My CEL has been coming on lately. I did a computer reading and got a P0135 - O2 Sensor Heater Circuit / Bank 1 Sensor 1. I replaced the O2 sensor with a Bosch sensor (can't find any Mopar sensors around where I'm from). Reset the computer (disconnected the battery for 10 minutes) and the CEL was gone and my Jeep was running better than it has been since last spring.
A week later, the CEL came back on and I checked it again. Same problem but I knew something else had to be going on. I did some looking around on the good ole interweb and found a great article on how to troubleshoot the O2 sensor. Long story short, I had a blown O2 sensor fuse (15 amp fuse in the engine compartment) that was preventing the heater circuit from operating. Replaced the fuse, checked the connection with my multimeter and voila! 12 volts right where it should be.
So, before going out and spending anywhere from $50 - $120 on a new O2 sensor, check the connections on the wire harness on the engine and on the sensor itself. Could save you some big bucks in the long run. The fuse, which came in a pack of four, only cost $4.
Here's the link to the article that I used: