I bought a 96 Tracker 4 dr 4x4 about 4 months ago. I've been using it as a daily, but I am unable to pass my smog here in Cali because the Cat monitor wont set on it. I've replaced both O2 sensors on it, I've tried another cat on it. I've driven it well over 800 miles while Ive had it and it just wont set. No DTCs, no pending codes, runs fine. All other monitors will set in a day with alittle driving. I have a Foxwell hand held scanner (Elite 614) and I can watch O2 activity and seems normal, S1 switching like it should, and S2 is steady (.06 to .30v). The only thing that Ive seen could be a problem is the long term fuel trip. When at temp, the short term is between -3 to +3. The long term is at +18 (ECU adding fuel for lean condition). When I rev it up, the LT comes down to about 10. I've checked for vacuum leaks with no luck. Any help or direction would be greatly appertained. 

 Follow up. I read on fixkick.com "If the MAF output  is stuck over 1.7–2.0 volts AT 800 RPM, then the MAF is bad ,but the RPM must be 800" I'll attach a photo of my findings. I just want confirmation to condemn the MAF before spending the money.

 I misread, my voltages where good. but I found this "Using the scan tool, check the values displayed for "MASS AIR FLOW RATE." The airflow rate should be 0.20–0.53 lb. per minute (1.5–4.0 grams per second) at idle". When I ran mine, at temp at idle, Im reading 0.01. And reved up to 2500 rpm its 0.02. While driving, at WOT its 0.09 lb. Under reading air coming into system? And O2 is catching it, raising LTFT?

 Replaced Walker rear O2 with Denso unit. Within 50 miles the monitor set and was able to smog. Smogged clean, NO blew 0 at 15 and 25 mph test. Thanks to all who contributed. LOOL

So what is the solution?? Just replaced Walker rear O2 with Denso unit??