420 Modena engine swap

check for faulty hydraulic follower for cylinder 2
check for over heating injector that is failing

Code 303 is a miss or lack of power on cylinder 3. If you know for a fact that cylinder is getting spark it could be an injector problem.
Code 420 means the catalytic converter is not performing up to standard which could be several things including the miss on cyl 3.

u sure its electrical--may be fuel system--many times the computer dozint recognize a new engine--may need to get it flashed or reprogrammd--dont replace anymore parts save ur $$

Well, at least someone else is having the same problem! I have had this problem occur 4 times in the last month, and have reported it to the dealership. Naturally, they can\'t find anything wrong, and if they can\'t duplicate the problem they can\'t repair it! I complained so much about the their lack of concern, that the service manager called GM and they claim NO ONE else had ever reported the problem with the later model Malibu\'s. I mean, come on! My Cobalt had this occur numerous times and the dealership finally replaced the defective parts. Why can\'t GM admit they have an overall problem with their ignitions? Geez........

Why do you people not even mention WHICH engine it is?
Anyway, on 4-cyl 2.5 L it is on the right side of the engine block (as seen from the driver's position ok?, you need to remove several accesory belts, alternator and a cover and you will see it.

You need to find out if any internal engine damage has occurred before you repair or replace anything. If it checks out ok & you decided to make the repairs, by all means replace the water pump and tensioner pulleys at the same time. The 2.7 litre engine had several flaws. Excessive engine sludge buildup that has led to complete engine failure, is well documented.
I hope this helps, reply back if you need further assistance.

Most car manufactures only warranty problems for a short time. If you can live with it I would try to. I have relaced these for customers before and the replacement ones crack also.

You could have an enefficient converter. Question is what caused it to go bad? I have attended seminars on code 420, Possible causes are lack of spark or clogged fuel injectors. there have been times where the spark plug wire lays on the engine and over time it wears a weak spot in the side of the wire, its not enough to throw a miss fire code but it keeps the full amount of spark from getting to the plug and not burning the all the fuel in the cyl. A leaky injector would also cause to much fuel to be deliverd into the cyl and not get burned.

Hope this helps.

Could be a clogged fule filter..
I have seen a clogged fuel filter cause this several times..
Not to much money to replace..

Good luck..