Take it to autozone they'll scan it free and give you the code. You can then google the code or come back here for feedback

 

Is your check engine light ON? If so, then you (or Autozone) can retrieve the code that is stored. Post the exact code(s) here for comment, not just a description of the code. With OBD, the devil is definitely in the details.

Oxygen sensors have a finite lifespan usually in the 100K to 150K range, depending on the efficiency of the engine.

If you need to replace one, unless you plan on selling the vehicle, replace both of them at the same time. The failure of the other one is likely to occur in the near future. Do em both. And use only NTK sensors, which are the original equipment sensor that your 99 XJ came with from the factory. NTK sensors are not expensive and are available at most parts stores.

Good luck!

 

The check engine light only monitors if the heater in the element is working. It does not monitor voltage coming out of it since it fluctuates. If you have a bazillion miles on your vehicle and it hasn't been changed, then yes replace it. They generate electricity and like solar panels, they wear out over time.

 

Oh, worse than that! The voltage they produce is in the microvolt range and just the age of the elements can change the output. Also the 02 sensors for CA emissions are more expensive than the Federal ones. But here's the amazing part. My 2000 4.0 WJ with CA emissions has 243K miles on it with the 4 original sensors and just one heater is out! Active data shows they are cycling within the normal range. NY will allow 1 monitor to be "not ready" and that's the one.

 

You don't hear this very often (in a positive context anyway.)