Engine reads bad coolant temperature sensor as well as bad catalytic converter?

The check engine reads bad coolant temperature sensor as well as bad cat converter and gas detected in exhaust. Changed temperature sensor twice and reads when it wants but has never over heated. Changed cat converter and it didn't even need a new one. For the past week it's been hard to start and smell of gas at start. I changed plugs. For three days check engine light went off run great and temp sensor works now. It's still running good but check engine light is back on. I take a lot of short drives to work and back two to three miles. I just tried sea foam in oil after oil change as well as Chevron Techron for two fuel ups now. Yesterday I found my gas gauge isn't reading right. Filled up and reading a quarter tank shy. Just want some suggestions maybe some help what I should do.