Circuit Description
In order to control emissions, a 3-way catalytic (TWC) converter converts any harmful exhaust emissions into harmless water vapor and carbon dioxide.
The powertrain control module (PCM) has the capability to monitor this process by using a rear heated oxygen sensor (HO2S) 2. The HO2S 2, located in the exhaust stream past the TWC converter, produces an output signal which indicates the storage capacity of the catalyst. This in turn indicates the catalysts ability to convert the exhaust emissions effectively. If the catalyst is functioning properly, the HO2S 2 signal will be far less active than the signal produced by the front oxygen sensor (O2S) 1.
Obstruction of the air reference and degraded HO2S 2 performance can result from any attempt to repair the above conditions.
The HO2S 2 must have a clean air reference in order for the HO2S 2 to function properly. This clean air reference is obtained by way of the HO2S 2 wires.
Conditions for Running the DTC
| • | The ignition 1 signal is greater than 10 volts. |
| • | The engine coolant temperature (ECT) is less than 45°C (113°F). |
| • | The intake air temperature (IAT) is less than 45°C (113°F). |
| • | The difference between the ECT and the IAT is less than 7°C (45°F). |
| • | The fuel level is more than 10 percent. |
| • | The rear O2S voltage is between 395-495 mV. |
Conditions for Setting the DTC
| • | The HO2S voltage does not go greater than 600 mV or less than 300 mV within 110-255 seconds after cold start. |
| • | The average flow must be less than 20 g/s during the test time. |
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
An intermittent could be the result of the following conditions:
| • | A faulty electrical connection |
| • | Moisture in the connector/wiring |
| • | Rubbed-through wire insulation |
| • | A broken wire inside the insulation |
Inspect for a faulty electrical connection, or for a damaged harness. Inspect the harness electrical connectors for the following conditions:
| • | Improper mating |
| • | Broken locks |
| • | Improperly-formed electrical connectors |
| • | Damaged terminals |
| • | A faulty terminal-to-wire connection |
| • | A damaged harness |
Obstruction of the air reference and degraded HO2S 2 performance could result from any attempt to repair the above conditions.
The HO2S 2 must have a clean air reference in order for the HO2S 2 to function properly. This clean air reference is obtained by way of the HO2S 2 wires.
Test Description
The numbers below refer to the step numbers on the diagnostic table.
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As the heater warms the O2S to operating temperature, the HO2S 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 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 heater ignition 1 voltage and ground supply.
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This step checks for proper PCM and signal circuit operation.
Step | Action | Values | Yes | No | ||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|
Schematic Reference: Engine Controls Schematics | ||||||||||||
1 | Did you perform the Diagnostic System Check-Engine Controls? | -- | Go to Step 2 | |||||||||
|
Important: The HO2S must be cool before proceeding with a diagnosis. The ignition must remain OFF for at least one-half hour before proceeding with a diagnosis.
Does the HO2S voltage increase or decrease from the initial voltage by at least the value specified? | 150 mV | Go to Step 3 | Go to Step 4 | |||||||||
3 |
Did you find and correct the condition? | -- | Go to Step 15 | Go to Diagnostic Aids | ||||||||
Does the test lamp illuminate? | -- | Go to Step 5 | Go to Step 6 | |||||||||
Ensure that the ignition is ON while the engine is OFF. Is the HO2S voltage less than the specified value? | 20 mV | Go to Step 8 | Go to Step 7 | |||||||||
6 |
Does the test lamp illuminate? | -- | Go to Step 10 | Go to Step 11 | ||||||||
7 |
Does the resistance measure less than the specified value? | 5 ohms | Go to Step 13 | Go to Step 12 | ||||||||
8 | Inspect for poor connections at the harness connector of the HO2S sensor. Refer to Testing for Intermittent Conditions and Poor Connections and Heated Oxygen Sensor Wiring Repairs in Wiring Systems. Did you find and correct the condition? | -- | Go to Step 15 | Go to Step 9 | ||||||||
9 | Replace the HO2S. Refer to Heated Oxygen Sensor Replacement . Did you complete the replacement? | -- | Go to Step 15 | -- | ||||||||
10 | Repair the open HO2S heater ground circuit. Refer to Wiring Repairs in Wiring Systems. Did you complete the replacement? | -- | Go to Step 15 | -- | ||||||||
11 |
Did you complete the repair? | -- | Go to Step 15 | -- | ||||||||
12 | Repair the circuit that measured high resistance. Refer to Wiring Repairs in Wiring Systems. Did you complete the repair? | -- | Go to Step 15 | -- | ||||||||
13 |
Did you find and correct the condition? | -- | Go to Step 15 | Go to Step 14 | ||||||||
14 | Replace the PCM. Refer to Powertrain Control Module Replacement . Did you complete the replacement? | -- | Go to Step 15 | -- | ||||||||
15 |
Does the DTC run and pass? | -- | Go to Step 16 | Go to Step 2 | ||||||||
16 | With a scan tool, observe the stored information, Capture Info. Does the scan tool display any DTCs that you have not diagnosed? | -- | System OK | |||||||||
