GM Service Manual Online
For 1990-2009 cars only

Circuit Description

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 within each HO2S heat the sensor. The powertrain control module (PCM) controls the HO2S heater control circuit. The HO2S heater diagnostic monitors the current draw through the HO2S output driver module (ODM) when the engine is running. This allows the system to enter Closed Loop earlier and the control module to calculate the air-to-fuel ratio sooner.

The PCM commands the heater ON or OFF to maintain a specific HO2S operating temperature range. The PCM determines the temperature by measuring the current flow through the heater. When the heater is in the ON state, the PCM will pulse the heater OFF for a duration of 50 milliseconds, once per second. when the heater is in the OFF state, the PCM will pulse the heater ON for a duration of 50 milliseconds, once per second.

If the PCM detects there is a circuit fault present on the HO2S 2 heater low control circuit, DTC P0056 for bank 2 sensor 2 will set.

The HO2S 2 has the following circuits:

    • A HO2S 2 high signal circuit
    • A HO2S 2 low reference circuit
    • A HO2S 2 heater ignition voltage circuit
    • A HO2S 2 heater low control circuit

DTC Descriptor

This diagnostic procedure supports the following DTC:

DTC P0056 HO2S Heater Control Circuit Bank 2 Sensor 2

Conditions for Running the DTC

The following conditions must be met:

    • The engine is cranking or running.
    • The system voltage is between 11-18 volts.

Conditions for Setting the DTC

    • The PCM detects that the HO2S 2 heater control circuit status is incorrect for more than 2 seconds.
    • This diagnostic runs continuously.

Action Taken When the DTC Sets

    • The control module illuminates the malfunction indicator lamp (MIL) on the second consecutive ignition cycle that the diagnostic runs and fails.
    • The control module records the operating conditions at the time the diagnostic fails. The first time the diagnostic fails, the control module stores this information in the Failure Records. If the diagnostic reports a failure on the second consecutive ignition cycle, the control module records the operating conditions at the time of the failure. The control module writes the operating conditions to the Freeze Frame and updates the 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: Powertrain Control Module Connector End Views or Engine Controls Connector End Views

1

Did you perform the Diagnostic System Check - Vehicle?

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Go to Step 2

Go to Diagnostic System Check - Vehicle

2

  1. Start the engine.
  2. Allow the engine to idle at operating temperature. Refer to Scan Tool Data List .
  3. Observe the affected HO2S Htr. Circuit Status parameter with a scan tool.

Does the parameter display Fault?

--

Go to Step 5

Go to Step 3

3

  1. Turn OFF the ignition.
  2. Turn ON the ignition, with the engine OFF.
  3. Observe the affected HO2S Htr. Circuit Status parameter with a scan tool.

Does the parameter display Fault?

--

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 Testing for Intermittent Conditions and Poor Connections

5

Inspect the fuse that powers the heated oxygen sensor (HO2S) for an open.

Is the fuse open?

--

Go to Step 6

Go to Step 7

6

Test the HO2S ignition 1 voltage circuit for a short to ground. Refer to Circuit Testing , Wiring Repairs , and Heated Oxygen Sensor Wiring Repairs .

Did you find and correct the condition?

--

Go to Step 20

Go to Step 14

7

  1. Turn OFF the ignition.
  2. Disconnect the HO2S 2.
  3. Important: Use a known good ground. DO NOT use the HO2S heater low control circuit or the HO2S low signal circuit.

  4. Probe the HO2S ignition 1 voltage circuit, on the engine harness side, with a test lamp that is connected to a good ground. Refer to Probing Electrical Connectors .
  5. Turn ON the ignition, with the engine OFF.

Does the test lamp illuminate?

--

Go to Step 8

Go to Step 17

8

Connect a test lamp between the HO2S ignition 1 voltage circuit and the HO2S heater low control circuit, on the engine harness side.

Does the test lamp illuminate?

--

Go to Step 11

Go to Step 9

9

Start the engine.

Does the test lamp illuminate?

--

Go to Step 15

Go to Step 10

10

  1. Turn OFF the engine.
  2. Turn ON the ignition, with the engine OFF.
  3. Measure the voltage from the HO2S heater low control circuit, on the engine harness side, to a good ground with a DMM. Refer to Circuit Testing .

Is the voltage more than the specified value?

1 V

Go to Step 12

Go to Step 13

11

Test the HO2S heater low control circuit for a short to ground. Refer to Circuit Testing , Wiring Repairs , and Heated Oxygen Sensor Wiring Repairs .

Did you find and correct the condition?

--

Go to Step 20

Go to Step 16

12

Test the HO2S heater low control circuit for a short to voltage. Refer to Circuit Testing , Wiring Repairs , and Heated Oxygen Sensor Wiring Repairs .

Did you find and correct the condition?

--

Go to Step 20

Go to Step 16

13

Test the HO2S heater low control circuit for an open. Refer to Circuit Testing , Wiring Repairs , and Heated Oxygen Sensor Wiring Repairs .

Did you find and correct the condition?

--

Go to Step 20

Go to Step 16

14

Important: Perform the following test on the HO2S that is supplied voltage by the suspect circuit.

Test the HO2S ignition 1 voltage circuit, on the sensor side of the HO2S connector, for a short to ground. Refer to Circuit Testing .

Is the sensor shorted to ground?

--

Go to Step 18

Go to Testing for Intermittent Conditions and Poor Connections

15

Test for shorted terminals and poor connections at the HO2S. Refer to Testing for Intermittent Conditions and Poor Connections , Connector Repairs , and Heated Oxygen Sensor Wiring Repairs .

Did you find and correct the condition?

--

Go to Step 20

Go to Step 18

16

Test for shorted terminals and poor connections at the powertrain control module (PCM). Refer to Testing for Intermittent Conditions and Poor Connections and Connector Repairs .

Did you find and correct the condition?

--

Go to Step 20

Go to Step 19

17

Repair the open or high resistance in the HO2S ignition 1 voltage circuit. Refer to Wiring Repairs and Heated Oxygen Sensor Wiring Repairs .

Did you complete the repair?

--

Go to Step 20

--

18

Notice: Refer to Heated Oxygen Sensor Resistance Learn Reset Notice in the Preface section.

Replace the HO2S. Refer to Heated Oxygen Sensor Replacement - Bank 2 Sensor 2 .

Did you complete the replacement?

--

Go to Step 20

--

19

Replace the PCM. Refer to Control Module References for replacement, setup, and programming.

Did you complete the replacement?

--

Go to Step 20

--

20

  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 21

21

Observe the Capture Info with a scan tool.

Are there any DTCs that have not been diagnosed?

--

Go to Diagnostic Trouble Code (DTC) List - Vehicle

System OK