A couple of years ago I had to get the engine replaced in my 2000 2.5 and had a friend of a friend do it. On the way home the check engine light came on. I had the codes checked and P0135 and P0141 came up saying the heating element circuit was malfunctioning on the up and downstream o2 sensors. I didn’t worry about it much as I wasn’t using it much. I decided to finally work on the problem so I can get it inspected and registered and stop worrying about getting pulled over. I ordered the o2 sensors for it and the connections are different so I’m assuming he replaced the engine with a different year. I also noticed the engine is not running at the temp it used to, runs about 160. I checked the fuses and they look okay.
Is there a way to tell what year the engine is? Asked my buddy to put me in touch with his friend but the guy is nowhere to be found.
What else should I check? Could the engine being a different year be causing a problem?
The reason your jeep is running cold is more than likely because incorrect parts were used. There is most likely a 160* or 180* thermostat installed when it should be a 195*. That alone can cause the engine to run rich and which could trigger O2 sensor codes.
For O2 sensors, upstream and downstream are different so hopefully you didn’t buy identical ones. They should be the same for all TJ 2.5L. Make sure you buy them at a quality parts store like Napa, Rockauto.com, or the dealer. Autozone, ebay, and amazon will not be your friends.
I’d pick up a 195* thermostat, swap it out(can be done in the driveway with minimal coolant lost), clear your codes, and see if that solves both the CEL and the running cold. It will at least solve the latter problem.
Article source: http://www.wranglerforum.com/f210/o2-sensor-issues-2051145.html