car starts and idles but cuts out and dies when I put it in gear and try to drive.

I have had it scanned on a computer, the only code came up was an o2 sensor. I have changed the crank shaft position sensor, the throttle position sensor, the idle air control valve or sensor, & the map sensor. This was all on a mechanichs advice the same one that I payed to come scan it. Now he says it could be the electronic control module. If that is so where is it located? Are there any other things it could be? The mechanic said an o2 sensor wouldn't cause this but I don't know. Please help!!!!!!