so i have a 05 r53 with only one modification i have an invidia cat back exhaust with the middle resinator cut out. the problem i have is ever since i cut the resinator out the check engine light came on. now just to let everyone no i use the car everyday and it seems to run fine iv ben puttting in regular gas like a dumb *** but should that affect the car, if anyone can help with this problem or y that code is on that would be great thanxs. also it rained the other day and the code went off, than today it came back on. could it be a bad o2 senssor or a leaky fitting i have a peice of straight pipe clamped on soo any help would be great

 

Check for vacuum leak/exhaust leaks. Could also be a dirty MAF sensor.

-Jorge

 

If you have not done so, search for "0172" in the forum. Lots of hits, several possibilities.