How's the cat converter working? Is it working?

No codes? Well, the converter won't necessarily set off a code and when mine went bad, it ran fine but smelled NASTY RICH! Yours could be clogged, which would explain the loss of revability...and the smell

Also, check for vacuum leaks, TB unit, hoses, etc.....

Check the timing, too. It should be okay, but do so if you sense the belt jumped a tooth.

If I guessed right, tip the mechanic real good and maybe he's worth keeping for giving a refund like that! Most would back off and rip you a new one to boot!