Too much shop think here. So, in your mind, limit yourself to a small box of basic tools. Now, tell the owner how to fix his or her vehicle.
If someone wants to work on their car, they will have to purchase tools. Period. Otherwise your guessing with diagnosis. You can do things and diagnose things without tools, but only to an extent. Not shop think. “Shop think” IS the diagnosing process.
Check Vacuum leaks with a can of carb. cleaner.
Listen for Fuel Injector solenoid operation with a screwdriver.
Test light or DVOM for check injector pulse.
$20 code reader to wipe DTC History to check for misfire travel when moving coils.
That’s a pretty cheap way to do it.
Besides the fact that most autoparts stores will let you rent specialty tools for free.
Anyway, check the relay. Eric has a great video on how to do that.