Although modern OBD II technology has simplified catalytic converter diagnostics, it’s still important to understand the basics of catalytic converter operation when dealing with a P0420 or P0430 diagnostic trouble code (DTC). DTCs P0420/P0430 indicate that ­catalyst efficiency is below predetermined thresholds on cylinder banks 1 and 2, respectively. In the following text, I’ll explore the basics of catalytic converter chemistry and summarize how common catalytic converter failures can be diagnosed. See Photo 1.
Photo 1: Although this catalytic converter didn't set a P0420 DTC, it accumulated enough contamination on the substrate to cause significant exhaust backpressure during a snap-throttle test.

LAMBDA NUMBERS
Understanding Lambda is important because the ­catalytic converter neither creates or destroys atoms. Instead, it simply changes how atoms are assembled into chemical compounds. The term “Lambda” represents a stoichiometric or chemically correct air/fuel (a/f) ratio of 14.7 unit weights of “pure” gasoline to 1.0 unit weights...