Ahh, the old discussion about fault codes being the diagnosis of the problem. Remember, before 1996 we had to diagnose and repair cars without fault codes to tell us anything about misfires, and often we had to diagnose and repair poor running cars with no data from the car.
In a nutshell @JimFrost , a code P0303 tells you nothing more than cylinder #3 is not firing properly. It could be caused by a coil, a spark plug, a coil boot, a fuel injector, an internal mechanical problem, a mouse chewing through a wire, a faulty engine computer, a loose vacuum hose, a coolant leak, and I’m sure 20 other things. The guys at Autozone are just giving you the most likely cause, probably from a database containing info gleaned from those of us who actually understand and repair cars and do specific testing before replacing parts.
Now if you had a fault code P0353, that would indicate that the engine computer had detected a fault with the primary ignition circuit and would lead you to suspect that the coil was faulty.
An actual scan tool with full live data capability and interactive controls would give a seasoned technician the ability to have the problem 90% nailed down before even opening the hood. On most late model cars, I know whether the problem is ignition, fuel, or mechanical just by looking at all the data the car has to offer. The code readers they use at the parts store? I wouldn’t waste $50 on them.
The problem for the DIY’er is that it’s often easier and more cost-effective to pull a fault code and replace part the store suggests.