Running a diagnostic test on your car typically costs between $40 and $150 at independent repair shops. Dealerships usually charge more, ranging from $100 to $200 depending on the brand and complexity. Many shops apply this fee toward repairs if you choose to have the work done there. For a free option, auto parts stores offer complimentary code scans in their parking lots, or use our OBD-II code lookup tool to interpret codes yourself. Learn how to do diagnostics yourself or use your phone as a scanner. See also: full diagnostic costs, what scanners mechanics use, and current check engine light repair statistics.