ON the ignition, with the engine OFF.
  2. With a scan tool, observe the affected HO2S voltage for 2 minutes.

Does the HO2S voltage go from a bias voltage to above or below the specified range?

300-600 mV

Go to Diagnostic Aids

Go to Step 3

3

Inspect the HO2S fuse for an open.

Is the HO2S fuse open?

--

Go to Step 11

Go to Step 4

4

  1. Raise the vehicle. Refer to Lifting and Jacking the Vehicle in General Information.
  2. Disconnect the HO2S electrical connector.
  3. Probe the ignition 1 voltage circuit at the PCM side of the HO2S electrical connector using the test lamp connected to a known good ground. Do not use the HO2S heater ground or the HO2S low signal circuits.

Does the test lamp illuminate?

--

Go to Step 5

Go to Step 7

5

Connect the test lamp between the HO2S ignition 1 voltage and the HO2S heater ground.

Does the test lamp illuminate?

--

Go to Step 6

Go to Step 8

6

Measure the resistance between the HO2S ignition 1 voltage and the HO2S heater ground at the HO2S pigtail using the DMM.

Is the HO2S resistance within the specified range?

2-50 ohms

Go to Step 9

Go to Step 10

7

Repair the open in the HO2S ignition 1 voltage circuit to the HO2S. Refer to Heated Oxygen Sensor Wiring Repairs in Wiring Systems.

Did you complete the action?

--

Go to Step 12

--

8

Repair the open in the HO2S heater ground circuit. Refer to Heated Oxygen Sensor Wiring Repairs in Wiring Systems.

Did you complete the action?

--

Go to Step 12

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9

  1. Test for a poor connection at the HO2S harness terminals. Refer to Testing for Intermittent Conditions and Poor Connections in Wiring Systems.
  2. If you find a poor connection, repair the terminal as necessary. Refer to Connector Repairs in Wiring Systems.

Did you find and repair the condition?

--

Go to Step 12

Go to Step 10

10

Replace the HO2S. Refer to Heated Oxygen Sensor Replacement - Bank 2 Sensor 1 .

Did you complete the action?

--

Go to Step 12

--

11

  1. Locate and repair the short to ground in the HO2S ignition 1 voltage circuit. Refer to Heated Oxygen Sensor Wiring Repairs in Wiring Systems.
  2. Replace the open fuse.

Did you complete the action?

--

Go to Step 12

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12

  1. Use the scan tool in order to clear the DTCs.
  2. Disconnect the scan tool.
  3. Turn OFF the ignition for 30 seconds.
  4. Start the engine.
  5. Operate the vehicle within the Conditions for Running the DTC as specified in the supporting text.
  6. Install the scan tool.

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?

--

Go to Diagnostic Trouble Code (DTC) List

System OK