Has anyone ever heard of putting some kind of putty or dope on a leaky o2 sensor to let it harden around the o2 sensor to stop it from leakin?
thank you

There is no point, as o2's get VERY hot and whatever you use will just burn off. Replace the sensor, because if it's leaking, the sensor isn't functioning and will screw up your fuel metering causing fuel mileage/performance to fall in the toilet.

The mechanic says the putty or dope is made for exhausts and won't burn off
my check engine light is not on yet.

much easier to change it now than when it does go bad u wont have putty all over it