Was having an ignition issue on my 2000 Chevy Blazer LS. All the symptoms lead me to believe it's a starter issue. I take off the starter and have it tested twice at the parts store. It passes both times. Put the starter back on and hook up the battery to a jump box and it fires right up.

The next morning I go out and it's giving me the same symptoms. Pull the battery out to have it tested and it tests good also. Hook it back up and still same problems. While trying to verify contact on the solenoid, I bypass it entirely and the starter motor engages. After jumping the solenoid I've started it with no issue a half dozen times.

Should I trust the machine at the store? If I had to jump the solenoid does that mean the starter is definitively defective?

 

Take a look at your ignition switch, not the cylinder.

 

You haven't said what the symptoms are, but if it's not cranking, I agree the switch could be the problem.

 

After jumping the solenoid, it will crank over with the ignition switch if (when) it happens again I'll start with the switch.