Hi. 1997 XJ Sport 5 speed 238xxx miles, failed CA smog twice for high NOx.

The numbers:
15mph
Hydrocarbons: 73ppm CO: 0.7%
CO2: 13.1% O2: 1.4%
NO: 2242ppm (FAIL)

25mph
Hydrocarbons: 53ppm CO: 0.8%
CO2: 13.5% O2: 0.7%
NO: 2452ppm (PASS)

Checked by test/repair station:
Negative intake/exhaust leak
CAT in fine condition (4 years old, not recommended to be replaced)
O2 sensor fine

THE ONLY THING THEY FOUND: Low compression in cylinder 4. Compression reading from 1-6:
175psi, 180psi, 180psi, 130psi, 180psi, 180psi 180psi

Recently replaced: Fuel injectors (stock units, rebuilt)
Thermostat (temps 210-215ish)

The noticed slight misfire while idling (so did another shop some time ago), but otherwise Jeep runs perfectly, no issues at all. Doesn't burn oil, good MPG, plenty of power.

It looks like the compression specs for my engine are 120-150psi, which would mean my number 4 cylinder is actually the only one in spec, except that it's outside the 30psi max variation spec. Is there something wrong with Number 4, or is my entire engine at too gigh compression? For some unknown reason? I've onwed this Jeep for over 10 years and 120,000 miles, never had the head off, never overheated, always passed smog fine, only replaced the CAT once etc etc.

I'm tired of getting pulled over for expired tags, please help!

 

I had a similar problem, based on my research hi NOX is caused by excessive combustion temps. To pass I ran a 180 degree thermostat in conjunction with colder heat range spark plugs



Every forum I posted on told me it was a sensor problem, which it was not. I tried everything, and nothing worked till I switched the plugs.

Can't guarantee it will work for you, but it's a cheap option, under 20$ for the plugs and thermostat

 

Here is my fail specs...



I passed with flying colors after the mods in my previous post. Once I passed I put the hotter plugs back in.