Well you will need an experienced workshop or mechanic to diagnose it for ya.

Unfortunately there is not a full proof way. Sometimes the ecm light comes on, other times, they don't.

Last week, had a gc8 come with with idle problems. No ecu chk light. Was dirivng ok over 3000rpm but was blowing black spoke at idle. Turned out to be the AFM.

Previous week had an V6 Sti come in, ecm chk light was on, showed the afm failure on my carman diagnostic screen: car was misfiring and wouldnt want to rev past 4000rpm --- ecu was in lymph mode. Popped new afm in, problem solved.

As a workshop, we only charge customers the replacement part if the part solves their problem.