You don’t have to wait for the check engine light to get a car diagnostic test. If you hear or feel something “off,” mechanics can use diagnostic testing to uncover the problem. However, many technicians recommend taking your car in for a diagnostic check at least once a year, which will uncover minor problems that can’t be seen or heard (and that won’t trigger the check engine light).
Diagnostic testing is also a useful tool when you’re checking out a used car. Both private sellers and professional car dealers should agree to a diagnostic test before you commit to buying, and if they don’t, that’s a sign you should walk away from the deal.