96 misfire and 2 error codes

Hi all, I'm new to the forum and found it while trying to research some error codes my mechanic gave me. I have a 1996 Century that was misfiring on two cylinders, 1 and 4. I've had #4 replaced once in this car about a year and a half ago, and I only have about 75,000 miles on the car. About a week ago it started misfiring again, and I needed the plugs changed anyways, so I took it in to have him look at it. Apparently the misfire cleared up while he was looking at it, and he changed the plugs like I asked, and it hasn't been back yet. I have only had the car for about 5 hours though, so I guess I haven't giving it enough time.
Their are two error codes though, that he said didn't have anything to do with the misfires. P1651 and P1652. I've looked online, but I can't find anywhere with a listing for what there are, and he said he didn't find much either. So if anyone knows a)what these codes mean or b)why I'm having such a huge problem with injectors I would greatly appreciate it.

thanks
paul

Try this for the error codes:
http://www.troublecodes.net/OBD2/Pcodes.shtml#1600

As for the injectors make sure you run good quality injector cleaner through the fuel system every oil change. Also change the fuel filter regularly to keep the crud from blocking the injectors. Should help eliminate the problem.