Bought a house in May that came with a window air conditioner connected to a 220V outlet. The air conditioner is gone with no plan to replace so I want to remove the outlet. I went to the breaker, flipped the double pole 20A breaker to "Off" and then use my power line tester to make sure the wire was not live and to my surprise, the line is still live. I confirmed I am turning off the correct breaker. What am I missing?
I'm not clear how you traced the wire, unless it's exposed the whole way?
If you do have access to the wire, even if it's just when it enters the panel, you should use a non-contact voltage detector to check for power. When you flip the breaker, it should go off.
There's a possibility that there are two feeds to the outlet. I would start by going through all breakers one-by-one (or several at a time, to narrow it down) to see if the power shuts off.
Keep in mind that you can get 220 by bridging two 120V breakers, and so one single breaker may not turn off all power. This setup is not legal, but it sounds like it may not be wired correctly anyway - don't make any assumptions when it comes to bad wiring!
Worst case, shut off all breakers, and confirm the power goes out. Then start turning things back on (an assistant and/or non-contact detector that beeps will help you here).
I personally would not touch any of those wires while my meter still read voltage, and I recommend no one else does either.
Best advice is to "pull" the breaker from the bus bar. In my experience, a breaker can become locked due to the contacts being welded internally and WILL NOT disconnect power/voltage from the circuit.
I had a problem like that, I tested my NEMA 10/30R 125/250V dryer outlet with the breaker off and still got a reading of 220V. I called the electrician that wired the house, and he read 0 volts with his tester and then read 220V with mine. My tester was a Sperry multi-meter and his was a Knopp.
The Sperry Multi-Meter
The K60 Knopp tester
Because the Knopp tester is solenoid driven you cannot get any back-feed from anything on the panel. The multi-meter can pick-up voltage back-feed and show up as voltage on your tester. This can also be a problem with proximity testers.
The older style testers like this is made by a lot of manufacturers.
Also the Knopp is considered a hi-impedance tester and the multi-meter that I had is a low-impedance tester.
I'm not saying that it's OK to presume the tester is at fault and I don't know what tester that you used. You should always check and double check and if in doubt, do like you are doing and ask questions. If you are still not satisfied, call an electrician.
Also, what reading do you get from one side when measured to ground compared to the other side to ground?