CA smog: FAIL NOx

93 4.0.
NO (ppm) 989 measured. 527 max allowed at 25 mph. FAIL

NO (ppm) 128 measured. 415 max allowed at 15 mph. Pass

All other tests pass with low numbers.

What would cause these readings?

Smog tech says dont replace cat its pretty new and all other aspects are well with in specs.

Bad. cat.
cat not hot enough before going for test. ( DRIVE THE CAR FOR AT LEAST ONE HOUR BEFORE GOING FOR THE TEST You want that cat RED HOT)
If this does not apply. then keep reading


low fuel pressure.
Bad spark
that reading means you engine is running too hot (lean condition). I.E. you have low fuel pressure. your coolant is really old and crappy. spark is bad. wires are bad. spark plugs are bad. blocked fuel lines. bad regulator. blocked fuel filter.
the list goes on. will need more diagnostic elements to fully fix this one. ( spend the $20 and make yourself an permanent inline fuel pressure gauge there is a guide online. its invaluable...)
I bet you spark plugs look like crap


check those. its one of those
My jeep ran like 600 nox. after a 20 min drive.
after an hour of driving I took her in. and it put out almost 0's


If all else fails. Put some De-natured alcohol in the gas tank and you will pass with almost 0's ** but I did'nt tell you that .. and this is not fixing the issue.

Also, e85 is available easily now. Your fuel system was not designed for it, and I am in no way suggesting you run it all the time...but alcohol runs cooler. Running half a tank of e85 has passed vehicles with Nox issues many times.

High NOx is most commonly caused by a lean condition or bad cat. I would do a tune up first (plugs, wires, cap, rotor and fuel filter) if its due then re-check it, make sure the car is HOT this will help a marginal cat. E85 WILL cause your NOx reading to increase out of the tailpipe in most cases. The only time E85 will cause your NOx to decrease is if you have a lean condition from hot running engine or if you have a newer car and the long term fuel trim is too high, E85 may bring it down. On your '93 E85 will make your NOx go up.

Thanks for the replies.

This is my niece's Jeep so I don't know if she fully warmed it up before the test.

I did pull the plugs to check them and they are Bosch Platinum that looked a bit carbonized at the base that easily scraped off.

I've never used a Platinum plug before. The center electrode is covered up so I'm not sure how to evaluate how they are running.

I know my Renix doesn't like a non copper plug, what about these later models?

This 93 runs cool and has a new radiator.

How do you tell if the pistons are carbonized and causing the high NOx?

My 91 hated the NGK iridium plugs and then purred when I wen't back to the Champions. Hated to the point of so rough I didn't want to drive it.

my2monkeys said:

Thanks for the replies.

This is my niece's Jeep so I don't know if she fully warmed it up before the test.

I did pull the plugs to check them and they are Bosch Platinum that looked a bit carbonized at the base that easily scraped off.

I've never used a Platinum plug before. The center electrode is covered up so I'm not sure how to evaluate how they are running.

I know my Renix doesn't like a non copper plug, what about these later models?

This 93 runs cool and has a new radiator.

How do you tell if the pistons are carbonized and causing the high NOx?

Click to expand...

Unsure what you mean ?
if your combustion chamber looks like crap run seafoam through it....
but I think you need more info.
cleaning it does not fix the issue.....
you want to do a compression test if your worried the lean condition is being caused by low compression....


spark plugs. check the spark. it should be blue/white. not yellow

clydefrog said:

High NOx is most commonly caused by a lean condition or bad cat. I would do a tune up first (plugs, wires, cap, rotor and fuel filter) if its due then re-check it, make sure the car is HOT this will help a marginal cat. E85 WILL cause your NOx reading to increase out of the tailpipe in most cases. The only time E85 will cause your NOx to decrease is if you have a lean condition from hot running engine or if you have a newer car and the long term fuel trim is too high, E85 may bring it down. On your '93 E85 will make your NOx go up.

Click to expand...

Not to be argumentative, because your points are accurate and valid, but on my 88 E85 did lower Nox quite a bit

JohnX said:

Not to be argumentative, because your points are accurate and valid, but on my 88 E85 did lower Nox quite a bit

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Your correct, it can make it go down, its rare but it can. Sometimes if you have carbon build up, fuel can adhere to the carbon in the combustion chamber thus increasing the temp and creating a lean condition. E85 in this case, will in fact lower the the NOx as it burns cooler. This is super common on some of the newer GDI motors (Particularly Audi and VW) and also can happen to the older engines as well.

Thanks for the info, I like adding real world stuff like this to my knowledge base for future use.

isnt the e85 a really good cleaner too? could be cleaning the injectors and valves too.

I think if your having to run E85 to pass smog you got other issues....

You want your engine temp to be on the cool side of specification - but you want the
cat to be hot (it's the heat in the cat that rips apart the nitrogen molecules from the oxygen molecules and thus reduces NOx out of the tailpipe).

High engine temp - specifically, high combustion chamber temp, will raise the NOx levels. Your cooling system needs to be in good shape. Having your car tested on
a cool day helps a bit, too.

Does your engine "ping" while accelerating uphill?? Could be due to excess carbon
buildup in the combustion chambers and piston tops (carbon deposits increases the
compression ratio, and combustion temps). Every now & then, I put a tablespoon of
Seafoam into each cylinder, through each spark plug hole (warm engine; using a very long, thin, funnel which I made myself) and let sit overnight. Drive normally the next
day. Do not be tempted to add more and a tablespoon per cylinder as you'll risk
hydrolocking the engine (= real bad). Do this 3-4 times, every other day, for a week.
After a week (maybe, two weeks), the carbon will soften and the pinging will very slowly
go away.

Some folks have had luck pouring Seafoam into the engine while running. I have done
this sometimes but it always makes me nervous to pour in the stuff and listen to my
engine cough, stumble and sputter. I worry about what all that commotion does to the
rod bearings.

Dragonslayer said:

Some folks have had luck pouring Seafoam into the engine while running. I have done
this sometimes but it always makes me nervous to pour in the stuff and listen to my
engine cough, stumble and sputter. I worry about what all that commotion does to the
rod bearings.

Click to expand...

Pour slower down brake booster line and keep rpms up and it won't stutter. Burn most of the easy stuff off and pour some more down to make it stall and let it soak. Works wonders.