Why does my Galaxy S5 OEM fast charger overheat?

[AC Question]

I bought a Samsung Galaxy S5 unlocked from the carrier Verizon and use it on the MetroPCS network. The phone when I first got it charged 100%, but the charger port that plugs into the wall (not the phone) would get hot. I then upgraded my firmware to Marshmallow and it still charged that way. A couple days ago, I left the phone charging and came back to it at 71% and it wasn't charging. I then grabbed the plug and it was so hot you could smell the electrics inside burning. Surprisingly, the phone port was not hot (thank goodness) and I have switched to a different wall plug that uses about 0.6A so it's a little slower, but it doesn't overheat. I have used a USB Voltage tester and it does not seem to show anything out of the ordinary when I plug the phone into the OEM charger, but I haven't left it in for an hour to find out since I don't want to risk damaging my phone. I have ordered a 9V wall plug, but can someone please verify with me that it is a wall charger problem and not a phone problem? I would hate to have to go through the hassle of sending it back.