my 96 accord failed for high co . it was around 4 percent. the mechanic hooked his obd2 scann tool and found nothing wrong.the car runs great and has a brand new cat and o'2 sensor. what is going on???i put on 2 cats.still wont pass.grooss polluter staus.

 

if that high theres somthing seriously wrong with your car ,start with getting rid of and aftermarket performance mods ,if it's 100% stock car I would check the basics first like pulling the plugs to see how each cylinder is burning if it's even ,timing ,and maybe it's a bad PCM if it's California car it's backed with a 14 year /140,000 mile extended emission warranty.

 

your right,i think it might have a bad ecu.Didnt these have a defect????im refering to the ecu.