So I have a 1993 YJ with the 2.5. I have failed emssions a few different times for High CO and High HC. and am hoping for a few ideas.
Test 1: Failed for high HC Reading 2.69, limits are 2.5. Hooked it up to the sun maching and discovered the cat wasnt lighting off so I replace it. (previous owner had punched it out)
O2 sensor was reading a negative voltage of -0.2 to -0.8
Replace 02 sensor, spark plugs, fuel filter, airfilter, cap and rotor, removed throttle body and cleaned that as well as the intake and no vacuum leaks.
(all for good measure)
Test 2: Failed for high CO Reading was 27.6 and limits are 25.
all the numbers on the scope machine are well below the limits ie, snap throttle, idle, and 2500rpm
the only thing i it seems i have issues with is under decel, the engine goes full rich, causing the reading to spike and HC goes through the roof. I tried multiple O2 sensor, yet the reading still read the -0.2 through -0.8 If I unplug the O2 sensor the check engine light does not illumaniate, with engine on.
Im thinking computer. any ideas