Diagnostics is like an art form, a lot of people pretend to know how to diagnose but it takes an expert to do it efficiently. Sometimes when diagnosing a check engine light or stop engine light it can lead you down a vague path, if you have pressure differential codes and find the tubes are plugged with soot you might’ve found the problem but not the root cause. The root cause could be the 7th injector or the DPF filters. Without a complete understanding of the system this engine light could come back again and again, so always consult a trustworthy and reliable mechanic. Even diagnosing something as small as a trailer marker light not working can become a long and expensive job if you don’t know where and what to check. An inexperienced mechanic may start tracing wires without first checking the fuses or junction boxes thus causing you more down time and a bigger bill than needed.