I just finished going through a bunch of issues with my own Renix XJ. This is my first post on the forum here, and my first XJ by the way, so take my advice with a grain of salt. :laugh3: I did, however, do quite a bit of research to learn how the Renix system is setup as I was fairly clueless. Wrenching on my own rig/car is nothing new to me, just this early Cherokee threw me for a loop.
Here are some excellent resources that helped me to diagnose, test and ultimately repair my jeep and successfully get the Renix running in Closed Loop.
For sensor testing, location and explanation:
LungHD Renix Sensor Diagnosis
Here's a site from a member of another jeep forum, there's a bunch of great tips and tricks on this one:
Cruiser54's Page
I went ahead and tested every sensor on my Renix Jeep and found that I had a bad Coolant Temp Sensor (CTS) and what I seemed to be a dead O2 sensor. The CTS was reading around 11,000 ohms on a fully warmed up engine, which tells the ECU the engine is operating around the freezing point in F and caused the Jeep to run very rich, and was possibly the reason closed loop was never reached. I say "possibly" because I replaced the O2 sensor at the same time, either way, the CTS being off by that much would definitely throw the AFR way out of whack. The Jeep fired right up, idled noticeably smoother and the O2 sensor now cycles from around .7v up to 4.8v at idle when it would stay at 4.8v constantly. I also had to fix my ground leads in the harness up near the firewall as outlined on the Cruiser54 site in
Tip #6. When I would touch that harness my idle would change pretty drastically and is now rock solid.
Sorry for the long post. You could easily just have a cheap sensor that failed early on, or there is another failed or failing sensor the Renix depends on before it will run in closed loop.