Oxygen Sensor: Testing and Inspection

SCAN TOOL DIAGNOSIS
The Tech 1 has several positions that will indicate the state of the exhaust gases, HO2S voltage, short term fuel trim, and long term fuel trim.

Normal Tech 1 voltage varies between 10 mV to 1000 mV (0.01 and 1.0 volt) while in "Closed Loop." DTC 13 and 63 will set in one minute if voltage remains between 350 mV and 550 mV, but the system will go "Open Loop" in about 15 seconds.

The PCM self-diagnostic circuit looks for trends in HO2S voltage. If a trend is detected, the MIL (Service Engine Soon) comes ON."

A rich exhaust trend causes the sensor voltage to stay above 700 mV for too long. This sets DTC 43 or DTC 65.

Lean exhaust causes voltage to stay below 300 mV for too long. This sets DTC 44 or 64. If voltage stays between 350 mV and 550 mV for too long, the PCM sets DTC 13 or 63. This indicates an open HO2S circuit.

RESPONSE TIME
Not only is it necessary for the HO2S to produce a voltage signal for rich or lean exhaust, it is also important to respond quickly to changes. The PCM senses the response times. If the HO2S responds slowly, the customer may complain of poor fuel economy, rough idle or lack of performance. It may also set false PCM DTCs because the PCM uses oxygen sensor voltages for system checks.

HEATED OXYGEN SENSOR CONTAMINANTS

Carbon
Black carbon or soot deposits result from over-rich mixtures. However, carbon does not harm a sensor. Deposits can be burned off in the engine by running it at least part throttle for two minutes.

Silica
Certain RTV silicone gasket materials give off vapors that contaminate the oxygen sensor. The sand-like silica particles from the RTV silicone embed in the molecules of the sensor element and plug up the surface. This results in "lazy" HO2S response and engine control. The sensor will also have a white-like appearance.

There is also a possibility of silica contamination caused by silicone in the fuel. Careless fuel handling practices with the transport containers can result in unacceptable concentrations of silicone in the fuel at the pump.

There is also a possibility of silica contamination caused by silicone in lubricants used to install vacuum hoses on fittings.

Lead
Lead glazes sensors. It can be introduced when regular, or leaded fuel, is burned. It is difficult to detect lead contamination by visual inspection.

Other Substances
Oil deposits will ultimately prevent sensor operation. The sensor will have a dark brown appearance. Causes of high oil consumption should be checked. The additives in ethylene glycol can also affect sensor performance.

This produces a white-like appearance. If antifreeze enters the exhaust system, you will likely encounter other, more obvious, symptoms of cooling system trouble.

MULTIPLE FAILURES
If you encounter multiple or repeat HO2S failures on the same vehicle, consider contamination.

Leaded fuel, silica contamination from uncured, low-grade (unapproved) RTV sealant, and high oil consumption are possible.

A problem in the HO2S circuit or fuel system should set a DTC 13 or 63 (open circuit), DTC 44 or 64 (lean indication) or DTC 45 or 65 (rich indication). Refer to applicable chart if any of these DTCs were stored in memory. Testing and Inspection