Unless you know exactly what's wrong, don't be tempted by chain repair shops, like Aamco and Mr. Transmission (for transmissions) or Midas and Meineke (for mufflers). Go back to your dealership or regular mechanic and describe exactly what you see, hear, or smell. Get a repair order and make sure it describes the specific problem (not "check engine performance"). Ask questions before you sign it, and find out if the work is guaranteed (90 days is usual).