O2 sensor or CPU

I have a 96 4.0 auto that runs good but will not pass smog check. Emmisions are excellent but it fails the OBD check. It does not have any codes in memory but it does not complete all it's self diagnosis checks. I have hooked up a scanner and watched it complete all but the HEGO sensor check. I have replaced the HEGO sensor and checked all the wiring. I am now wondering if it is the CPU causing my headaches. Anyone have any ideas of what this could be or what I should check?

I just had my 95 pass smog yesterday. I went and looked at the printout and it doesn't show any OBD check. Could that be a unique thing in your area?

I believe OBD is only checked on 96 and newer vehicles.

Matt S is correct. All 96 and newer vehicles need to pass OBD II check. Does anyone know if the 96 XJ is exempt from the HEGO self check? Some Chrysler products around that period are. I am getting desperate now.