failed emission test advice to correct these numbers

First off 90 XJ 4.0 (9 monthes on rebuild) K&N cleaned this morning, 5.0 injectors, Catco cat and Flowmaster muffler(3 months old) approx 152000 miles or approx 7000 since rebuild

Heres the #s

HC ppm CO% CO2 O2% RPM
Limit Reading Result Limit Reading Result Result
220 50 Pass 1.20 3.09 Fail 16.20 0.0 2508
220 24 Pass 1.20 0.00 Pass 14.30 1.1 937

Everything I have seen says O2 sensor could be the problem. I have never changed so most likely is the original. Also how hard is it to change the O2 sensor.

high CO (if I'm reading it right - may be worth some reformatting) ... sounds like there's more fuel than air... or the cat isn't burning well...

O2 sensor is kinda, maybe suspect, but look for something along the lines of a dirty air filter -- really, the #1 cause of a CO failure at high RPM (after passing at low RPM)

what Ford injectors? - did you increase displacement on the rebuild?

The booger here is that high CO2 is a sign of a healthy engine...and a good indicator of overall cumbustion efficiency (you'rs seems pretty happy and with the low HC value thigs seem OK)... so things are looking fine there, but the CO (again if I'm reading it right) is a bit high -

If you'd'a failed at lower RPM as well things could be due to a little extra injector, a bad CTS or bad IAT sensor (either/both reading cooler, or open circuited) -- the Cat not being lit (given that the HC value increased as well) --

This could be filthy oil or rings that don't settle well, but look for air restrictions in the intake or something that'd get you more fuel than you'd want at speed...

Make sure the cat's properly warmed up before testing. Also, sorry, don't know what it could be, but if the O2 sensor needs replacing, its not difficult. The only hard part is bending over to get screwed outta sixty or so bones when you buy a new one. But, even though thats the case, I would suggest getting a new one if yours is old. Those puppies are pretty important to emmisions, economy, power output and engine health. Good Luck.

Hey Harvey. Email still work?