I just took the car (95 z28) to get tested and it pretty much failed across the board. I had the cats replaced last year right before I got tested and it passed then with no problems. I'm confident I can fix it, Im just not sure where to start. Here are the numbers:

First number is the reading, second is the max allowed

25/25 Test

HC ppm 187 - 114 max
CO% .69 - .63 max
MOx ppm 894 -796 max
RPM 1871 - 2500 (pass?)
CO+CO2% 14.73 - 6.00 min

50/15 Test

HC ppm 127 - 117 max
CO% .23 - .65 max (only thing that passed)
NOx ppm 1073 - 879 max
RPM 1806 - 2500 max
CO+CO2% 14.6 - 6.0 min

Like I said, it passed with no problem last year after new cats (failed the first time then, too). Where should I start?

Also, no check engine light ever.

Thanks guys!

 

The fact that you are running high on all three (CO, HC, NOx) would point to a problem with the cat. Running rich will usually cause high CO and HC, and low NOx. Running lean will generally cause high NOx and low CO and HC.

You mentioned you had the cats replaced. Do you have a CA emissions 95 A4, because that was the only 95 that had dual cats. What replacement did you use?

There is no diagnostic in OBD-I for cat performance, so you wouldn't get a code/SES for a faulty cat.

 

The S in cats was a mistype. My mistake. I bought the car last October and it failed emissions right after I bought it. The original dealer took the car to the local Chevy dealer who put a replacement stock cat on it. After the replacement, the car passed easily. Now, barely a year later, its failing again.

 

Also, the car does have longtubes. Thought I should mention that.

 

Long tubes probably put the cat quite a ways back from the engine. You have to make sure you get it good and hot before they run the emissions check.

 

Good idea. I drove about 5 miles to the bank, left the car off for about 10 minutes, then drove straight to the emissions place which was only about another mile or so. I know it happens, but I'll be pretty ticked if that new cat didn't last more than a year. Sigh, the joys of owning a 15 year old car..

 

Does it help narrow it down that the HC and CO goes down on the second test, but NOx goes up slightly?

Also, would the cat that was replaced last year have any sort of warranty since the work was done by Chevy (if it is indeed bad already)? I'm having trouble finding a good answer to that one.

 

Ask the dealer what the warranty is.

It may have heaned out slightly on the second test, which would account for CO and HC going down slightly and NOx going up slightly. But will all three readings still high, I'd suspect the cat. Give the dealer the data and ask him to stnd behind the install.