High NOx fails emissions but no codes - 1997 850 R

Post by **matthew1** » 14 Dec 2025, 15:10

As I understand it, high NOx is usually vacuum leak. As I know from firsthand experience, vacuum leaks generally throw codes. I've only put 40-50 miles on this car, so not a lot of engine time. But previous owner gmh just 4 weeks ago took it in and it failed.

What am I missing?

Post by **scot850** » 14 Dec 2025, 16:18

When were to O2 sensors last changed? Like you say, the only other possibility would be a vacuum leak somewhere, so best way to tackle is a cheap smoke tester.

Neil.

Post by **erikv11** » 14 Dec 2025, 16:57

hopefully not the cat! viewtopic.php?t=21105

In other words I don't know, would check fuel trims and consider a front O2. But that was a good read!

Post by **matthew1** » 14 Dec 2025, 17:26

Thanks guys. The lack of codes is odd. I've had both O2 sensors go bad and vacuum leaks on essentially the same car, and both threw codes and CEL. I'll loop back with gmh and ask if he's tried Seafoam, spark plugs, premium fuel, MAF cleaning, or fuel filter.

Post by **abscate** » 14 Dec 2025, 18:06

Slighty elevated NOx.

Try a real hard hot run to make sure cat is efficient

Post by **volvolugnut** » 14 Dec 2025, 18:07

Only one thing to do - move to Oklahoma where there are no emissions tests!
volvolugnutr

Post by **erikv11** » 15 Dec 2025, 12:54

Iowa has no emissions tests.

Matt wouldn't even need to move - I'd get the car the here!

Post by **matthew1** » 15 Dec 2025, 15:38

Most areas of Colorado do not have emissions testing. But not the area I'm in.

Fuel system cleaner and replacing fuel filter are the only items on my list above that were not tried. I did a look-see today and the vac tree looks good, unused ports capped. Of the three vac elbows I could see, they all look and feel good, like they'd been replaced in the last 15 or fewer years. Sometime in the past an owner had replaced a few pieces of the system with silicone line, including the infamous P0172 elbow, much like I did with my 850 T-5 in 2012. viewtopic.php?p=233700#p233700

I forgot to ask him about O2 sensors so waiting for that answer. Thank goodness he was a member here/actively knew about and practiced maintenance. If this was a "mystery car" this would all be more difficult.

454cid · Post by **454cid** » 15 Dec 2025, 17:23

Has the car been run enough to finish all it's tests that would even trigger a light? I have a bad SAS system on my 96, and when I reset the codes it takes awhile for the car to turn the light back on.

Post by **matthew1** » 26 Dec 2025, 16:40

454: I've run it a good bit, but the problem is that it failed emissions 3x for the previous owner.

Could it be a bad gas cap?

The thing that kills me about this is that so many or absolutely all the things that could be causing it throw codes, and there are no codes stored.

Note: I have not had it re-tested yet. I've done fuel treatment and am now working on getting O2 sensors to swap those. The ones in it are swimming in PB Blaster as I type.

I have a fear that yes, it is going to end up being the catalytic converter. Colorado is unluckily a CARB state, so this scenario will likely be a bridge too far due to OEM cat prices.

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