GM Service Manual Online
For 1990-2009 cars only

Circuit Description

The heated oxygen sensors (HO2S) are used for fuel control and catalyst monitoring. Each HO2S compares the oxygen content of the surrounding air with the oxygen content of the exhaust stream. When the vehicle is started, the control module operates in an Open Loop mode, ignoring the HO2S signal voltage while calculating the air-to-fuel ratio. The control module supplies the HO2S with a reference, or bias, voltage of approximately 450 mV. While the engine runs, the HO2S heats up and begins to generate a voltage within a range of 0-1,000 mV. This voltage will fluctuate above and below the bias voltage. Once sufficient HO2S voltage fluctuation is observed by the control module, Closed Loop is entered. The control module uses the HO2S voltage to determine the air-to-fuel ratio. An HO2S voltage that increases above bias voltage toward 1,000 mV indicates a rich fuel mixture. An HO2S voltage that decreases below bias voltage toward 0 mV indicates a lean fuel mixture.

The heating elements inside each HO2S heat the sensor to bring the sensor up to operating conditions faster. This allows the system to enter Closed Loop earlier and the control module to calculate the air-to-fuel ratio sooner.

The HO2S utilizes the following circuits:

    • A signal circuit
    • A low reference circuit
    • An ignition 1 voltage circuit
    • A heater control circuit

If the ECM detects the HO2S 1 voltage signal is too low during Power Enrichment (PE) mode, this DTC sets.

DTC Descriptor

This diagnostic procedure supports the following DTC:

DTC P1171 HO2S Circuit Low Voltage During Power Enrichment Sensor 1

Conditions for Running the DTC

    • DTCs P0106, P0107, P0108, P0117, P0118, P0122, P0123, P0171, P0172, P0201-P0204, P0300, P0336, P0337, P0351, P0352, P0401, P0402, P0404, P0405, P0406, P0441, P0443, P0488, P0502, P0506, P0507, and P01404 are not set.
    • The engine coolant temperature is more than 60°C (140°F).
    • The ignition 1 voltage is more than 10 volts.
    • The air fuel ratio is less than or equal to 13.5:1.
    • The engine is in Power Enrichment (PE) mode.
    • DTC P1171 runs continuously once the above conditions are met for 2 seconds.

Conditions for Setting the DTC

The ECM detects that the HO2S 1 voltage is less than 350 mV for 11 seconds during Power Enrichment (PE) mode.

Action Taken When the DTC Sets

    • The control module illuminates the malfunction indicator lamp (MIL) when the diagnostic runs and fails.
    • The control module records the operating conditions at the time the diagnostic fails. The control module stores this information in the Freeze Frame/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.

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 - Engine Controls?

--

Go to Step 2

Go to Diagnostic System Check - Engine Controls

2

Observe the DTC Information with a scan tool.

Is DTC P0131 or P0134 also set?

--

Go to Diagnostic Trouble Code (DTC) List

Go to Step 3

3

  1. Start the engine.
  2. Allow the engine to reach the normal operating temperature.
  3. Operate the engine above 1,200 RPM for 2 minutes.
  4. Observe the HO2S 1 voltage parameter with a scan tool.

Does the voltage remain below the specified value?

350 mV

Go to Step 5

Go to Step 4

4

  1. Observe the Freeze Frame/Failure Records for this DTC.
  2. Turn OFF the ignition for 30 seconds.
  3. Start the engine.
  4. Operate the vehicle within the Conditions for Running the DTC. You may also operate the vehicle within the conditions that you observed from the Freeze Frame/Failure Records.

Did the DTC fail this ignition?

--

Go to Step 5

Go to Intermittent Conditions

5

  1. Turn OFF the ignition.
  2. Disconnect the HO2S 1 sensor.
  3. Turn ON the ignition, with the engine OFF.
  4. Measure the voltage from the HO2S 1 signal circuit to a good ground with a DMM.

Is the voltage within the specified range?

350-550 mV

Go to Step 7

Go to Step 6

6

Test the HO2S 1 high signal circuit for a short to ground or a short to the HO2S 1 low reference circuit. Refer to Circuit Testing and Wiring Repairs in Wiring Systems.

Did you find and correct the condition?

--

Go to Step 12

Go to Step 9

7

  1. The HO2S 1 may be contaminated or detecting a lean condition. Inspect for the following conditions:
  2. • HO2S wiring or connector water intrusion.
    • Silicon-contaminated H02S.
    • Vacuum leaks
    • Exhaust leaks, including leaks around the HO2S sealing surface. Refer to Symptoms - Engine Exhaust in Engine Exhaust.
  3. Repair any of the above or similar engine conditions as necessary.

Did you find and correct the condition?

--

Go to Step 12

Go to Step 8

8

Test for an intermittent and poor connections at the harness connector of the HO2S 1 sensor. Refer to Testing for Intermittent Conditions and Poor Connections and Connector Repairs in Wiring Systems.

--

Go to Step 12

Go to Step 10

9

Test for an intermittent and poor connection at the engine control module (ECM). Refer to Testing for Intermittent Conditions and Poor Connections and Repairing Connector Terminals in Wiring Systems.

Did you find and correct the condition?

--

Go to Step 12

Go to Step 11

10

Replace the HO2S 1. Refer to Heated Oxygen Sensor 1 Replacement .

Did you complete the replacement?

--

Go to Step 12

--

11

Replace the ECM. Refer to Engine Control Module Replacement .

Did you complete the replacement?

--

Go to Step 12

--

12

  1. Clear the DTCs with a scan tool.
  2. Turn OFF the ignition for 30 seconds.
  3. Start the engine.
  4. Operate the vehicle within the Conditions for Running the DTC. You may also operate the vehicle within the conditions that you observed from the Freeze Frame/Failure Records.

Did the DTC fail this ignition?

--

Go to Step 2

Go to Step 13

13

Observe the Capture Info with a scan tool.

Are there any DTCs that have not been diagnosed?

--

Go to Diagnostic Trouble Code (DTC) List

System OK