Andrew 1800

Hate to join a forum with a question but I need some advice. Just bought a 2012 challenger 1800 se with the 255. Took it out yesterday and it tops out at 35mph and around 6500-6800 rpm. Now the guy I bought it from had it checked and it does need a air temp sensor. Will this cause the low speed and rpm? Also the start button light is yellow. Not sure I that indicates some kind of safe mode or if it is supposed to be yellow.

Has warranty til mid 2017 but the dealer said they want to charge me the 50 decidable, which is fine, but another 100 to just check out if that is the problem. Thoughts?

Thanks a bunch.