Using an infrared thermometer, check the temperature of the converter's front and rear weld rings, to ensure the converter has "lit off." Depending on their sizes, most converters begin to light off around 350º F, and are fully lit around 500º F.
Under normal conditions, the rear weld ring may reach temperatures which are as much as 150º F higher than the front weld ring. If the rear weld ring reaches temperatures in excess of 150º F higher than the front weld ring, the engine may have an emissions problem.
Keep in mind that the converter's rear weld ring temperature is directly related to the amount of work the converter is performing. Therefore, elevated temperatures may indicate an emissions issue.
If the rear weld ring is significantly cooler than the front, the converter may not be lighting off. This may indicate the converter has failed, or that the exhaust mixture is not correct - the symptom of an underlying emissions issue.
Typically, converter temperatures will not exceed 1200º F on a properly running engine. Periodic operation above 1600º F can negatively affect the precious metals coating on the substrate, reducing its efficiency. Excessive temperatures can reduce the converter's durability, or - if high enough - destroy the converter's matting or substrate.
Damaged matting and melted substrates typically occur at temperatures exceeding 1700º F. It is possible to test for a cracked substrate or damaged matting by tapping on the converter housing. Using a rubber mallet, "thump" the shell, listening for loose components.
A bronze / blue rainbow discoloration of the shell typically indicates elevated temperatures. If the converter is removed, look through the substrate to observe whether the small passageways are melted or collapsed. The substrate may actually appear normal at either end, since the substrate melts internally.