First off, as a new member I just wanna say hi to everyone. I look forward to becoming a part of this community and learning A LOT

Now that that's out of the way, I just bought a 2000 SI and the CEL keeps coming on, at first i was getting two errors, one was an emissions control code due to a gas tank that rusted through which I just had replaced. The second error was P0420, which from research I discovered is Catalyst System Efficiency Below Threshold. I understand that this can be caused by a number of things, bad O2 sensor or a bad cat as well as others. The car came with a header and a performance exhaust on it. There is an O2 sensor directly in front of the cat and one behind it. I'm wondering if the sensor before the cat should even be there. A friend of mine has a 96 civic and it has an O2 sensor in the header. Should mine be the same way? Could the sensors beings so close together cause the CEL? I was looking underneath and it looks like I will need some sort of extension for the harness if i was to move the O2 sensor the the header.
Any help will be greatly appreciated!

 

In general, P0420 is not caused by a bad O2 sensor. More often than not it's caused by a bad cat, although an exhaust leak can also throw this code.

 

Yea, that was one of the other things that came up when i was researching. Im pretty sure the cat is fine, it looks like a relatively new exhaust system, and I checked it over for leaks as well. I understand the two sensors need to have enough of a difference in readings or it will throw this code, thats why im wondering if they are too close together, especially with the performance exhaust

 

The locations of the O2 sensors sound fine. When an engine runs too rich, a new cat can be killed very quickly, even though it looks new on the outside.

 

Should i try replacing thr O2 sensors first? I know someone that has a few that i could try.. If thats not it and it is the cat what would be causing the engine to run too rich. I dont want to replace it and ruin another one it the engine is running rich.

 

Start with a tune up (plugs, wires, cap, rotor, ignition timing). Does the engine have any other mods that would make it run rich?

 

Already replaced the plugs and ran seafoam through it, it has a short pipe with a filter on it insted of the stock air filter but still has the stock intake pipe on it, i plan on getting a full AEM cold air for it as soon as i get more money. Also it has a new distributer cap on it. Im waiting to hear about a set of performance wires.. What to you mean by rotor and how would i check/fix the ignition timing.

 

Stock fuel pump and injectors?

You should first Seafoam treat the IM and then replace the plugs as Seafoam can foul the spark plugs. Did you use NGK plugs?

There's a diagram in the FAQs sticky showing how to set the ignition timing.

The rotor is inside the distributor.

 

I did run the seafoam before i replaced the plugs and there is a brand new distributer cap on it so i shouldnt have to check that should i?

 

I would try 2 diffrent o2 first (replace both)... it could be running rich causing the 2nd sensor to see the cat not working correctly... Find someone with a snapon scanner with a scope and have them watch the 2nd o2. I would also do a compression test and check the PCV valve also make sure your Emission system is up to par since your gas tank did rust through. make sure your fuel return is actually working.. If something is blocking the return line or the regulator, that would increase your fuel pressure and and make your injectors inject to much fuel causing you to run rich