Having some problems passing the NOX portion of the emmisions test. This is the first time the car has failed emmisions since I bought it back in '04. Failing only the Nox part of the test. No changes have been made in the last year since it passed. I should also mention that the original cat was replaced about a year and half ago because the insides had fallen apart. I'll be honest , the replacement is of unknown brand or origin and looks really small compared to the original. The car passed with this Cat a year ago. Car is not showing an SES light. Bulb check is good. Results from the tests are below. All other testing parameters were well within specs for this test. NOX has been the only issue. Any opinions, help or suggestions would be appreciated. Notice that after the o2 sensor change that it passed the high speed test, barely, but failed the low speed test.

At this point I am suspecting the Cat. I simply don't have the test equipment to go any further.


Details- 94 lt1 / obd1 / m6 / SLP cat back exhaust . Lingenfelter intake. Jet 180 thermostat and fan switch. BBK headers and Y pipe. 150K. Stock computer tune.


Test 1

High speed results-FAIL-NOX- 914 out of 796/low speed 466..out of 879



Test 2- driven for an hour to get engine temps up.

High speed results-FAIL-NOX-1011 out of 796/low speed 518 out of 879



Test 3- New 02 sensors. As far as I know Old ones were OEM.

High speed results-PASS-NOX-795...........-796/ FAIL- low speed 983 out of 879

 

I agree, it is probably your cat. I ran into that same problem, but I still had my original cat. I failed with the aftermarket one a couple times, then cobbled the stock one back on and passed with 20x better HC, 10x better CO and 2x better NOX. When I was having problems, I called an emissions shop and the guy told me the aftermarket cats skimp on the expensive metals because they don't have to last 7 or how many ever years the stock ones do. I also learned from a former GM tech at work that if you need a replacement cat, go with the Brown or Walker brands - they are good quality OEM replacements.

 

NOx is controlled by your EGR system; might want to look at that. I'm running a Carsound Cat (very small) and I have no problem passing smog with my supercharged 95. Here's a link to Shoebox's site with EGR diagnostic suggestions: http://shbox.com/1/4th_gen_EGR.html

 

I live in houston and passed with flying colors. I have a 94 camaro with dual Random Technology 2 1/2 inch In/Out cats. I kept blowing out the 3 inch In/Out aftermarket cats.

 

This is what I am hearing about these "shop" brand converters. The one on the car now seems abnormally small for this car. The install leaves a lot to be desired . When they unbolted the OEM converter they tossed the bolt flanges and welded everything in with bits and pieces to adapt to the aftermarket exhaust. I should have paid more attention when they put it in. Really difficult to find a shop you can trust to do good work. Thanks for the tips on replacement brands.

 

Thanks.

I did check the EGR for function and clear internal passages. System functioning normally. EGR valve was replaced around 5 years ago. Not many miles on it. I'll put maybe 6000 miles a year on the car if I'm lucky.