Making Sense Of Catalytic Converters

What's the difference between two-way and three-way catalytic converters? And how does the newer technology (three-way), combined with onboard engine management systems, affect the emissions diagnostic process?

There's no easy answer to either question. In fact, the growing sophistication of onboard engine management and emissions systems has forced technicians to develop a vast new range of diagnostic skills as they track down seemingly invisible performance issues that cause vehicles to fail state mandated tailpipe tests.

In a majority of the difficult cases facing today's technicians, oxides of nitrogen (or NOx) are at the heart of the problem. The ability to reduce NOx emissions is the key difference between a three-way converter (the term simply refers to the three regulated gasses, carbon monoxide, hydrocarbons and NOx)  and a two-way unit, which converts only hydrocarbons (HC) and carbon monoxide (CO).

TWO-WAY CONVERTERS

Two-way converters, introduced in the 1970s, use an oxidation process that converts HC and CO molecules into water and carbon dioxide by oxidizing, or "burning," them with the assistance of a precious metals catalyst. It's important to understand that the catalyst chemistry required to convert HC and CO is most effective with a lean air/fuel mix because this condition provides ample oxygen to "burn" the pollutants.

THREE-WAY CONVERTERS

Three-way converters, introduced in the 1980s, use two catalyst processes reduction and oxidation and a sophisticated oxygen storage control system to convert all three regulated gasses. But here's where it gets complicated: The catalyst chemistry required to clean up NOx is most effective with a rich air/fuel mix. To operate properly, therefore, a  three-way catalytic converter first must convert NOx (with a rich air/fuel bias), then HC and CO (with a lean bias).

This seemingly impossible task is accomplished in a variety of ways: Older three-way converters, which typically are run on engines with a slightly rich air bias (for the NOx conversion), introduce additional oxygen between the reduction and oxidation stages, thereby creating a lean condition for the oxidation catalyst. Older three-way converters are easily identified by their air tube at the center of the unit.

Modern three-way units, found on most vehicles manufactured since the late 1980s, rely on an advanced catalyst chemistry that stores and releases oxygen on a single substrate, and an oxygen monitoring and control system (using one or more 02 sensors) that causes the engine to oscillate between lean and rich conditions to ensure proper reduction of NOx, HC and CO. This oscillation, combined with the oxygen storage and release on the catalyst surface, enables the converter to perform two otherwise mutually exclusive functions using the same catalyst brick. 

Engine-management systems, onboard computer software and the development and use of three-way catalytic converters have resulted in no shortage of diagnostic nightmares for exhaust specialists. In fact, many of today's most proficient technicians have received extensive training in emissions diagnosis and repair.

When trying to pinpoint an emissions related problem, always begin with the engine; the catalytic converter can not do its critical job unless the engine is providing it with the proper air-to-fuel mix. A misfire condition, loose spark-plug wire or other issue will overwhelm the catalyst's capability to reduce regulated emissions.

That's not always an easy diagnosis, of course: Consider, for example, a late model vehicle that appears to be running well yet fails a tailpipe test for NOx. The potential causes include a misfire condition, an EGR adjustment issue, a bad oxygen sensor, an air leak in the upstream exhaust tubing or manifold, high cylinder compression due to excessive carbon deposits, improper spark advance and a failed catalyst.