Circuit Description
Heated oxygen sensors (HO2S) are used to minimize the amount of time required for Closed Loop fuel control operation, and to allow accurate catalyst monitoring. The oxygen sensor (O2S) heater greatly decreases the amount of time required for fuel control sensor HO2S 1 to become active. The O2S 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) will run the heater test only after a cold start, and only once during an ignition cycle. A cold start is determined by the engine coolant (ECT) and intake air temperature (IAT) at the time of startup. When the engine is started, the PCM will monitor the HO2S voltage. When the HO2S voltage indicates a sufficiently active sensor, the PCM measures how much time has elapsed since startup. If the PCM determines that too much time was required for the HO2S 1 to become active, this DTC will set. The time required for the HO2S to reach operating temperature is based on the ECT at start-up and the average mass air flow (MAF) since startup. A higher average airflow or higher startup ECT equals a shorter time to HO2S activity.
Conditions for Setting the DTC
| • | No EGR, Transaxle, TP sensor, EVAP system, misfire, IAT sensor, MAP sensor, fuel trim, fuel injector circuit, ECT sensor, or MAF sensor DTC(s) are present. |
| • | 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. |
| • | (VIN 1) - Average mass airflow for the sample period is less than 18 gm/s. |
| • | (VIN K) - Average mass airflow for the sample period is less than 19 gm/s. |
| • | HO2S 1 voltage remains within 150 mV of the bias voltage (about 450 mV) for a longer amount of time than it should. The amount of time ranges between 30 and 140 seconds depending on engine coolant temperature at startup and average Mass 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 and Failure Records data. |
Conditions for Clearing the MIL/DTC
| • | The PCM will turn OFF the MIL during the third consecutive trip in which the diagnostic has been 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 the scan tool. |
Diagnostic Aids
Check for the following conditions:
| • | Poor connections at the PCM or the HO2S |
| Inspect harness connectors for backed out terminals, improper mating, broken locks, improperly formed or damaged terminals, and poor terminal to wire connection. |
| • | Damaged harness |
| Inspect the wiring harness for damage. If the harness appears to be OK, observe the display on the scan tool while moving connectors and wiring harnesses related to the sensor. A change in the display will indicate 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
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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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.
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Checks the HO2S heater ground circuit.
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Checks for an open or shorted HO2S heater element.
Important:: 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). 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 DTC to set with the HO2S heater operating normally. Check the HO2S signal and the HO2S low circuits before replacing the sensor.
Step | Action | Values | Yes | No |
|---|---|---|---|---|
1 | Was the Powertrain On-Board Diagnostic (OBD) System Check performed? | -- | ||
|
Important:: If the engine has just been operating, allow engine to cool for about one half hour before proceeding.
Does HO2S voltage go from bias voltage to greater than the first specified value or less than the second specified value within 2 minutes? | 600 mV 300 mV | Go to Diagnostic Aids | ||
3 | Check the fuse for the HO2S 1 ignition feed circuit. Is the fuse blown? | -- | ||
Important:: Use a known good ground. Do not use the HO2S 1 heater ground circuit or the HO2S 1 low circuit. Does the test lamp illuminate? | -- | |||
Connect the test lamp between the HO2S 1 ignition feed circuit and the HO2S 1 heater ground circuit. Does test lamp illuminate? | -- | |||
Is the HO2S heater resistance within the specified values? | 4-6 ohms | |||
7 | Repair open HO2S 1 ignition feed circuit to HO2S 1. Refer to Heated Oxygen Sensor Wiring Repairs . Is action complete? | -- | -- | |
8 | Repair open HO2S 1 heater ground circuit. Refer to Heated Oxygen Sensor Wiring Repairs . Is action complete? | -- | -- | |
9 |
Was a problem found? | -- | ||
10 |
Was a problem found? | -- | ||
11 |
Was a problem found? | -- | ||
12 |
Was a problem found? | -- | ||
13 |
Was a problem found? | -- | ||
14 | Replace HO2S 1. Refer to Heated Oxygen Sensor Replacement . Is action complete? | -- | -- | |
15 | Locate and repair short to ground in HO2S 1 ignition feed circuit and replace the fuse. Refer to Wiring Repairs . Is action complete? | -- | -- | |
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 greater than the first specified value or less than the second specified value within 2 minutes? | 600 mV 300 mV | System OK |
