Both technicians are correct. Fuel trims are a valuable diagnostic aid if you know what impacts them and how to decipher what they're telling you. But many techs I've met don't understand what fuel trims are to begin with. They don't understand how or why the ECM makes changes to them, or what factors impact those decisions. If you aren't comfortable with how the ECM manages fuel delivery, how will you be able to comprehend what's happening when fuel trims go awry?
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Shore up your foundation
Let's be real with one another. We don't know what we don't know, or what we've learned incorrectly, until a situation arises and brings the weakness to light. Back in the old days, we could afford to make mistakes because the vehicles of the day more easily tolerated them.
The same is not true today. So, no matter how long you've been turning a wrench, no matter what school you've attended, attend training and continue to feed your mind and build your skills. Question everything an instructor tells you, be sure you understand the "why" when presented with new material, and never give up on moving forward. The technologies waiting for us ahead demand it.