What is a check engine light diag

They need to do a diagnostic test to find out why the Check Engine light is on. The Engine Computer detected a problem, set a diagnostic fault code, and turned the light on to tell you. They will read those codes to see which circuit or system needs further diagnosis. Those codes never say to replace parts, but about half the time a new part will solve the problem. The other half of the time it will be a wiring problem like corroded connector terminals or a broken wire, or it can be due to a loose hose clamp and things like that.