For 1990-2009 cars

### Circuit 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 of the exhaust stream. When the vehicle is first started the powertrain control module (PCM) operates in an Open Loop mode, ignoring the HO2S signal voltage when calculating the air-to-fuel ratio. The PCM supplies the HO2S with a reference, or bias, voltage of about 450 mV. The HO2S generates a voltage within a range of 0-1,000 mV that fluctuates above and below bias voltage once in Closed Loop. A high HO2S voltage output indicates a rich fuel mixture. A low HO2S voltage output indicates a lean mixture. Heating elements inside the HO2S minimize the time required for the sensors to reach operating temperature, and provide an accurate voltage signal. The PCM controls the HO2S 1 heater low control circuit with a low side driver. The HO2S 1 heater diagnostic monitors the current draw through the HO2S 1 low side driver when the engine is running. This DTC will set if the current level is not within the calibrated range.

Each HO2S 1 has the following circuits:

• | HO2S 1 high signal

• | HO2S 1 low reference

• | HO2S 1 heater ignition voltage

• | HO2S 1 heater low control

• | Low reference loop

### Conditions for Running the DTC

• | DTCs P0101, P0102, P0103, P0107, P0108, P0112, P0113, P0116, P0117, P0118, P0121, P0122, P0123, P0125, P0128, P0201, P0202, P0203, P0204, P0205, P0206, P0410, P0440, P0442, P0443, P0446, P0449, or P1441 are not set.

• | The system voltage is between 9-18 volts.

• | The engine has been running for at least 4 minutes.

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

• | The engine speed is between 650-2,500 RPM.

• | The mass air flow (MAF) is between 4-26 g/s.

### Conditions for Setting the DTC

The HO2S 1 Heater Current is not within the calibrated range.

### Action Taken When the DTC Sets

The PCM illuminates the malfunction indicator lamp (MIL) on the second consecutive ignition cycle that the diagnostic runs and fails.

The PCM records the operating conditions at the time the diagnostic fails. The first time the diagnostic fails, the PCM stores this information in the Failure Records. If the diagnostic reports a failure on the second consecutive ignition cycle, the PCM records the operating conditions at the time of the failure. The PCM writes the operating conditions to the Freeze Frame and updates the Failure Records.

### Conditions for Clearing the DTC

• | The PCM turns OFF the MIL lamp after 3 consecutive ignition cycles that the diagnostic runs and passes.

• | A current, DTC last test fails, clears when the diagnostic runs and passes.

• | A DTC stored in history clears after 40 warm-up cycles, if no failures are reported by this or any other emission related diagnostic.

• | A scan tool can be used to clear the DTC.

### Test Description

The numbers below refer to the step numbers on the diagnostic table.

1. Check the heater current with the engine at operating temperature, and allow time for the heater current to stabilize. The HO2S 1 heater current is initially higher until the sensor reaches full operating temperature.

2. This step ensures that the ignition voltage circuit to the HO2S 1 is not open or shorted. The test lamp should be connected to a good chassis ground, in case the HO2S 1 low reference or HO2S 1 heater low control circuit is malfunctioning.

3. This step checks the HO2S 1 heater ground circuit. The engine must be running for the PCM to enable the heater low control driver to provide a path to ground.

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

1. Allow the engine to idle at operating temperature.
2. Monitor the HO2S 1 Heater Current parameter on a scan tool.
3. Allow the current to stabilize.

Is the HO2S 1 Heater Current within the specified range?

0.25-0.85 A

Go to Step 3

Go to Step 4

3 |

1. Observe the Freeze Frame/Failure Records data 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 as specified in the supporting text or as close to the Freeze Frame/Failure Records data that you observed.

Does the DTC fail this ignition?

--

Go to Step 4

Go to Intermittent Conditions

4 | Important: Use a known good ground. Do not use the HO2S 1 heater low control circuit or the HO2S 1 low reference circuit.

1. Disconnect the HO2S 1 electrical connector.
2. Turn ON the ignition, with the engine OFF.
3. Probe the HO2S heater ignition voltage circuit at the HO2S 1 electrical connector, on the PCM harness side, with a test lamp connected to ground.

Does the test lamp illuminate?

--

Go to Step 6

Go to Step 5

5 | Test the HO2S 1 heater ignition voltage fuse. Is the fuse open? | -- | Go to Step 11 | Go to Step 12
6 |

1. Turn OFF the ignition.
2. Connect a test lamp between the HO2S 1 ignition 1 voltage circuit and the HO2S 1 heater low control circuit.
3. Start the engine.

Does the test lamp illuminate?

--

Go to Step 7

Go to Step 8

7 |

1. Leave the test lamp connected between the HO2S 1 heater ignition voltage circuit and the HO2S 1 heater low control circuit.
2. Turn OFF the ignition.
3. Turn ON the ignition, with the engine OFF.

Does the test lamp illuminate?

--

Go to Step 9

Go to Step 13

8 |

1. Turn OFF the ignition.
2. Disconnect the PCM connector containing the HO2S 1 heater low control circuit. Refer to Powertrain Control Module Replacement .
3. Measure the resistance of the HO2S heater low control circuit between the PCM harness connector, and the HO2S harness connector using a DMM. Refer to Circuit Testing in Wiring Systems.

Is the resistance within the specified range?

0-5 ohms

Go to Step 15

Go to Step 10

9 | Test the HO2S 1 heater low control circuit for a short to ground. Refer to Wiring Repairs in Wiring Systems. Did you find and correct the condition? | -- | Go to Step 18 | Go to Step 15
10 | Test the HO2S 1 heater low control circuit for an open or high resistance condition. Refer to Wiring Repairs in Wiring Systems. Did you find and correct the condition? | -- | Go to Step 18 | --
11 | Test the HO2S 1 heater ignition voltage circuit for a short to ground and replace the fuse. Refer to Wiring Repairs in Wiring Systems. Did you find and correct the condition? | -- | Go to Step 18 | Go to Step 13
12 | Test the HO2S 1 heater ignition voltage circuit for an open or high resistance condition. Refer to Wiring Repairs in Wiring Systems. Did you find and correct the condition? | -- | Go to Step 18 | --
13 | At the disconnected HO2S 1 pigtail connector test the HO2S heater ignition voltage circuit for continuity between the following circuits of the HO2S:

• | The HO2S 1 high signal

• | The HO2S 1 low signal

• | The HO2S 1 outer shell or a good ground

Do any of the circuits measure other than infinite resistance?

--

Go to Step 16

Go to Intermittent Conditions

14 | Check for poor connections at the harness connector of the HO2S 1. Refer to Testing for Intermittent Conditions and Poor Connections in Wiring Systems. Did you find and correct the condition? | -- | Go to Step 18 | --
15 | Check for poor connections at the harness connector of the PCM. Refer to Testing for Intermittent Conditions and Poor Connections in Wiring Systems. Did you find and correct the condition? | -- | Go to Step 18 | Go to Step 17
16 | Replace the HO2S 1. Refer to Heated Oxygen Sensor 1 Replacement . Did you complete the replacement? | -- | Go to Step 18 | --
17 | Replace the PCM. Refer to Powertrain Control Module Replacement . Did you complete the replacement? |  | Go to Step 17 | Go to Step 2
18 |

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 as specified in the supporting text.

Does the DTC run and pass?

--

Go to Step 19

Go to Step 2

19 | 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