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1991 Jeep Cherokee - Won't pass smog

Hi all. I have a 1991 Jeep Cheroke Sport. It has 200k miles on it and after proper, general, care, it still starts and runs really good. I recently failed the California Smog Plus because of just past max high NOX readings at 25 MPH. I had the catalytic converter changed about 10 years, or guessing 70k miles ago. I am seeing a lot about oxygen sensor and catalytic convert replacement to fix this. Does my catalytic converter sound young enough to not have it replaced and try just repacing the oxygen sensor first and pay for the cheaper 2nd smog retest, avoiding the more expensive route, then doing a 3rd retest after the cat change. The risk of the cost of the retests without paying the more expensive cat conv change sounds like it might be worth it if I am being realistic. Thoughts/experience? Thanks!

Since your Jeep has an EGR circuit, the catalytic converter plays no role in NOx reduction.

High NOx occurs when the combustion temperatures become elevated.

The EGR circuit introduces a small amount of inert gas into the combustion chambers to lower the combustion temperatures, which reduces NOx emissions.

So the things that can cause high combustion temperatures are, an inoperative EGR circuit, an engine that’s running hot, an engine that is running lean, or an engine that has excess carbon deposits in the combustion chambers.


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If you are somehow attached to this vehicle and wont be getting a new one for whatever reason… You could always use that fuel additive that guarantees you to pass a smog test. Since you almost passed without it… I’m sure you will pass with it.

Otherwise 1991 was a long time ago and cars n trucks have come a long way since then. Something 10-20 years newer are actually quite affordable and dare I say cheap as can be. May be time to think about an upgrade.