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 between 100 mV to 1,000 mV.
The Alternative Fuels ECM utilizes a Closed Loop air/fuel metering system when the HO2S is at operating temperature. During normal Closed Loop fuel control operation, the Alternative Fuels ECM will add fuel when the HO2S indicates a lean exhaust content. When the HO2S indicates a rich exhaust content, the Alternative Fuels ECM will subtract fuel. 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 that fuel is being added to the mixture. Negative percentage values indicate that fuel is being subtracted from the mixture.
This DTC indicates 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 (ECT) is more than 49°C (120°F). |
| • | The Alternative Fuels ECM is operating in Closed Loop fuel control. |
Conditions for Setting the DTC
The Alternative Fuels ECM 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 Alternative Fuels ECM will illuminate the MIL and store the DTC as History after 2 consecutive drive trips that the diagnostic runs and fails. |
| • | The Alternative Fuels ECM 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 Alternative Fuels ECM 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 with a scan tool. |
Test Description
The numbers below refer to the step numbers on the diagnostic table.
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This step determines whether the fault is present.
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If DTC P0171 and DTC P0174 is set at the same time, this indicates both banks of the engine are operating lean. Inspect the items that would cause both banks to operate lean.
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A vacuum leak causes DTC P0171 and DTC P0174 to set at the same time. Inspect all areas of the engine for a vacuum leak. Also inspect the PCV valve for being the correct one for this application. Make sure that the engine oil fill cap is in place and that it is tight. Verify that the engine oil dip stick is fully seated.
Step | Action | Values | Yes | No | ||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
1 | Did you perform the Diagnostic System Check Alternative Fuels - Engine Controls? | -- | Go to Step 2 | Go to Diagnostic System Check - Alternative Fuels (Engine Controls) | ||||||||||
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Important: If any DTCs are set, except P0171 and P0174, refer to those DTCs before proceeding with this diagnostic.
Is the Long Term FT Avg. less than the specified value indicated? | 45% LT | Go to Step 3 | Go to Step 4 | |||||||||||
3 |
Does the DTC fail this ignition cycle? | -- | Go to Step 4 | Go to Intermittent Conditions | ||||||||||
Are both banks of the engine operating lean? | -- | Go to Step 5 | Go to Step 6 | |||||||||||
Inspect for the following conditions:
Did you find and correct the condition? | -- | Go to Step 7 | Go to Fuel System Diagnosis | |||||||||||
6 | Inspect for the following conditions:
Did you find and correct the condition? | -- | Go to Step 7 | Go to Fuel System Diagnosis | ||||||||||
7 |
Does the DTC run and pass? | -- | Go to Step 8 | Go to Step 2 | ||||||||||
8 | With a scan tool, select Capture Info in order to observe the stored information. Does the scan tool display any DTCs that you have not diagnosed? | -- | System OK |
