I failed a smog test in CA (1992 Corvette) and the results came back that the EGR valve is not holding pressure. It failed on nitric oxide emissions measured at 989 ppm at 15 mph and 1329 ppm at 25 mph.

The repair for that is cheap, but the mechanic is also saying my cats are bad, which are mondo expensive as you well know. But what I'm wondering is if the cats could perform adequately if the EGR valve was working correctly. The mechanic didn't mention anything about back pressure, but the temperatures measured in the two cats are: #1 520*F in front, 217*F in rear, and #2 532*F in front and 202*F in rear. I understand that these temps are not good, but is it possible they would come down if the EGR was doing its job?

I've had never-ending problems with this car and I'm just trying to get it up to code so I can sell it and be done with it, so I'm not concerned with the long term health of the vehicle. I'll be sure to let the next buyer know about the cats though. I'm not a crook, I'm just trying to cut my losses down to a minimum.

2 Answers 2


The EGR bleeds exhaust into the intake, that essentially reduces the oxygen level of the intake which reduces the peak temperature of combustion. The nitrogen oxides are formed at the peak combustion temperature : That fits up well with your failed EGR and high NO levels. The cat converters are not involved. A working EGR should fix your NOx.


Those temperatures are not bad. You might be shocked at how hot things get inside of the combustion chamber. (What with the fire and the combustion and all.) If your cats are achieving a 300*F drop, I'd say they're doing their job. (In as much as whatever part of their job is a temp drop. Which makes some sense, but is not something I've heard discussed before.)

Another point here is that they're performing nearly identically, based on those numbers. I find it very unlikely that both have gone bad at the same time, and to the same extent.

If you feel that you must, change the EGR valve, then get it re-inspected, preferably at a different facility. The $30 for the inspection - assuming you have to pay it again; in Texas you can get it re-inspected for free if you fail the first one - is going to be a lot cheaper than a couple of cats.

Personally, I'd have someone else look at it first. It sounds a bit like this guy was blowing smoke.

  • That's wonderful news. Are my NO levels completely absurd though? It seems like I failed by a lot. Max allowed is 541@15mph and 591@25mph here, and my car measured at double that. Is it normal that a bad EGR can cause those sky high numbers?
    – Brimby
    Aug 29, 2018 at 18:53
  • 1
    @Brimby I don't know, but in practice it's an irrelevant question. The only place where NOx is generated is during combustion, and if you have the wrong mix of gases in the cylinder because your EGR valve isn't working properly, the wrong mix of gases is going to come out. Cure the disease (EGR) and the symptoms (NOx) will go away - unless something else is also wrong, but you won't see the symptoms of the second disease until you cure the first one. For all you know, your cats might also be shot - but take one step at a time!
    – alephzero
    Aug 29, 2018 at 20:07

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