Altima 95 overheating

My car overheats when I set the TEMP lever to the HOT position. It runs fine in the COLD position, with or without the A/C on.

I suspect it started after my mechanic replaced the radiator, maybe he connected something wrong.

I live in Texas, so it's actually no big deal, I almost never put in the Hot position. But it really puzzles me. Any ideas (Heater core, thermostat)?

was it doing it before you took to mechanic?

bring it back to the mechanic if this problem arises AFTER the job...

maybe there's still an air pocket in the system & he just needs to "re-burp" the system