Mechanic sez manifold is cracked - BS?

[Gil]

I've got a 2004 ML350 with 47k on the clock. It was throwing codes (O2 sensor and a right fuel bank problem, depending upon where you go to find code meanings online), so I brought it in. Mechanic calls me and sez its a cracked intake manifold. Used replacement will cost me about 1100 total, which seems reasonable if it needs to be done.

I didn't look at it but while driving I never noticed any hissing, rough idle or other problems. Thinking of going back down to have a look and maybe just JB weld it if the crack is small enough. I know there is a risk the JB weld could get into the intake system, though.

This intake is not made of plastic like some others, but aluminum. How likely is it that it cracked after so few miles (or at all, really)?

Thoughts?