Mechanic said "emissions sensor". Uh, ok, what sensor? What code? What did he say it would cost to fix?
My no info guesses based on info we don't have:
If he said it'll cost $150 to fix, that means it's an oxygen sensor. These are easy to replace, but depending on the cost, if you replace a completely good one, you'll spend more on sensors than paying him the $150.
If he said it'll cost $1500, then you likely have a P0420, cat efficiency too low code which can be the cat at worst but can be a buncha other things as simple as a leak in the exhaust.
In short, this is a particular problem that you should bring it to a competent mechanic who is going to actually diagnose the problem, find the reason, then have the solution to the dollar.
I am very familiar with many car problems. But I don't have the high end (thousands of dollars) OBD scanners to look at output of the ox sensors on the screen and such. For those problems, I bring it to a "real" mechanic and let them diagnose and fix the problem.