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 engine 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 about 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 voltage is low or is near bias voltage, a DTC sets.
DTC Descriptor
This diagnostic procedure supports the following DTCs:
| • | DTC P0131 HO2S Circuit Low Voltage Bank 1 Sensor 1 |
| • | DTC P0151 HO2S Circuit Low Voltage Bank 2 Sensor 1 |
Conditions for Running the DTC
| • | DTCs P0030, P0031, P0050, P0051, P0132, P0133, P0134, P0152, P0153, P0154, P0171, P0172, P0174, P0175, P1171, P1172, P1174, or P1175 are not set. |
| • | The ignition 1 voltage is between 11-16 volts. |
| • | The fuel system is in closed loop. |
| • | The engine speed is less than 6000 RPM. |
| • | The DTCs run continuously once the above conditions are met. |
Conditions for Setting the DTC
The ECM detects one of the following conditions:
| • | The ECM detects that the HO2S voltage is less than 20 mV for 15 seconds in closed loop. |
| • | The ECM does not detect closed loop within 40-125 seconds which depends on startup engine coolant temperature. |
| • | The ECM detects that the HO2S voltage remains between 352-499 mV for 15 seconds in closed loop. |
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. |
Test Description
The numbers below refer to the step numbers on the diagnostic table.
-
This step tests for a short to ground to the signal circuit of the HO2S.
-
This step tests the HO2S signal and the low reference circuit for an open. The HO2S voltage should go low.
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 | |||||||||||
2 |
Does the scan tool display Closed Loop? | -- | Go to Step 3 | Go to Step 4 | ||||||||||
3 |
Did the DTC P0131 or P0151 fail this ignition? | -- | Go to Step 4 | Go to Intermittent Conditions | ||||||||||
Is the voltage within the specified range? | 350-550 mV | Go to Step 5 | Go to Step 8 | |||||||||||
Is the voltage less than the specified value? | 20 mV | Go to Step 7 | Go to Step 6 | |||||||||||
6 |
Is the voltage less than the specified value? | 20 mV | Go to Step 9 | Go to Step 8 | ||||||||||
7 | Test or inspect for the following conditions:
Did you find and correct the condition? | Go to Step 14 | Go to Step 10 | |||||||||||
8 | Test the HO2S signal circuit for one of the following conditions:
Refer to Circuit Testing and Wiring Repairs in Wiring Systems. Did you find and correct the condition? | -- | Go to Step 14 | Go to Step 11 | ||||||||||
9 | Test the HO2S low reference circuit for one of the following conditions:
Refer to Circuit Testing and Wiring Repairs in Wiring Systems. Did you find and correct the condition? | -- | Go to Step 14 | Go to Step 11 | ||||||||||
10 | Test for shorted terminals and poor connections at the HO2S. Refer to Testing for Intermittent Conditions and Poor Connections and Connector Repairs in Wiring Systems. Did you find and correct the condition? | -- | Go to Step 14 | Go to Step 12 | ||||||||||
11 | Test for shorted terminals and poor connections at the engine control module (ECM). Refer to Testing for Intermittent Conditions and Poor Connections and Connector Repairs in Wiring Systems. Did you find and correct the condition? | -- | Go to Step 14 | Go to Step 13 | ||||||||||
12 | Replace the appropriate HO2S. Did you complete the replacement? | -- | Go to Step 14 | -- | ||||||||||
13 | Replace the ECM. Refer to Engine Control Module Replacement . Did you complete the replacement? | -- | Go to Step 14 | -- | ||||||||||
14 |
Did the DTC fail this ignition? | -- | Go to Step 2 | Go to Step 15 | ||||||||||
15 | Observe the Capture Info with a scan tool. Are there any DTCs that have not been diagnosed? | -- | System OK | |||||||||||
