System Description
Heated oxygen sensors (HO2S) are used for fuel control and post catalyst monitoring. Each HO2S compares the oxygen content of the surrounding air with the oxygen content in the exhaust stream. The HO2S must reach operating temperature to provide an accurate voltage signal. Heating elements inside the HO2S minimize the time required for the sensors to reach operating temperature. The engine control module (ECM) supplies the HO2S with a reference, or bias, voltage of about 450 mV. When the engine is first started the ECM operates in Open Loop, ignoring the HO2S voltage signal. Once the HO2S reaches operating temperatures and Closed Loop is achieved, the HO2S generates a voltage within a range of 0-1,000 mV that fluctuates above and below bias voltage. High HO2S voltage indicates a rich exhaust stream. Low HO2S voltage indicates a lean exhaust stream.
If the ECM detects an HO2S voltage that stays below a specified value, DTC P1171 will set.
DTC Descriptor
This diagnostic procedure supports the following DTC:
DTC P1171 Fuel System Lean During Acceleration
Conditions for Running the DTC
| • | DTCs P0122, P0123, P0131, P0132, P0133, P0134 are not set. |
| • | The ECT Sensor parameter is more than 20°C (68°F). |
| • | The Engine Run Time parameter is more than 20 seconds. |
| • | The Loop Status parameter is closed. |
| • | The Power Enrichment parameter is active. |
Conditions for Setting the DTC
The ECM detects that the HO2S 1 parameter is less than 300 mV for 5 seconds.
Action Taken When the DTC Sets
| • | The control module stores the DTC information into memory when the diagnostic runs and fails. |
| • | The malfunction indicator lamp (MIL) will not illuminate. |
| • | The control module records the operating conditions at the time the diagnostic fails. The control module stores this information in the Failure Records. |
| • | The driver information center, if equipped, may display a message. |
Conditions for Clearing the DTC
| • | 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 non-emission related diagnostic. |
| • | Clear the DTC with a scan tool. |
Diagnostic Aids
| • | A restricted fuel filter can supply adequate amounts of fuel at idle, but may not be able to supply enough fuel during heavy acceleration. Water or alcohol in the fuel may cause low HO2S 1 voltage during acceleration. |
| • | High resistance in the ignition coil control circuits may cause this condition. |
| • | Manifold absolute pressure (MAP) sensor-High resistance in the MAP sensor circuits may cause this condition. |
| • | Inspect for faulty or plugged fuel injectors. |
Test Description
The numbers below refer to the step numbers on the diagnostic table.
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When the system is operating correctly, the HO2S 1 voltage should toggle above and below the bias voltage. You may need to operate the vehicle within the Freeze Frame conditions and Conditions for Setting the DTC in order to duplicate the malfunction that was detected by the ECM.
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The specified value is what is measured on a correctly operating system.
Step | Action | Values | Yes | No | ||||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
Schematic Reference: Engine Controls Schematics Connector End View Reference: Engine Control Module Connector End Views or Engine Controls Connector End Views | ||||||||||||||||
1 | Did you perform the Diagnostic System Check - Vehicle? | -- | Go to Step 2 | |||||||||||||
Does the scan tool indicate that the heated oxygen sensor (HO2S) 1 voltage is less than the specified value? | 300 mV | Go to Step 4 | Go to Step 3 | |||||||||||||
3 |
Did the DTC fail this ignition? | -- | Go to Step 4 | Go to Diagnostic Aids | ||||||||||||
Does the voltage measure within the specified value? | 350-550 mV | Go to Step 6 | Go to Step 5 | |||||||||||||
5 | Test the HO2S 1 high signal circuit for a short to ground or a short to the HO2S 1 low signal circuit. Refer to Circuit Testing and Wiring Repairs . Did you find and correct the condition? | -- | Go to Step 11 | Go to Step 8 | ||||||||||||
6 |
Did you find and correct the condition? | -- | Go to Step 11 | Go to Step 7 | ||||||||||||
7 | Inspect for poor connections at the harness connector of the HO2S 1 sensor. Refer to Testing for Intermittent Conditions and Poor Connections and Connector Repairs . Did you find and correct the condition? | -- | Go to Step 11 | Go to Step 9 | ||||||||||||
8 | Inspect for poor connections at the harness connector of the engine control module (ECM). Refer to Testing for Intermittent Conditions and Poor Connections and Connector Repairs . Did you find and correct the condition? | -- | Go to Step 11 | Go to Step 10 | ||||||||||||
9 |
Important: Before replacing the HO2S, determine and remove any source of contamination. Replace the HO2S 1. Refer to Heated Oxygen Sensor 1 Replacement . Did you complete the replacement? | -- | Go to Step 11 | -- | ||||||||||||
10 | Replace the ECM. Refer to Control Module References for Replacement, Setup, and Programming. Did you complete the repair? | -- | Go to Step 11 | -- | ||||||||||||
11 |
Did the DTC fail this ignition? | -- | Go to Step 2 | Go to Step 12 | ||||||||||||
12 | Observe the Capture Info with a scan tool. Are there any DTCs that have not been diagnosed? | -- | System OK | |||||||||||||
