Lol sweg.
Your battery may not hold a charge/be bad. Starter may also be bad.
Oriellys/Autozone should be able to check if your battery and starter are bad.
Cobalt is good on mpg. When you find one kick its ass on the highway and take it to a mechanic you trust(if youre not a mechanic). If the owner is scared to take it to a mechanic...well.
Id sell you mine if you live anywhere near AR(ignore my florida name).
Are you sure you put the cap on the exact same way? Jk
Go to oriellys/autozone and tell them you have a check engine light on. Theyll check it for free.
Write down what code(s) it gives.