I live in Colorado and have a 2000 civic si. The car came with a full exhaust system including a cat. There are no emissions laws here and want to put a test pipe/resonator in place of the cat. My question is regarding the o2 sensor on the cat.
If I buy a test pipe with v a bung for the o2 sensor will my CEL go off??? Are resonators designed to work properly with stock ecu???

 

Not really an answer to your question, but when my cat became rusted, it eventually snapped off from bottoming-out on my buddies driveway. I completely removed the exhaust from the flexpipe to the muffler - the flexpipe was the only thing connected besides the manifold of course. I eventually replaced the whole exhaust system $600 bucks at O'Reily's for the flexpipe, cat, resonator and muffler. If I had time to spare I would of bought a nice aftermarket exhaust like
90-93 Honda Accord N1 Style 2.5" Performance Catback Exhaust - Titanium Tip

If my memory serves me properly the o2 sensor on my 1991 honda accord lx is on my exhaust manifold, and it never threw a code.

Don't know about yours, though.

 

I have dc headers, new cat, and dc cat back exhaust system. I'm gonna put a test pipe into the cat position. But my car has 2 o2 sensors. One is on the cat. The test pipe I'm ordering has a bung for that o2 sensor. I'll see what happens.

 

P0420 inefficient catalyst code. If the cat is not bad you won't see any power gain from putting a test pipe on (unless you have a built or turbo engine).

 

Removing 90% of my exhaust system improved performance in terms of throttle response and acceleration. Except gave me some gross MPG. COuld of just been my imagination, although my RPMs ran a little bit higher oddly, I think it was getting too much air.

 

So running a test pie isn't the same as running straight pie? Or running straight pipe doesn't increase performance as opposed to having the cat???
Because I ran straight pipe on my modified d16y8 (non turbo) and I noticed immediate improvement in throttle response.

 

Of course the D series was rebuilt 20 over bore so maybe I was letting that power free.

 

On my first stock h22a the difference on the dyno between a test pipe and stock cat was 1hp. repeated more than once. That was even using the stock ACCORD cat, which was for a 125 hp engine compared to the 200hp h22a.
On engines that are near stock there is minimal gain, unless louder is considered a gain.

 

I had a 1990 Acura RS with a 1996 gsr swap, already had a test pipe installed, when i switched to a cat I noticed zero difference. If anything the cat seemed more responsive, which is weird because I was expecting the opposite.

 

The cat could theoretically cause a restriction point whereby the exhaust pulse must accelerate in order to maintain the same volume of flow, that increase in velocity helps with the scavenging effect at lower RPM.

OP, if the cat works and your engine is stock or close to stock, just leave it be. If you want something high-flow you can get Mgnaflow spun-metallic core catalysts for less than $100 on eBay. I know these are performance cars and environmental quality is not the first concern but it hurts nothing to have the cat there.