SES is on with a code of p0420. I can clear it then it comes back on after a couple of days.
SES Light on with a code of P0420 on 2001 Pontiac Montana
check out this article on this code and what it mans: http://www.aa1car.com/library/p0420_dtc.htm
Check for cracks near the o2 sensor near the exhaust manifold.