THIS!!! When we got our 06 Monte back in 09, it had this exact same issue. Clear the codes, and it'd work for a day or so, until the vent solenoid threw the code again. Not sure how familiar you are with GMs remote start systems, but they are programmed so that the car will not start if there is a code in memory. See if you mechanic, or auto zone for that matter, can tell you what code it is storing.When the evap solenoid at the tank failed, it threw a code which I found later was why the remote wouldn't work - with an stored engine fault. If the codes are cleared, the remote will work until another, or the same, fault gets thrown again.