I get a P0421 code on my Evo.
It says Warm Up Catalyst Efficiency Below Threshold

I have searched the forum and it looks like the code appears when the reading between 2 O2 sensors is off and that I need to change the cat to fix it.

The problem is that I do not have a cat. I have a downpipe with a remote switch 3'' cutout

When I clear the code, it goes away and then comes back on after 3-4 days

 

The code will keep coming back because you have no catalytic converter. Your in Turkey were i doubt you have smog testing so i wouldn't worry about it. Have your car tuned professionally and the tuner should inform you more on it.

 

can the tuner completely clear it from the oem ecu??

btw, we do have smog testing here, you get it done every 2 years and they are very strict on it. catalytic converter is a must. aftermarket xenons, lowering springs, aftermarket mufflers, windows tinting. these are all against the regulations. I know people getting pulled over and cars are getting towed and sent to smog testing

 

well then i advised you to stop breaking the law and get a smog legal cat put in the car.
the code will keep coming back on, a tuner can disable the code or you can get the o2 sensor extender to keep the code from coming back on. BUT you are still cat less and still breaking the law and will not pass inspection.