My 2003 Volvo V70 had the alternator replaced recently. When I got the car back from the mechanic the check engine light came on. This is the first time the time the light came on since I have owned the car.
I took it back to the mechanic and he put the computer on it and said he was getting a reading there was a problem with the #4 cylinder. He suggested they replaced the plugs in the car and reset the computer and I should be all set. I approved the work to be done and they called the next day to say everything is good. No check engine light and the car test drove great.
I picked up the car and it was hot day so I put the AC on and within 5 minutes of driving the check engine light is back on and the car sputters like it is going to stall and has no power when accelerating. I took it back to the mechanic and he kept the car overnight to some diagnostics and called me in the morning to say everything checked out fine and the check engine light is off.
Obviously since I am writing this nether of those issues are resolved. Check engine light sporadically comes on and the car runs like crap when the AC is on.

Any suggestions of where to start to figure out what happened?

 

My first stop would be a new set of coil packs. Those are likely original to your car and your mechanic may not know that they have a service life.

I would have normally replaced all plugs anyway so just a set of coil packs will do. I've had some good success with aftermarket coil packs in the $8 to $9 per coil price point. I've bought some on eBay that were every bit as well made as the OEM from what I could see. I have not had any vehicles come back for those coils.

Food for thought. They're easy to change.

 

I have also seen fuel pressure issues (regulator or sensor going bad) also cause a CEL for a misfire. When the FPR on my S40 went, the CEL showed a P0303 (misfire #3).

 

Good point. Low fuel pressure and you have a lean condition in all cylinders. Some will fire, some won't.

 

Would the coil being bad or the fuel sensor issue show up on the computer the mechanic was using? They say that nothing is showing when they do a diagnosis.
Is it possible if the used an aftermarket alternator that might be the issue or as I read on another forum that they did tighten all connections enough that it may lose some power?

Probably stupid questions but the mechanic doesn't seem to have any answers for me

 

If your alternator is not charging, you will eventually lose fuel pressure but you would have other signs before it finally quits.

The misfire is almost always an ignition problem. There should be some misfire codes in the computer. If not, then you are back to basic diagnostics, powers and grounds.

 

Here's is the reading from scan
8010 battery voltage
9400 brake pedal sensor
280 a front heated oxygen sensor bank 1
801 a system relay
2340 injector cylinder 4
3503 misfire at least 1 cylinder
3543 misfire cylinder 4
Cleared codes, test drove and we can not duplicate the misfire

Same problem started shortly after this with the check engine light and rough running engine so I took it back. Here is what they did
Scanned codes P0304 we found ignition coil on cylinder 4 to be bad and the spark plugs to be worn out. We replaced the #4 coil and spark plugs, erased codes and test drove.
They told it should be fine.
Check engine light came back on within the first hour of driving.

Should all the ignition coils been changed when the did the plugs? Also it seems the car runs rougher the warmer the engine gets from running

 

The ignition coils should, in my opinion, be changed at your age and presumed mileage.

 

Ended up being the #5 fuel injector was bad. Got it replaced and has been running great since. No check engine light or running rough. Thanks for your input