For 1990-2009 cars

### Circuit Description

The powertrain control module (PCM) monitors the heated oxygen sensor (HO2S) activity for 100 seconds. During this test period the PCM counts the number of times that the HO2S signal voltage crosses the rich-to-lean and lean-to-rich thresholds. If the PCM determines that the HO2S did not switch enough times, this diagnostic trouble code (DTC) will be set.

A lean-to-rich switch is determined when the HO2S voltage changes from less than 300 mV to more than 600 mV. A rich-to-lean switch is determined when the HO2S voltage changes from more than 600 mV to less than 300 mV.

### Conditions for Running the DTC

• | DTCs P0101, P0102, P0103, P0107, P0108, P0112, P0113, P0117, P0118, P0121, P0122, P0123, P0125, P0128, P0201-P0206, P0410, P0440, P0442, P0443, P0446, P0449, or P1441 are not set.

• | The system voltage is more than 9 volts.

• | The engine is running in Closed Loop.

• | The engine is running for at least 3.3 minutes.

• | The engine coolant temperature (ECT) is more than 70°C (158°F).

• | The throttle position (TP) sensor angle is more than 2 percent.

• | The engine speed is between 1,300-3,000 RPM.

• | The mass air flow (MAF) is between 13-29 gm/s.

• | The above conditions are stable for 3 seconds.

### Conditions for Setting the DTC

• | The PCM monitors fewer than 25 rich-to-lean and 25 lean-to-rich switches for the HO2S 1.

• | The condition is present for a 100-second monitoring period.

### 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.

1. This step checks for correct sensor activity. When in Closed Loop fuel control, the HO2S voltage should rapidly swing above and below the bias voltage.

2. This step checks the PCM and the HO2S high signal and HO2S low signal circuits between the PCM and the HO2S connector for correct operation.

Step | Action | Values | Yes | No
--- | --- | --- | --- | ---
Schematic Reference: Engine Controls Schematics
1 | Did you perform Diagnostic System Check-Engine Controls? | -- | Go to Step 2 | Go to Diagnostic System Check - Engine Controls
2 |

1. Clear the DTCs using the scan tool.
2. Reset the fuel trim values.
3. Start and idle the engine until normal operating temperature is reached.
4. With the scan tool, observe the HO2S voltage for the sensor that applies to the DTC.

Does the HO2S voltage remain fixed within the range specified?

300-600 mV

Go to Step 4

Go to Step 3

3 |

1. The condition that set this DTC is not present. This DTC may have been set by one of the following conditions:

• | HO2S connector water intrusion

• | An intermittently open HO2S high signal circuit

• | An intermittently open HO2S low reference circuit

• | An HO2S low reference circuit with high resistance to ground

• | PCM ground circuits with high resistance

• | Poor HO2S or PCM connector terminal contact

• | An exhaust system leak, typically within 12 inches up-stream of the HO2S

• | A defective HO2S

2. Repair any of the above conditions as necessary.

Did you find and correct the condition?

--

Go to Step 11

Go to Intermittent Conditions

4 |

1. Turn OFF the ignition.
2. Disconnect the HO2S connector for the sensor that applies to the DTC.
3. Jumper the HO2S low reference circuit to a known good ground. Refer to Using Fused Jumper Wires in Wiring Systems.
4. Jumper the HO2S high signal circuit to a known good ground. Refer to Using Fused Jumper Wires in Wiring Systems.
5. Turn ON the ignition, with the engine OFF.
6. With a scan tool, observe the HO2S voltage for the sensor that applies to the DTC.

Does the voltage measure less than the value specified?

20 mV

Go to Step 8

Go to Step 5

5 |

1. Turn OFF the ignition.
2. Remove the jumpers from the HO2S terminals.
3. Disconnect the PCM connector containing the HO2S high signal and low reference circuits.
4. Using a DMM, first measure for continuity of the HO2S high signal circuit. Then measure for continuity of the HO2S low reference circuit.

Do the individual resistances of both circuits measure less than the specified value?

5 ohms

Go to Step 6

Go to Step 7

6 |

1. Check for correct PCM connector terminal contact.
2. Repair PCM terminals as necessary. Refer to Repairing Connector Terminals in Wiring Systems.

Did you find and correct the condition?

Go to Step 11

Go to Step 10

7 | Repair the high resistance between the HO2S low reference circuits and the PCM ground circuit. Refer to Wiring Repairs in Wiring Systems. Probable causes of the condition include the following:

• | Poor HO2S connector terminal contact

• | An open sensor harness low reference circuit--This requires HO2S replacement. Refer to Heated Oxygen Sensor 1 Replacement .

• | A sensor harness low reference circuit with high resistance--This requires HO2S replacement. Refer to Heated Oxygen Sensor 1 Replacement .

• | A poor PCM ground connection

• | High PCM ground circuit resistance

• | An ungrounded exhaust system

Did you complete the repair?

--

Go to Step 11

--

8 |

1. Check for exhaust system leaks upstream of the applicable HO2S. The leak may be very small and typically will be within 12 inches of the applicable HO2S.
2. Repair the exhaust as necessary.

Did you find and correct the condition?

--

Go to Step 11

Go to Step 9

9 | Replace the HO2S. Refer to Heated Oxygen Sensor 1 Replacement . Did you complete the replacement? | -- | Go to Step 11 | --
10 | Replace the PCM. Refer to Powertrain Control Module Replacement . Did you complete the replacement? | -- | Go to Step 11 | --
11 |

1. Use the scan tool in order to clear the DTCs.
2. Turn OFF the ignition for 30 seconds.
3. Start the engine.
4. Operate the vehicle within the Conditions for Running the DTC as specified in the supporting text.

Does the DTC run and pass?

--

Go to Step 12

Go to Step 2

12 | With a scan tool, observe the stored information, Capture Info. Does the scan tool display any DTCs that you have not diagnosed? | -- | Go to Diagnostic Trouble Code (DTC) List | System OK