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So here is the run down for everyone, I purchased a 94 Jeep Grand Cherokee Laredo with 260*** miles on it. It has a straight 6, auto, 2wd. I took it for a smog test and it failed the NOX part the acceptable limit is 930ppm and mine was about 1536ppm. So I took it home did an oil change and ran a fuel injection system cleaner through it change the spark plugs, spark plug wires, cap, and rotor. Took it to be retested and it is still about 1087ppm. As it has been explained to me NOX is related mostly to the combustion temperature in the engine and is usually regulated by the EGR system, but Jeep stop putting EGR systems on the straight 6 in 1990 so I can doing anything with that. Its not throwing any codes not even sure if it could don't know much about this vehicle in particular. So does anybody have any suggestion for things to try short of putting an entire exhaust on? The guy at the E-check station told me to check with Jeep to find what the acceptable threshold was from the factory because sometime there are errors in system and there threshold is lower than the factory standard and if that is the case I would be exempt. But I have not been able to find this information anywhere on the net. So I will probably contact Jeep on monday if I can't find anything.
 

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Wouldn't that be controlled by the 02 sensor? I failed once (I think it was NOX) and it was due to the 02 sensor. It's a little pricey, but did the trick.
 

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Wouldn't that be controlled by the 02 sensor? I failed once (I think it was NOX) and it was due to the 02 sensor. It's a little pricey, but did the trick.
If it were an O2 sensor would it throw a code or light the check engine light or is it too old?
 

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NOx should be a cause of high engine temperatures more than a bad O2 sensor. All that sensor does is read the oxygen content in the exhaust. If you failed a smog test then i would look at how well the engine is running. Clean throttle body, good gas, clean injectors, newer spark plugs, wires, cap and rotor. All those things can make the engine run cleaner and help out the smog level.
 

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I have a 93 zj with a 4.0 everytime i didn't pass it was due to the cat going bad ( 3 times) Two weeks ago I took it down for a smog check it didn't pass change the cat still didn't pass. So I am not sure where to start at.
 

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I have a 93 zj with a 4.0 everytime i didn't pass it was due to the cat going bad ( 3 times) Two weeks ago I took it down for a smog check it didn't pass change the cat still didn't pass. So I am not sure where to start at.
You can start by voting Libertarian the next time!
 

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If it were an O2 sensor would it throw a code or light the check engine light or is it too old?

Mine never did.

I've had a bad cat before too. Can't remember what the readings were. Whenever I've failed, the tech has given me good advice as to what to check and they have always been spot on.
 
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