I'm asking this for my buddy. He has a 98 civic ex and he recently decided to gut the cat. conv.. I told him to make sure he takes out the O2 sensor first but he didn't. Of course soon as he started it up the cell(PO420)came on. Plus he doesn't like the raspy tone now. So he goes and buys himself a high flow cat. I came by and cleared his code for him with my OBD2 scanner. I told him to go ahead and drive it for a 100 miles or so and see if the code comes back. Well it did, P0420, "catalyst system Efficiency below threshold." Now is his O2 sensor bad or is the high flow cat causing this problem. He needs this fixed soon he is trying to get inspected.
I really don't know. I think he got it off ebay. Its hard for me to convince him to by better performing parts. Its kinda like instead of him buying Sprite to drink he would prefer to buy the supermarket brand. Hey so do I sometimes but this is your car not a .69 2 liter
Mods: H&R sports, koni yellows, enkei 15's, y8 IM, apexi N1 catback, redded out tails, black housing heads, DIY front grill, all moldings painted, DcSport short shifter, type-r shift knob, makeshift AEM SRI, rear si sway bar, cross/slotted front rotors
when it says "catalyst system Efficiency below threshold" that means the emissions output is above what it should be. So the o2 sensors work just fine. its the cat causing the CEL to come on. most ebay cats will not pass emissions, I e-mail several of the people who sell them and almost all of them say "they pass visual inspection and no they work pass the test itself" so best stay with ones from muffler shops or dealers. it may be more money but its worth it.
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