I am getting a PO420 code when i scan my car. It is a 99 firebird. I have a "catalyst efficiency" problem. What the heck does that mean. I just did my plugs and wires and added a fuel inject cleaner. Is this a bad O2 sensor or maybe something else? Thank for any advice you may have.

 

I've heard of different peole getting the same error, and almost every case I heard of ended up in a cat replacement. The code pops up when the cat gets plugged, or degrades and doesn't perform (cat goes bad, in short).

 

thus, get an o2 sim and get a high flow cat. get them on ebay pretty cheap... Or just put a straight pipe in thats what id do!