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 AF ECU utilizes a Closed Loop air/fuel metering system when the HO2S is at operating temperature. 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 addition and subtraction to the air/fuel mixture may be indicated on a scan tool as the Long and Short Term Fuel Trim values. Short Term Fuel Trim values are adjustments to the fuel mixture that occur quickly. Long Term Fuel Trim values are adjustments to the fuel mixture that accumulate over longer periods of time. The Long and Short Term Fuel Trim values are displayed on a scan tool as both percent and counts. Positive percentage values indicate fuel is being added to the mixture. Negative percentage values indicate fuel is being subtracted from the mixture.
This DTC is used to indicate a fully rich fuel trim system that cannot compensate for a lean engine exhaust content.
Conditions for Running the DTC
| • | The engine is operating on alternative fuel. |
| The engine coolant temperature is more than 85°C (185°F). |
| • | The AF ECU is operating in Closed Loop fuel control. |
Conditions for Setting the DTC
The AF ECU monitors that the Short Term Fuel Trim is at plus 50 percent OR the Long Term Fuel Trim is at plus 45 percent for at least 5 seconds continuously.
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 numbers below refer to the step numbers on the diagnostic table.
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The engine may also have a higher than normal idle if a vacuum leak is present.
An exhaust system leak may be very small and cause the oxygen sensor to sense a false lean condition.
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When testing the circuits for continuity, the resistance should be less than 5 ohms. When testing the circuits for shorts, the resistance should measure infinite with all components disconnected from the circuit.
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This step determines if a base engine concern is causing the lean condition or if the lean condition is only present when operating on CNG.
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 | |||||||||||||||||||||
2 |
Are other DTCs stored? | -- | ||||||||||||||||||||||
3 | Perform the other DTC diagnosis before attempting diagnosis of this DTC. Is the other DTC diagnosis and/or repair complete? | -- | -- | |||||||||||||||||||||
This DTC may have been set by one of the following conditions:
Was a condition found and repaired? | -- | |||||||||||||||||||||||
5 |
Do the fuel trim values indicate a lean mixture condition is present or does a fuel trim DTC set? | -- | ||||||||||||||||||||||
6 | The fault is not present. Refer to Diagnostic Aids. Are any DTCs stored that have not been diagnosed? | -- | Go to Intermittent Conditions | |||||||||||||||||||||
7 | Perform the AF Fuel System diagnosis and return to this step. Refer to Fuel System Diagnosis . Was a CNG fuel system problem found and repaired? | -- | ||||||||||||||||||||||
Ensure the oxygen sensor, gasoline control module, and the AF ECU connectors are disconnected before testing the circuits. Was a wiring repair made? | -- | |||||||||||||||||||||||
The FIL will NOT illuminate due to the removal of the CNG 20 amp fuse. Do the fuel trim values indicate a lean mixture condition is present or does a fuel trim DTC set? | -- | |||||||||||||||||||||||
10 | Repair the possible causes of lean engine operation. Refer to the items in Step 4. Is the repair complete? | -- | -- | |||||||||||||||||||||
11 | Replace the GMS/MCV. Refer to Gas Mass Sensor/Mixture Control Valve Mounting Bracket Replacement . Is the replacement complete? | -- | -- | |||||||||||||||||||||
12 |
Does the scan tool indicate the DTC passed? | -- | System OK |
