Circuit Description
Heated oxygen sensors (HO2S) are used for fuel control and post catalyst monitoring. Each HO2S compares the oxygen content of the surrounding air with the oxygen content of the exhaust stream. When the vehicle is first started, the powertrain control module (PCM) operates in an Open Loop mode, ignoring the HO2S signal voltage when calculating the air-to-fuel ratio. The PCM supplies the HO2S with a reference, or bias, voltage of 450 mV. The HO2S generates a voltage within a range of 0-1000 mV, that fluctuates above and below bias voltage once in Closed Loop. A high HO2S voltage output indicates a rich fuel mixture. A low HO2S voltage output indicates a lean mixture. A heating element inside the HO2S reduces the time required for the sensor to reach operating temperature, and provide an accurate voltage signal.
The HO2S heater diagnostic runs continuously when the heater circuit is energized. The HO2S heater circuit is only energized when the engine is running or commanded ON by the scan tool. The DTC will set if the heater current is above or below a calibrated threshold for 200 seconds.
Conditions for Running the DTC
| • | The ignition 1 signal is more than 10 volts. |
| • | The engine coolant temperature (ECT) is more than 45°C (113°F). |
| • | The airflow is less than 40 g/s. |
| • | the engine run time is more than 60 seconds. |
| • | The HO2S 2 voltage is between 395-495 mV. |
Conditions for Setting the DTC
The HO2S 2 heater current is not within the calibrated range for more than 200 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 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.
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As the heater warms the oxygen sensor (O2S) to operating temperature, the HO2S 2 will output a voltage in relation to the amount of oxygen in the exhaust. With the ignition ON and the engine OFF, the exhaust usually contains a high oxygen content. With the engine OFF the HO2S 2 voltage will usually decrease from the initial, or bias, voltage due to the high oxygen content.
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This step checks for proper HO2S 2 heater ignition 1 voltage and ground supply.
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This step checks for proper PCM and signal circuit operation.
Step | Action | Value(s) | Yes | No |
|---|---|---|---|---|
Schematic Reference: Engine Controls Schematics | ||||
1 | Did you perform the Diagnostic System Check-Engine Controls? | -- | Go to Step 2 | |
|
Important: Allow the engine to cool for about one-half hour before proceeding with
this diagnosis.
Does the HO2S 2 signal voltage go from bias voltage to more than or less than the specified range? | 350-550 mV | Go to Step 3 | Go to Step 4 | |
3 |
Does the scan tool indicate that this DTC failed this ignition? | -- | Go to Step 4 | Go to Intermittent Conditions |
Does the test lamp illuminate? | -- | Go to Step 5 | Go to Step 7 | |
Does the test lamp turn ON and OFF with each command? | -- | Go to Step 10 | Go to Step 6 | |
6 | Does the test lamp remain illuminated with each command? | -- | Go to Step 8 | Go to Step 9 |
7 |
Did you find and correct the condition? | -- | Go to Step 14 | Go to Intermittent Conditions |
8 | Test the HO2S 2 low control 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 14 | Go to Step 11 |
9 | Test the HO2S 2 heater low control circuit for an open, high resistance or short to voltage. Refer to Circuit Testing and Wiring Repairs in Wiring Systems. Did you find and correct the condition? | -- | Go to Step 14 | Go to Step 11 |
10 | Inspect for poor connections at the harness connector of the HO2S 2. 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 14 | Go to Step 12 |
11 | Inspect for poor connections at the harness connector of 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 14 | Go to Step 13 |
12 | Replace the HO2S 2. Refer to Heated Oxygen Sensor Replacement . Did you complete the replacement? | -- | Go to Step 14 | -- |
13 | Replace the PCM. Refer to Powertrain Control Module Replacement . Did you complete the replacement? | -- | Go to Step 14 | -- |
14 |
Does the DTC run and pass? | -- | Go to Step 15 | Go to Step 2 |
15 | With a scan tool, observe the stored information, Capture Info. Does the scan tool display any DTCs that you have not diagnosed. | -- | System OK | |
