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 the pre-catalyst oxygen sensors to become active. An oxygen sensor heater is required by catalyst monitor sensor to maintain a sufficiently high temperature which allows accurate exhaust oxygen content readings further from the engine. The PCM will run 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 will monitor the HO2S voltage. When the post 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 post HO2S to become active, this DTC will set. The time it should take the HO2S to reach operating temperature is based on the engine coolant temperature at start-up and the average Mass Air Flow since startup (higher average airflow or higher startup engine coolant temperature = shorter time to HO2S activity).
Conditions for Running the DTC
| • | No Misfire, CKP, Fuel injector, MAF, TP, EVAP, IAT, MAP, Fuel trim, EGR or ECT DTCs. |
| • | System voltage is between 9 and 18 volts. |
| • | Intake air temperature (IAT) is less than 35°C (95°F) at startup |
| • | Engine coolant temperature (ECT) is less than 35°C (95°F) at startup. |
| • | IAT and ECT are within 6°C (11°F) of each other at start-up. |
| • | The average mass air flow for the sample period is less than 22.7 gm/s. |
Conditions for Setting the DTC
| • | Post HO2S voltage remains within 150 mV of the bias voltage (about 450 mV). |
| • | The amount of time depends on engine coolant temperature at startup and the average air flow since startup. |
Action Taken When the DTC Sets
| • | The PCM will illuminate the malfunction indicator lamp (MIL) during the second consecutive trip in which the diagnostic test has been run and failed. |
| • | The PCM will store conditions which were present when the DTC set as Freeze Frame/Failure Records data. |
Conditions for Clearing the MIL/DTC
| • | The PCM will turn OFF the malfunction indicator lamp (MIL) during the third consecutive trip in which the diagnostic has run and passed. |
| • | The history DTC will clear after 40 consecutive warm-up cycles have occurred without a malfunction. |
| • | The DTC can be cleared by using a scan tool. |
Diagnostic Aids
Inspect for the following:
Many situations may lead to an intermittent condition. Perform each inspection or test as directed.
Important: : Remove any debris from the connector surfaces before servicing a component. Inspect the connector gaskets when diagnosing or replacing a component. Ensure that the gaskets are installed correctly. The gaskets prevent contaminate intrusion.
| • | Loose terminal connection |
| - | Use a corresponding mating terminal to test for proper tension. Refer to Testing for Intermittent Conditions and Poor Connections , and to Connector Repairs in Wiring Systems for diagnosis and repair. |
| - | Inspect the harness connectors for backed out terminals, improper mating, broken locks, improperly formed or damaged terminals, and faulty terminal to wire connection. Refer to Testing for Intermittent Conditions and Poor Connections , and to Connector Repairs in Wiring Systems for diagnosis and repair. |
| • | Damaged harness--Inspect the wiring harness for damage. If the harness inspection does not reveal a problem, observe the display on the scan tool while moving connectors and wiring harnesses related to the sensor. A change in the scan tool display may indicate the location of the fault. Refer to Wiring Repairs in Wiring Systems for diagnosis and repair. |
| • | Inspect the powertrain control module (PCM) and the engine grounds for clean and secure connections. Refer to Wiring Repairs in Wiring Systems for diagnosis and repair. |
If the condition is determined to be intermittent, reviewing the Snapshot or Freeze Frame/Failure Records may be useful in determining when the DTC or condition was identified.
Test Description
The numbers below refer to the step numbers on the diagnostic table:
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The HO2S should be allowed to cool before performing this test. If the HO2S heater is functioning, the signal voltage will gradually increase or decrease as the sensor element warms. If the heater is not functioning, the HO2S signal will remain near the 450mV bias voltage.
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This step 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 faulty.
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This step tests the HO2S heater ground circuit.
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This step tests for an open or shorted HO2S heater element. The heater element resistance will vary according to HO2S temperature (a hot HO2S heater element will measure a much higher resistance than a HO2S heater element at room temperature). It is important to allow the HO2S to cool before measuring HO2S heater element resistance.
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An open HO2S signal or low circuit can cause the HO2S heater to appear faulty. inspect these circuits before replacing the sensor.
Step | Action | Values | Yes | No |
|---|---|---|---|---|
1 | Did you perform the Powertrain On-Board Diagnostic (OBD) System Check? | -- | ||
2 |
Important: If engine has just been operating, allow engine to cool for about one half hour before proceeding.
Does HO2S voltage vary from bias voltage to more than the first specified value or less than the second specified value? | 600 mV 300 mV | Go to Diagnostic Aids | |
3 | Inspect ignition feed fuse for the HO2S. Is the fuse open? | -- | ||
4 |
Important:: Use a known good ground. Do not use the HO2S heater ground circuit or the HO2S low circuit.
Does the test lamp illuminate? | -- | ||
5 | Connect a test lamp between HO2S ignition feed and HO2S heater ground. Does the test lamp illuminate? | -- | ||
6 |
Does the resistance measure within the specified range? | 5-10 ohms | ||
7 | Repair the open HO2S ignition feed circuit. Refer to Wiring Repairs in Wiring Systems. Did you complete the repair? | -- | -- | |
8 | Repair open HO2S heater ground circuit. Refer to Wiring Repairs in Wiring Systems. Did you complete the repair? | -- | -- | |
9 | Inspect 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? | -- | ||
10 |
Did you find and correct the condition? | -- | ||
11 | Inspect for a poor HO2S signal or low circuit connection 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? | -- | ||
12 | Inspect for a poor HO2S low circuit connection 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? | -- | ||
13 | Inspect for a poor HO2S signal circuit connection 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? | -- | ||
14 | Replace the HO2S. Refer to Heated Oxygen Sensor Replacement Did you complete the replacement? | -- | -- | |
15 | Repair the short to ground in HO2S ignition feed circuit and replace the faulty fuse. Refer to 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 HO2S voltage go from bias voltage to more than the first specified value or less than the second specified value? | 600 mV 300 mV | System OK |
