Civic SI throwing codes

[PRO®]

I am about to purchase a 99 civic SI but it has a CEL and is throwing P0420 and PO131. The guy I am buying it from says he believe it has a b16 bottom and a b18 top, dc headers, and an exhaust. Any ideas what it could be?

[Kaiser]

Yep. 0420 is Catalyst Efficiency out of range, 0131 is Oxygen Sensor reading low.

Likely causes:
A.) Bad Catalytic Converter (If it is this, get the engine checked over by a shop to make sure it's well tuned)
B.) Bad Oxygen sensor (It's not likely this, as the sensor has to die a very specific way for it to look like a bad cat and not a dead sensor)

Had those EXACT codes thrown for my 2002 Suzuki Aerio and it was the cat. Take it to a muffler/exhaust shop and have them look at it. It will not pass emissions as it is right now, so if you're looking at buying it, remember it's the seller's responsibility to provide a passed emissions certificate. You might wanna ask how long the CEL's been on and all, and consider if you're gonna buy it that you might not be able to tag it till you get that fixed, though it's not a huge deal.

[Decepticon™]

Hey man i looked up those codes at my work and im pretty sure a cat. and O2 sensor should fix the the problem. Good luck. hope everything works out.
Peace.Love.Honda

[Barefoot]

i fixed this problem today replace the cat.

[Andy099]

how color si? price? lemme know if you change your mind

[PRO®]

Does it matter what brand 02 sensor I get, or can I just buy any brand from Summit Racing/Autozone etc. Autozone has the bosch/denso universal sensors for about 68 bucks, and the O.E. sensors for $114(Bosch)/$123(denso). I forgot to mention it has a high flow cat...will that affect my emissions?

how color si? price? lemme know if you change your mind

its a black si, but im not selling it, im buying it

[preferredduck]

i get an o2 code in my si once every few mos. i read a service bulletin that the o2 randomly reads low sometimes. you may not need an o2 at all just a cat.

[PRO®]

I replaced the 02 sensor before the cat this morning, which got rid of the P0131 code, but it is still showing the P0420 code. The autozone printout shows 4 probable causes(catlytic converter, engine misfire, large vacuum leak, engine oil leakage) I was wondering if the high flow-cat has anything to do with this.