Engine dies when climate control shut off
 

2001 Lesabre with automatic climate control. Previous owner put a bypass pulley & shorter serpentine belt since AC compressor is seized.
I was driving and could smell exhaust when waiting at a red light, so I pressed the climate control "OFF" button, which seems to have caused the engine to die.
I shifted to "P", turned the key and the engine fired right back up.
As soon as I got home I tried to get the same thing to happen, but rather than dieing, the engine rpm just dropped off a bit.
Has anyone else experienced something like this and what did you do to correct it?
Thanks

Unfortunately, we don't know everything the previous owner did.,

This will require some detailed checking, and we at this time have no idea what he might have altered aside from the , "by pass"