Circuit Description
Heated oxygen sensors are used to minimize the amount of time required for closed loop fuel control operation and to allow accurate catalyst monitoring. The oxygen sensor heater greatly decreases the amount of time required for fuel control sensor HO2S 1 to become active. The oxygen sensor heater is required by catalyst monitor sensor HO2S 2 to maintain a sufficiently high temperature. This allows accurate exhaust oxygen content readings further from the engine.
The powertrain control module (PCM) runs the heater test only after a cold start (determined by engine coolant and intake air temperature at the time of startup) and only once during an ignition cycle. When the engine is started the PCM monitors the HO2S voltage. When the Bank HO2S voltage indicates a sufficiently active sensor, the PCM looks at how much time has elapsed since startup. If the PCM determines that too much time was required for the HO2S 2 to become active, the DTC is set. The expected time for the HO2S to reach operating temperature is based on the engine coolant temperature at start-up and the average Mass Air Flow since startup (more total airflow or higher startup engine coolant temperature = shorter time to HO2S activity).
Conditions for Running the DTC
| • | None of the following DTCs are present: |
| - | Throttle position (TP) sensor DTCs |
| - | Evaporative emissions (EVAP) DTCs |
| - | Intake air temperature (IAT) sensor DTCs |
| - | Manifold absolute pressure (MAP) sensor DTCs |
| - | Exhaust gas recirculation (EGR) pintle position DTCs |
| - | Engine coolant temperature (ECT) sensor DTCs |
| - | Crankshaft position (CKP) sensor DTCs |
| - | Mass air flow (MAF) sensor DTCs |
| - | Misfire DTCs |
| - | Fuel trim DTCs |
| - | Fuel injector circuit DTCs |
| • | The closed loop commanded air/fuel ratio is between 14.5 and 14.8 |
| • | The system voltage is between 9 and 16 volts. |
| • | The IAT is less than 35°C (95°F) at startup. |
| • | The ECT is less than 35°C (95°F) at startup. |
| • | The IAT and the ECT are within 6°C (11°F) of each other at start-up. |
| • | The average MAF for the sample period is less than 24 g/s. |
Conditions for Setting the DTC
The HO2S 2 voltage remains within 150 mV of the bias voltage (about 450 mV) for a longer amount of time than necessary. The amount of time ranges between 50 and 130 seconds depending upon The ECT at startup and average air flow since startup.
Action Taken When the DTC Sets
| • | The PCM illuminates the malfunction indicator lamp (MIL) during the second consecutive trip in which the diagnostic test has run and failed. |
| • | The PCM stores the conditions which were present when the DTC set as Freeze Frame and Fail Records data. |
Conditions for Clearing the MIL/DTC
| • | The PCM turns the MIL off during the third consecutive trip in which the diagnostic has run and passed. |
| • | The history DTC clears after 40 consecutive warm-up cycles have occurred without a malfunction. |
| • | The DTC may be cleared by using the scan tool Clear Info function or by disconnecting the PCM battery feed. |
Diagnostic Aids
Inspect for the following conditions:
| • | A faulty connection at the PCM. Inspect the harness connectors for backed out terminals, improper mating, broken locks, improperly formed or damaged terminals, and faulty terminal to wire connections. |
| • | Inspect the wiring harness for damage. If the harness appears to be OK, observe the display on the scan tool while moving the connectors and the wiring harnesses related to the sensor. A change in the display indicates the location of the malfunction. |
Reviewing the Fail Records vehicle mileage since the diagnostic test last failed may help determine how often the condition that caused the DTC to be set occurs. This may assist in diagnosing the condition.
Test Description
The number(s) below refer to the step number(s) on the Diagnostic Table.
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The HO2S should be allowed to cool before performing this test. If the heated oxygen sensor (HO2S) heater is functioning, the signal voltage gradually increases or decreases as the sensor element warms. If the heater is not functioning, the HO2S signal remains near the 450 mv bias voltage.
-
Ensures that the ignition feed circuit to the HO2S is not open or shorted. The test lamp should be connected to a good chassis ground, in case the HO2S low or HO2S heater ground circuit is malfunctioning.
-
Tests the HO2S heater ground circuit.
-
Tests for an open or shorted HO2S heater element.
Important: The heater element resistance varies according to the HO2S temperature (a hot HO2S heater element measures a much higher resistance than a HO2S heater element at room temperature). Allow the HO2S to cool before measuring HO2S heater element resistance.
-
An open HO2S signal or low circuit can cause the DTC to set with a good HO2S. Test the HO2S signal and the HO2S low circuits before replacing the sensor.
Step | Action | Value(s) | Yes | No |
|---|---|---|---|---|
1 | Did you perform the Powertrain On-Board Diagnostic (OBD) System Check? | -- | ||
|
Important: If the engine has just been operating, allow the engine to cool for about 1/2 hour before proceeding.
Does the HO2S voltage vary from bias voltage to more than the first specified value or less than the second specified value within 2 minutes? | 600 mV 300 mV | Go to Diagnostic Aids | ||
3 | Inspect the fuse for the HO2S 2 ignition feed circuit. Is the fuse open? | -- | ||
Important: Use a known good ground. Do not use the HO2S 2 heater ground or the HO2S 2 low circuit. Does the test lamp illuminate? | -- | |||
Connect the test lamp between the HO2S 2 ignition feed and HO2S 2 heater ground. Does test lamp illuminate? | -- | |||
Is the resistance within the specified range? | 4-6 ohms | |||
7 | Repair the open HO2S 2 ignition feed circuit to the HO2S 2. Refer to Heated Oxygen Sensor Wiring Repairs in Wiring Systems. Did you complete the repair? | -- | -- | |
8 | Repair the open HO2S 2 heater ground circuit. Refer to Heated Oxygen Sensor Wiring Repairs in Wiring Systems. Did you complete the repair? | -- | -- | |
9 |
Was a problem found? | -- | ||
10 |
Was a problem found? | -- | ||
11 | Check for a poor HO2S 2 signal or low circuit terminal connection at the HO2S 2 harness connector and replace any terminal(s) if necessary. Refer to Connector Repairs in Wiring Systems. Was a problem found? | -- | ||
12 | Inspect for a faulty HO2S 2 low circuit terminal connection at the PCM and replace the terminal if necessary. Refer to Connector Repairs in Wiring Systems. Was a problem found? | -- | ||
13 | Inspect for faulty HO2S 2 signal circuit terminal connection at the PCM and replace the terminal if necessary. Refer to Heated Oxygen Sensor Wiring Repairs in Wiring Systems. Was a problem found? | -- | ||
14 | Replace the HO2S 2. Refer to Heated Oxygen Sensor Replacement . Did you complete the repair? | -- | -- | |
15 | Repair a short to ground in the HO2S 2 ignition feed circuit and replace a malfunctioning fuse. Refer to Heated Oxygen Sensor Wiring Repairs in Wiring Systems. Did you complete the repair? | -- | -- | |
16 |
Important: If engine has just been operating, allow engine to cool for about one half hour before proceeding.
Does the HO2S voltage vary from bias voltage to more than the first specified value or less than the second specified value within 2 minutes? | 600 mV 300 mV | System OK |
