I started out working on the Sentra changing a upstream 02 sensor ,, because there was a P2A00 code ,,, now it won't start , this has not been a issue before so I'm trying to figure out what I did wrong . The funny part it acted like the starter wasn't engaging with the motor ,, had a whizzing sound , So I consulted a mechanic who's opion was the starter relay must have gone out in the IPDM . So I changed it out but I haven't synced it together with the ECM yet . Still learning that process . But ended up with all these codes from nowhere ( Were not there before I started at that time } ,, Now have P2A00 ,, P0123 ,, P0223,, P0603,,, they been in History of car ,, ,Then I tried checking power , groond , Cleaned the carburetor { throttle Body } .. I did leave the throttle body intact with the hoses still connected and plug , then just pulled aside instead of removing as I did before in previous times ,,,, But another mechanic said it sounded like possibly the ECM because of the codes , P0123 and P 0223 running consecutively ,, so ordered a new preprogrammed ECM to vin number , security , and key , but something was wrong , had to return . So I put the Original ECM back on ,, Do I have to sync the IPDM with the ECM in order for it work ??