Catalytic Converter Failure on the Rise?

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We recently had several older Volvos (1981-1990) require new catalytic converters to pass local emissions testing. We've found that the new enhanced emission testing, now performed on a dyno in most states for cars 1981 and newer, is much tougher to pass. Often a new catalytic converter is the only way to get the car to pass. In the past it was possible to pass emissions test with a properly tuned car, however the new procedure tests under load and the catalytic converter must be working properly to reduce emissions to acceptable levels.

Converters function like an incinerator, burning excess emissions, which produces a lot of heat in the exhaust. The converter can handle this as long as there are "normal" amounts of pollutants in the exhaust. When too much unburned fuel or other contaminants enter the exhaust because of a misfiring spark plug, overly rich fuel mixture, or a leaky exhaust valve or head gasket, the converter's operating temperature can soar, causing internal meltdown. This results in partial or complete blockage.

Converters can also fail because of contamination. As a converter ages, the catalyst gets "tired" because of a gradual accumulation of contaminants on its surface. The process can be accelerated by oil burning (worn valve seals, guides and/or rings), internal coolant leaks (cracked head or block, or leaky head gasket), and sulfur deposits normally found in gasoline.

As contaminants build up inside the converter, HC, CO and NOX emissions rise. On a late-model, well-tuned engine with a properly functioning converter, HC and CO emissions should be nearly zero. If an emissions test reveals higher than normal emissions, the converter may need attention. There's no way to rejuvenate a dead or damaged converter, so replacement is your only option.

2021 Update

One of the curious factors in the rise of catalytic convertor "failures" is WHERE they are happening.  

(The same thing can be said for oxygen sensors.)

States where repair shops are responsible for emissions testing seem to have a much higher replacement rate for convertors.

In Oregon, the emissions testing is done by a state agency and we seem to have a much lower failure rate that in other states where the testing and repair is done by the same facilities.  The testing is the same.  So what gives?

Maybe it's because most states have a minimum value threshold where if you spend enough money, you get a pass even if your car doesn't?  

Makes you wonder...