So my car is an 03 lancer, has had a p0421 code for a while. I replaced the cat and both o2 sensors and the problem still isnt fixed. Any ideads?

 

How many sensors on your car? I thought it was 3 not 2 sensors

 

the ONLY thing that will cause that is a failed cat.

NOT failed o2 sensors.

 

He never answered me back it means he solved the problem.

He said he replaced the Cat Back but still has a problem he probably replaced all 02 sensor.

 

naw it hasnt been fixed. Im still pondering of what it could be.. I tested the tps sensor, checked the spark plugs countless things. Any other suggestions. One thing i noticed which is VERY odd is the downstream o2 sensor is before the cat and not after it. I find that very odd. How is it supposted see if the cat is working or not?

 

This code that keeps coming back could be the following.
Aftermarket Cat installed. (These cars need originals)
heated O2s wiring problem check wiring to PCM / ECM
if all checks out good you could well have a bad PCM / ECM
Your cat is up on the exhaust manifold right under the first O2s
if i remember correctly.

 

you replaced the wrong cat.

the one thats monitored is in the exhaust manifold

 

X2 on this.