Since the oxygen sensor is the heart of the closed loop control system, proper operation is critical to efficient emission control. There are several factors which can cause the oxygen sensor signal to degrade and they include the following:
• Silicon contamination from chemical additives, some RTV sealers, and contaminated fuel.
• Lead contamination can be found in certain additives and leaded motor fuels.
• Carbon contamination is caused by excessive short trip driving and/or malfunctions resulting in an excessively rich mixture.
The effects of sensor degradation can range from a subtle shift in air/fuel ratio to a totally inoperative closed loop system. With respect to driveability and emissions diagnosis, a silicon contaminated sensor will cause the most trouble.
When silicon burns in the combustion chamber, it causes a silicon dioxide glaze to form on the oxygen sensor. This glaze causes the sensor to become sluggish when switching from rich to lean, and in some cases, increases the sensor minimum voltage on the lean switch. This causes the fuel system to spend excessive time delivering a lean mixture.
It is often difficult to identify a sensor which is marginally degraded, and in many cases, vehicle driveability may not be effected significantly. With the advent of IM240 emissions testing, however, marginal sensor degradation may cause some vehicles to fail the NOx portion of the loaded mode test.
The impact of a slightly lean mixture has a dual effect on emissions. A leaner mixture means higher combustion temperatures so more NOx is produced during combustion. Additionally, because less carbon monoxide is available in catalyst feed gas, the reducing catalyst efficiency falls off dramatically. The end result is a vehicle which may fail an IM240 test for excessive NOx.
As previously mentioned, the O2S signal voltage must fluctuate above and below 0.45 volts at least 8 times in 10 seconds at 2500 rpm with the engine at operating temperature. During the rich swing, voltage should exceed 550 mv and during the lean swing should fall below 400 mv. O2S signal checks can be made using the Autoprobe feature of the Diagnostic Tester, digital multimeter, or 02S/RPM check using the Diagnostic Tester. Refer back to the oxygen sensor tests in the closed loop control section for specific test procedures.
Now that we understand the effects of O2S degradation on catalyst efficiency, let's look at the effects of a catalytic converter failure. Keep in mind, there are many different factors that can cause its demise.
• Poor engine performance as a result of a restricted converter. Symptoms of a restricted converter include; loss of power at higher engine speeds, hard to start, poor acceleration and fuel economy.
• A red hot converter indicates exposure to raw fuel causing the substrate to overheat. This symptom is usually caused by an excessive rich air/fuel mixture or engine misfire. If the problem is not corrected, the substrate may melt, resulting in a restricted converter.
• Rotten egg odor results from excessive hydrogen sulfide production and is typically caused by high fuel sulfur content or air/fuel mixture imbalance. If the problem is severe and not corrected, converter meltdown and/or restriction may result.
• IM emission test failure may occur if catalyst performance falls below its designed efficiency level. Perform additional tests to confirm that the problem is in fact converter efficiency and not the result of engine or emission sub-system failure. Never use an emission test failure as the only factor in replacing a catalytic converter! If you do, you may not be fixing the actual cause of the emission failure.
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