Jeep: 1999 Jeep Wrangler o2 Sensor, technical service bulletins, jeep wrangler

Question
I recently put new exhaust system on my 199 Wrangler it has the straight 6cyl. Since i did this i keep getting a heater circuit malfunction on the front sensor and a high voltage code on the rear sensor. I replaced the front sensor last month, what is going on with my jeep?

Thanks for the help  

Answer
Hi Andrew,

The first thing you should do is check the fuses in the PDC. TSB#18-09-99 references common DTC's caused by an open fuse in the circuit. Both O2 heater circuits are powered by fuse #27 (10A), and can be blown by a faulty heater, or even a stressed harness (like when replacing exhaust). The codes stated happen to be Oxygen Sensor Heater Circuit Failure and Shorted to Voltage (among others) -- both of which you seem to have.

If you find it blown, just replace it and the codes will eventually clear themselves (3 Good Trips to turn the light off, and 40 Warm-Up Cycles to clear the codes). If it blows again, then you have to trace the short to ground somewhere in the heater circuit, but it should be relatively easy, since the harness is very visible in that portion. The first step in the Diagnostic Method, after confirming the issue, is checking for Technical Service Bulletins, and it just so happens that this TSB seems to address your particular concern. Add that to the fact that this started right after the exhaust replacement, and it makes perfect sense.

If it's *not* that, then it's possible that there is an open circuit or poor connection (a lot can happen if you forget to unplug both connectors -- been there, done that!), which would emulate a blown fuse (no B+) in the heater circuit. Don't rule out the possibility that one or both sensors are bad as well, regardless of how new they are. I don't know which one you've replaced, but if none of the above fixes this, I'd take a good look at the other O2 sensor (which might need proper attention with service info and a scantool). In most cases (where a physical change has created a seemingly unrelated problem), a thorough visual inspection is your greatest asset.

Good luck!