For some reason there appear to be two different version of CATS with different locations for the downstream O2 sensors (2000 to 2002 4.0 anyway). Mine are located about midway down the body of the converter, another version has them in the usual location after the converter. If you look at the "49 state" direct fit CATS on eBay you can see both versions.


If I get the California legal universal fit, which requires a custom inlet and outlet, then they are in the conventional locations.


Any idea why they are different, and any opinions on how a "downstream O2" sensor works correctly to measure catalyst performance if it goes into the middle of the converter ???


I don't need CATs right now, but looking to the future if needed.

 

AJ26 engines have 4 standard Zirconium O2 sensors, two before the cat, and two after the cat.

AJ27 engines have 2 wideband upstream O2's before the cat, and 2 Zirconium O2's at mid-cat.

NA cars got the AJ27 for 1999, SC didn't get the AJ27 until 2000.
That's what screws some folks up. That's also why you hear so much about people ordering O2 sensors and getting the wrong ones. Parts houses can't seem to get passed the idea the sensors didn't change cleanly by model year.

Cheers,

 

Well that is certainly good information from xjrguy.


So, now I am wondering if I were to get the California approved Magnaflow universal CAT which has the "downstream" O2 located after the CAT, will it function correctly, or is the ECU expecting a reading that would be different than the "mid-CAT" O2 ?