I have a 1999 Pontiac Grand Am with 105,000 miles on it. I bought the car from a used car lot last year and they had to put a new engine in it. I hadn't changed the spark plugs on it until recently. My son-in-law replaced the spark plugs for me and now the #5 plug misfires. He can't figure out the problem. It also does not get a lot of power such as getting the car up to the speed limit to take it on the freeways. Also, the " Service Engine Soon" light stays on and will not go off. When I get to a stop light or sign, the light indicating that I need oil comes on and as soon as I go the light goes off. I know I don't need oil because I just got an oil change when my son-in-law changed the spark plugs.
Is it advisable to get a tune up on the car and what is causing the light to stay on? Any help will be appreciated. Thanks.