Does an EMT diagnose

No. EMTs never diagnose. We don't have the equipent nor supplies to diagnose any condition or disease. We treat for signs and symptoms that we can see. If I can test your Blood Glucose and see that it's too low, I can give you oral glucose (treatment-short term), but I can not diagnose you with hypoglycemia or the opposite, Diabetes. I can't test your BG and see that it's too low or too high and say "hey dude, you're diabetic!" It's not my place or my job.

After becoming a paramedic, you can do some diagnostics (like with EKGs and other equipment) but it's still not your place to tell a paitent "you're having a heart attack". That's what protocols are for. If you see a certain rhythm, or test a BG, your protocols tell you exactly what to do for that rhythm or that measure.