I bought a 2004 Chevrolet Trailblazer LT EXT 6 cly. and when i test drove vehical all was good, the next day after i purchased vehical it started idling rough at stops and red lights. I took to Mechanic and it was throwing 3 codes which all three pointed to O2 Sensor, took it back to lot where i purchased and they replaced o2 sensor and when i got home it started idling rough again. since then i have figured out it only does it when heat and air is running. any suggestions? also i figured out the the Check Engine light has been disabled because when i turn key on all lights come on except the check engine light, any suggestions on getting this light re-enabled?