Need help have 03 evo I have a aftermarket exhaust system from down pipe back but I'm getting check engine light with p0420 code which Cadillac detereation need help fixing this cause I thought there was only one converter on evos plz any advise is helpful thxs

 

Night.

You should probably change the title of your thread to say Catalytic converter. Also, if you haven't tried searching... P0420 is common enough.

There are 2 O2 sensors, but only one catalytic converter. This code essentially pertains to the front O2 sensor (in the O2 housing) reading something, and the aft O2 sensor (which would be in the cat) disagreeing.

Did this code pop because you JUST changed something? Or was nothing changed, and it just came on... it can be a number of things, but on Evos, it's most commonly because your catalytic converter has been removed, and you need to space your rear O2 sensor out, or disable your check engine light.

 

It is catalytic converter. Your rear O2 sensor may be on its way out.

These could be the cause:
Leaded fuel was used where unleaded was called for
A damaged or failed oxygen sensor (HO2S)
Downstream oxygen sensor (HO2S) wiring damaged or connected improperly
The engine coolant temperature sensor is not working properly
Damaged or leaking exhaust manifold / catalytic converter / muffler / exhaust pipe
Retarded spark timing
The oxygen sensors in front and behind the converter are reporting too similar of readings
Leaking fuel injector or high fuel pressure
Cylinder misfire
Oil contamination

My advise is to check the items in this list.

Do you have a test pipe or are you catted still?

 

Test pipe and I also just changed the front o2sensor cause it threw a code so I replaced and the light went off for a week then came on and it gave that po402 code

 

If you delete the cat, you get the code. Buy a rear o2 spacer or tune the code out. Done.

 

There's your problem. You have a test pipe. Get yourself one of these http://www.ebay.com/itm/121112069096?redirect=mobile put it on the test pipe and then put the o2 sensor into that and your cel will go away.