Ever since putting the header on I have had a CEL for the upstream o2 sensor. I have noticed that when it is cooler outside the CEL goes away... why is that? Could it be that the sensor isn't bad after all?

Hey... answer my question!!! hehe

Strange..... Might just be a loose connection or a shorted wire. When the temp changes the wire may expand just enough to make contact or something. I would maybe spray some contact cleaner on the connection or make sure its not corroded at all...

Just my 2 cents.... Good Luck!

Chad

Your sensor is malfunctioning. I just replaced mine which was giving the CEL on and off for about 4 months (I was lazy to replace it). But within the last month (before I replaced it), we had a week of cool air, and the CEL stayed off the whole week, then it got hot the next week, and I got the Cel again. I replaced my O2 sensor with the Bosch one at Autozone, and I've not had a Cel since. CAR DRIVES DAMN GOOD TOO!!!! WOOHOO!!! You can get the sensor for $60 to $100. I bought the $100 one that was marked for our car, but it had the wrong sensor in the box, so my buddy (who worked at Autozone) had me try the NEON one, which seemed to be the same electrical connection, just a shorter wire, and it's been working for a month now with no Cel. That one was $60

watch out bosch sucks, it will fail soon
get mopar o2 sensor only