What may cause harsh shifting

dashboard engeen and SERV ((P)) light on

need a drawing to show how to remove. where is it located.i have removed radio but dont see it

It's in the back of the car it does this

I just bought a 2005 LS. After 2nd day engine light came on and has stayed on. Owners manual says gas cap might not be seated right or installed correctly. I filled up tank, put cap on and it and found it will not click at all. Can I buy an after market locking cap that will click. I think this might be the cheapest course to take first. Any help though in case. BTW, there is no dealer warranty and I'm a bit pissed they didn't disclose this problem. They had car a month in their inventory and I think this is something they knew about. On the plus side this car only has 42,000 plus miles on it.