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 exceeds a calibrated amount.

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

The engine is running.

### Conditions for Setting the DTC

The PCM detects that the heater control circuit current is greater than the capacity of the PCM internal driver.

The above condition is present for more than 20 seconds.

### 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. Inspect 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. A DTC fail in this step verifies the problem is present with the HO2S 1 disconnected.

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 |

1. Turn OFF the ignition.
2. Disconnect the HO2S 1.
3. Start the engine.
4. Allow the engine to idle at operating temperature.
5. Monitor the scan tool for at least 30 seconds.

Does the DTC fail this ignition?

--

Go to Step 5

Go to Step 6

5 | Test the HO2S 1 heater low control circuit for a short to voltage. Refer to Wiring Repairs in Wiring Systems. Did you find and correct the condition? | -- | Go to Step 10 | Go to Step 7
6 | 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 10 | Go to Step 8
7 | 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 10 | Go to Step 9
8 | Replace the HO2S 1. Refer to Heated Oxygen Sensor 1 Replacement . Did you complete the replacement? | -- | Go to Step 10 | --
9 | Replace the PCM. Refer to Powertrain Control Module Replacement . Did you complete the replacement? | -- | Go to Step 10 | --
10 |

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 11

Go to Step 2

11 | 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