When I turn-on AC or defroster, motor idles rough then stalls.
My mechanic says it is not related to CHECK ENGINE light or exhaust O2 sensor? Any ideas? I have an 02 Ford Explorer.
The problem might be with the Idle Air Control valve. This valve adjusts the engine idle depending on the load impossed on the engine at idle. So for example in your case, when the AC is turned on the IAC valve fails to bump the engine idle up to compensate for the load impossed by the AC compressor on the engine. This then causes then engine to idle rough and stall.
Tester
Does th AC work? It isn’t out of the question that the AC compressor has fallen to pieces. If it works then, this isn’t the answer.
It’s also possible that the AC compressor is bound up. Usually I’d expect this to snap the belt, but it can stall the engine too.
“My mechanic says it is not related to CHECK ENGINE light”
I’m not a mechanic, but I don’t use them any more because I don’t have any I trust. Should I assume from the statement above that your check engine light is on? There’s nothing wrong with the answers so far, but if your check engine light is on then there are error codes stored in the computer. They look like “P1234” - ask your mechanic what they are and report them.