Can't pass CA smog - High NOx - '99 5VZ

I'm gonna take the truck into a STAR smog repair tech to get it diagnosed, but I'd like some guidance so I at least have the vocabulary to talk to the tech about it. I've failed 3 times, and replaced my catalytic converter at the advice of a mechanic (not a smog specialist) who claimed everything else checked out so it "had to be the cat". NOx reading was substantially worse after replacing the cat. So I'm out almost $1k and have nothing to show, but I'm sitting on a fix-it ticket for expired tags so it's gotta get handled somehow.

Truck is a 1999 Prerunner, 5VZ-FE. I've read these engines can have an EGR system but I can't visually see anything running from the exhaust manifold to the intake and my smog paperwork says EGR Not applicable. As far as I can tell, O2 sensors have never been replaced (I know that's probably the next step).

Numbers attached:

 

My advice? Move out of California!

I am a Navy brat who was born in NorCal, and I feel that it is my duty as a lifelong car enthusiast to hate that state based purely on the fact that they are such nazis about emissions testing! California is unkind to car guys (and girls)!

 

Some things to check, clean, or change will be the MAF, spark plugs, and vacuum leaks.

One question I have for you is, are you driving straight to the smog place from home or are you driving around town to get the cat as hot as possible before going in for a smog? Also, are you turning the truck off while you wait for your turn to smog or are you letting the truck idle while you wait?

 

I'll leave eventually when I'm looking to buy property, I don't think I'll ever be able to afford to own anything here.

But I have no beef with the emissions standards. There are 40 million people in this state, and 20 million of them live in the 5 big counties of SoCal (LA, OC, Riv, SBn, SD). If we deregulate emissions we're cooked. It's inconvenient and frustrating for someone with an older vehicle, and I admit it's unfair if you live in a rural county that's less populated, but Socal would be ruined if we could all dump whatever we wanted out of our tailpipes. I enjoy the quality of life I have here enough to deal with making my truck compliant.

 

1st test on 11/23/24 I drove straight from home, pretty much no warm-up, about 1.5 miles to the smog station. Shut the car off for about 20 minutes while I waited.

2nd test on 01/09/25 I drove about 40 miles (stopped at a different station along my commute). I think I let the truck idle while I waited, can't remember for certain.

3rd test on 02/27/25 I drove about 40 miles (same deal) the day after I got my cat replaced. Again, can't remember if I let it idle or not while I waited.

What's the optimal strategy in terms of running temp/cat temp? Should I get it nice and warm and make sure I'm idling before the test?

 

Because you don't have an EGR valve I would suggest cleaning your MAF and throttle body as there's a possibility you don't have a sufficient air fuel mixture leading to higher combustion chamber temps (cause of NOx).

 

NO or NOx is a direct result of high combustion temeratures, EGR is used to introduce a tiny bit of exhaust gas into the intake to reduce combustion temperatures. There is a work around... I am sending you a PM.

 

I had a similar problem. Ended up running a can of seafoam through the tank, drove it an hour on the highway to make sure it’s good and hot, and airing up my rear tires to ~50psi to reduce rolling resistance on the rollers. I let it idle while waiting for my test. The test tech told me to keep it rev’d around 2500 while I waited to make sure it was hot. Was able to pass on my second try.

Now I live in a cali county without a smog requirement because it’s too rural.

 

Greats tips, I appreciate the advice!

 

LMFOL!
so true

got a race friend that moved from MA to CA
he has a similar 98 Impreza with an engine swap like mine
went to this 'smog inspection' place(rookie's 1st trip)
inspector there busted out a picture book with every manufacturer's Stock Engine Bay!!!!!
(all years/makes/models)
of course did not even pass the 'eyeball' test

i couldnt believe it

 

My aunt and uncle have lived in Monterey for more than forty years. My uncle has two Volkswagen Buses, a 1971 and a 1977. The 1971 is exempt from emissions due to age, but the 1977 unfortunately is not. The 1977, a Westfalia camper, was originally equipped with a fuel injected motor and a catalytic converter.

Early fuel-injected VW motors are notoriously finicky, and the previous owned replaced the original motor with no catalytic converter. Most states wouldn’t give a shit. The smog station refused to touch based simply on the fact that the VIN says it was originally fuel-injected, and said it wouldn’t pass anyway because it doesn’t have cats.

The Westfalia runs fine and is hardly a smoke-belching piece of shit. My uncle simply registers it at his brother’s house in Texas!

 

Freakin mechanical fuel injection!!!!
The worst……ask me how I know

good for your uncle

Funny, that is exactly what my friend did to “by some time “
MA renewed plates/registration…..coming up on his 2yrs(?) thou
He is a “by the book” kinda guy

to be legal and pass smog…..
Owner can do engine swap, but every part of the engine HAS to be from the swapped engine. He bought a donor, had to swap fuel tank/charcoal canister system/filler tube/gas cap/some EGR crap in engine bay among other things
Lucky, all the above parts bolt in to the Subaru.
Has been chasing an evap code for months
He is 100% more technical than me when comes to these things…..that’s why I call him all the time when I have a Subie problem
Smoke tested every single inch of the swapped system, in sections even

I just went to his wedding in May
After 4months of chasing…..
He said threw in the towel, has the GC8 at a specialist now

so much work to have/own/drive an old car
In CA.

 

If you can contribute to op’s issue besides telling op to move out of CA, don’t derail the thread with nonsense. Simply move on and create your own “why you don’t live in CA” thread.

OP, vacuum leak might be the culprit. As stated, get the cat hot by driving on the highway for a length of time and let the truck idle while you wait at the smog place. A cat is designed to function when hot.