97 GMC Sonoma failes emissions test (inefficient catalytic converter)

My 97 Sonoma failed the emissions test for the code “inefficient catalytic converter.” So I went to a local muffler shop and had an aftermarket converter installed. I was told to drive it a couple days after the computer was reset, and I did. But, it still fails the emissions test with the same cat code. What went wrong?”

Well, theoretically it could be any of a number of things, from a bad oxygen sensor to an exhaust leak to an intake leak to any of a number of other possibilities. BUT, from my experience with those GMCs and aftermarket catalytic converters, I’d guess the new cat just isn’t working correctly. I’ve had that happen to me in the past, and I won’t use anything but OEM cats on those GMCs now. Something about how the aftermarket cats are made, they just don’t synch correctly with the vehicle’s fuel injection system and keep tripping the code. The vehicles seem to run perfectly fine with the aftermarket cats, but they won’t let you pass the emissions test because they keep tripping the inefficient cat code. I’d advise trying an original equipment cat and see what happens.