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 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 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:

The ECM monitors the rich-to-lean and lean-to-rich transition time. A transition is defined as the HO2S voltage changes from above 600 mV to below 300 mV or from below 300 mV to above 600 mV. If the ECM detects that the transition time is too long, this DTC sets.

This diagnostic procedure supports the following DTC:

DTC P0133 HO2S Slow Response Sensor 1

The ECM detects that the HO2S 1 rich-to-lean average response time is less than 90 milliseconds or lean-to-rich average response time is more than 125 milliseconds.

Or

The ECM detects that the HO2S 1 rich-to-lean or lean-to-rich switches are less than 10.

Or

The ECM detects that the HO2S 1 signal spends twice the amount of time in the rich region as compared to the lean region or the HO2S 1 signal spends twice the amount of time in the lean region as compared to the rich region.

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

2. This step determines if the condition is present. This test may take 5-10 minutes for the diagnostic to run.

4. An exhaust leak 6-12 inches away from the HO2S can cause a DTC to set.

5. Certain RTV silicone gasket materials release vapors that can contaminate the HO2S. Silicone in the fuel can also cause silicone contamination. If the sensors appear contaminated by silicone, and if all the silicone sealant is a non-silicone base, advise the customer to try a different fuel company. A missing fuel filler restrictor indicates that the customer may have used leaded fuel.