Well, today I finally took my 1999 Blazer to the shop to have a new catalytic converter installed. For those of you who may not have read my earlier thread (it got zero replies, so I seemed to have stumped everyone else as well), I had been getting a P0420 CEL code (catalyst efficiency below threshold).

When the mechanic got the Blazer on his lift, he called me aside to show the cat that was on it. He pointed out that it had been replaced before; the previous owner not only replaced it with an OBD1 cat, but also managed to install it backwards. As soon as I got it home after the replacement was installed, I hooked up my OBD2 scanner and cleared the codes. So far so good; it hasn't thrown another code yet. I don't think the problem of the miss at take-off is quite fixed yet (it rained today and still seemed to be doing it), but at least the computer seems happy.

I was just wondering if anyone might be able to speculate about the effect the old setup might have. I didn't get a chance to look at the old cat before I left the shop (the shop kept it). Whats the difference between an OBD1 and OBD2 converter that would make it incompatible? What happens when you install it backwards? I'm not sure how they're built inside; could it have a sort of check valve effect, limiting air flow in one direction more than the other?

 

ive always wondered this too. i do alot of exhaust work at my shop and the majority of cats we buy are universal and theres is no direction of flow on them they can go in either way. even the factory ones you cut out if you look in them they look the same both way i dont know why direction would matter but im not gonna find out.

also i been told about that obd1 cats cant be used on obd2 cars. theres probably just more material, newer technology in them. im sure you could use it the other way around though