from Goss’ Garage by Pat Goss

Catalytic converters are mostly misunderstood, ignored, and improperly tested. Even though the converter is crucial to a car’s performance and emissions status, testing is usually nothing more than a guess or reading a code. Because codes don’t tell what is wrong many replacement converters were never needed. This is mostly due to a lack of knowledge and unreliable, flawed test procedures. Flawed tests yield flawed results.

Catalytic converters work much like blowing air across not-quite-burning wood to make it flare into flames. In the converter air is directed through a ceramic honeycomb coated with metals like platinum, palladium, and rhodium. Similar to blowing on coals, air causes the converter to become super hot inside. The high temperature creates a catalytic reaction between the “Noble” metals, oxygen and exhaust gas. This converts noxious exhaust byproducts into mostly non-harmful gasses and water.

Converters are excellent for reducing emissions but create substantial confusion among motorists possessing “enough knowledge to be dangerous.” Especially regarding oil consumption, “My car uses a lot of oil but it isn’t leaking because there are no drips under the car, and it obviously isn’t burning oil because there’s no smoke coming from the tailpipe. Where is it going?”

Diagnosing mysterious oil loss is actually quite simple. Actually, there are only two ways for oil to leave an engine, leaking and burning. Oil leaks produce puddles so that’s no mystery. However, the lack of puddles and the lack of smoke from the tailpipe often triggers a diagnostic dilemma leading to bizarre theories to support impossible causes for disappearing oil and bizarre theories lead to bizarre expenses.

Fortunately, the “no smoke” mystery really isn’t a mystery either. Catalytic converters operate at such high temperatures that as long as they are working, they easily burn and dissipate exhaust smoke. This makes it possible for an engine to consume alarming quantities of oil without a trace of exhaust smoke.

Failed emissions due to excessive Hydrocarbon (HC), Carbon Monoxide (CO) or Oxides of Nitrogen (NOX) are also frequently caused by a deteriorated catalytic converter. Problem is, how to determine if the converter is causing the emissions failure.

Converters consume oxygen in the catalytic process, which provides the basis for a simple, two minute converter efficiency check referred to as an “Oxygen Capture” test. Using an exhaust gas analyzer the percentage of oxygen coming from the tailpipe of a fully warmed-up vehicle is measured at idle. Once the oxygen reading has stabilized the throttle is rapidly snapped full open and instantly closed. Following the throttle snap there should be a slight increase in the amount of oxygen coming from the tailpipe. The allowable “slight increase” is typically 1.2%. If the oxygen increase exceeds the allowable limit the converter is almost always bad.

In a healthy converter the catalytic reaction consumes large quantities of oxygen. Therefore an excessive increase in oxygen at the tailpipe, following the snap test, indicates the converter has lost its ability to capture and use oxygen. Without capturing and consuming oxygen the catalytic reaction stalls, emissions climb, the check engine light comes on and the computer stores a code. The oxygen capture test is quick, simple, and highly reliable and helps avoid buying an unnecessary and expensive converter. Always ask for an oxygen capture test before buying a replacement converter, it’s two minutes that could save you several hundred dollars.