Circuit Description
The Heated Oxygen Sensor (HO2S) is used in order to determine the oxygen content of the engine's exhaust. The oxygen content of the exhaust indicates when the engine is operating lean or rich. When the engine is operating lean, the exhaust will have more oxygen content. Once at operating temperature, the HO2S will produce a voltage in relation to the oxygen content of the exhaust. The HO2S requires an operating temperature of 360°C (600°F) to produce voltage. The HO2S will produce more voltage when the exhaust is rich and less voltage when the exhaust is lean. The HO2S operating voltage range is around 0.10 volts (100 mV) to 1 volt (1000 mV).
The gasoline control module (VCM/PCM or ECM) supplies a bias (or reference) voltage to the HO2S. This bias voltage is around 450 millivolts. The AF ECU monitors the bias voltage as well as the voltage produced by the HO2S.
During normal Closed Loop fuel control operation, the AF ECU will add fuel (enriches the mixture) when the HO2S indicates a lean exhaust content. When the HO2S indicates a rich exhaust content the AF ECU will subtract fuel (leans-out the mixture). This oscillation above and below the bias voltage (sometimes referred to as activity or switching) can be monitored with the HO2S signal voltage.
The oxygen sensor contains a heater. The heater is necessary in order to quickly warm the sensor to operating temperature. The heater will also maintain operating sensor temperature during extended idle conditions. A functioning oxygen sensor heater will allow the sensor to produce voltage in a short amount of time. A sensor at operating temperature will cause the HO2S signal voltage to go above or below (depending upon exhaust oxygen content) the bias voltage.
This DTC is used to indicate an HO2S signal voltage that has not left the cold sensor bias voltage range in an acceptable amount of time.
Conditions for Running the DTC
| • | The engine is operating on alternative fuel. |
| • | No AF ECU ECT DTCs are set. |
| • | The engine start-up coolant temperature is less than 33°C (91°F). |
| • | The engine has been operating for at least 5 seconds. |
| • | The mass air flow is less than 35 grams per second. |
| • | The battery voltage is more than 8 volts. |
| • | All other AF ECU DTCs for this HO2S sensor have ran and passed. |
| • | This DTC has not tested this ignition cycle. |
| • | The AF ECU determines the HO2S was cold at ignition key ON. The average HO2S signal voltage must be within the bias voltage range of 0.30 volts (300 mV) to 0.60 volts (600 mV) in order for the AF ECU to determine the HO2S was cold. |
Conditions for Setting the DTC
The AF ECU monitors an HO2S signal voltage that did not leave the cold sensor bias voltage range 0.30 volts (300 mV) to 0.60 volts (600 mV) in an acceptable amount of time. The time varies from 50 seconds to 110 seconds based upon mass air flow.
Action Taken When the DTC Sets
| • | A first failure of this DTC will NOT illuminate the MIL and will store as Last Test Failed but not as History. |
| • | The AF ECU will illuminate the MIL and store the DTC as History after TWO consecutive drive trips that the diagnostic runs and fails. |
| • | The AF ECU will record operating conditions at the time the DTC sets. This information will be stored as Freeze Frame data. |
Conditions for Clearing the MIL/DTC
| • | The Last Test Failed status will clear when the DTC runs and passes. |
| • | The AF ECU will turn OFF the MIL after 3 consecutive drive trips that the DTC runs and passes. |
| • | The History status will clear after 40 consecutive WARM-UP cycles with NO failures of ANY DTC. |
| • | All DTC Information can be cleared using a scan tool. |
| • | Interruption of Control Module (AF ECU / VCM / PCM or ECM) power or ground MAY clear DTC Information. Clearing DTC Information with this method is inconsistent and should not be performed. |
Diagnostic Aids
Fault Not Present
Fault Not Present indicates the condition that caused the DTC to set is intermittent and not currently present. Refer to Intermittent Conditions for additional information. If any additional DTCs were stored proceed to the applicable DTC table in the order stated in the OBD System Check.
Test Description
The number(s) below refer to the step number(s) on the diagnostic table.
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As the heater warms the oxygen sensor 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 (bias) voltage due to the high oxygen content.
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This step tests for a proper HO2S heater ignition feed and ground supply.
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This step tests for proper control module and signal circuit operation.
Step | Action | Value(s) | Yes | No | ||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|
1 | Did you perform the AF Powertrain On-Board Diagnostic (OBD) System Check? | -- | Go to Alternative Fuels (AF) Powertrain On Board (OBD) System Check | |||||||||
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Important: The HO2S must be cool before proceeding with this diagnosis. The ignition must remain OFF for at least one half hour before proceeding with the diagnosis.
Establish communication with the gasoline control module (VCM/PCM or ECM) for this step. Does the HO2S voltage increase or decrease from the initial voltage by at least the specified value? | 150 mV | |||||||||||
3 |
Does the AF ECU indicate the same HO2S voltage as the gasoline control module, plus or minus the specified value? | 45 mV | ||||||||||
4 | The condition that set this DTC is not present. This DTC may have been set by one of the following conditions:
Repair any of the above conditions as necessary. Was a condition found and repaired? | -- | ||||||||||
5 | The fault is not present. Refer to Diagnostic Aids. Are any DTCs stored that have not been diagnosed? | -- | Go to Intermittent Conditions | |||||||||
Turn the ignition ON, leaving the engine OFF. Does the test lamp illuminate? | -- | |||||||||||
Ensure the ignition is ON while the engine is OFF. Is the voltage less than the specified value? | 20 mV | |||||||||||
8 | Connect the test lamp between the HO2S heater ignition feed circuit terminal (engine harness side) and a known good ground. Ensure the ignition is ON while the engine is OFF. Does the test lamp illuminate? | -- | ||||||||||
9 |
Are both resistances less than the specified value? | 5 ohms | ||||||||||
10 | Replace the HO2S sensor. Refer to Heated Oxygen Sensor Replacement . Did you complete the repair? | -- | -- | |||||||||
11 | Repair the open HO2S heater ground circuit. Did you complete the repair? | -- | -- | |||||||||
12 | Repair the lack of power on the HO2S heater ignition feed circuit. If the fuse is open repair the short to ground on the circuit. An open fuse or lack of power to the fuse may cause other DTCs or symptoms to be present. Did you complete the repair? | -- | -- | |||||||||
13 | Repair the circuit that measured high resistance. Did you complete the repair? | -- | -- | |||||||||
14 |
Ensure the oxygen sensor, gasoline control module and the AF ECU connectors are disconnected before testing circuits. Was a wire repair made? | -- | ||||||||||
15 | Replace the AF ECU. Refer to Engine Control Unit Replacement . Did you complete the repair? | -- | -- | |||||||||
16 |
Was a terminal repair made? | -- | ||||||||||
17 |
Important: The following control module replacement procedures must all be completed before attempting to operate the vehicle.
Important: In order to perform the variation learn procedure, a switch to gasoline operation is necessary. Remove the CNG 20 amp fuse in order to perform the switch. All of the above procedures are located in the gasoline engine service manual. Did you complete the repair? | -- | -- | |||||||||
18 |
Important: The HO2S must be cool before proceeding with this diagnosis. The Ignition must remain OFF for at least one half hour before proceeding with the diagnosis.
Does the voltage increase or decrease from the initial voltage by at least the specified value? | 150 mV | System OK |
