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Old 04-21-2012, 10:09 PM
Phoenix, AZ
Join Date: Mar 2008
Posts: 16
accuracy of AZ emissions test in question

1995 C220 been through emissions 7 times but big difference this time
AZ applicable standards: Hydrocarbons (HC) 0.80 Carbon Monoxide (CO) 12.00 Oxides of Nitrogen (NOX) 2.00
My problem is with the NOX: 2.63 is highest 2.01 2 times and we get to today crazy results:

HC 0.65 CO 6.45 NOX 2.02 failed nox again by .02
They were almost closed and I asked can you please just retest it again now
the guy said drive it around get another ticket and we'll retest.
Drove out of bay back to entrance and into bay again about 5min total time if that.
This time a different guy did the test he seemed to be the knowledgeable one.
He did the test different as on the slow test he got up to speed faster than anyone else
and on the high speed test he creaped up to speed taking longer heres the results:
HC: 0.96 fail CO: 17.96 Fail NOX: 1.60 Pass HC was never over 0.69 CO was never over 7.00
How could these reading be accurate at all. I dont know a lot about emissions but this
just dont seem right by any means. Any clarification would be welcomed and appreciated

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Old 04-22-2012, 09:45 AM
cth350's Avatar
Registered User
Join Date: Dec 2000
Location: Long Island, NY
Posts: 3,801
Part of it is engine dynamics. Hold a steady speed for a bit with steady air intake and fuel delivery, you get steady emissions (and numbers) from the tail pipe.

Real driving has very little steadiness to it and the car's reaction to pressing your foot on the gas, or going up/down a grade or just engine load from power steering & A/C changes.

In short, when you stomp on the gas, you get more unburned fuel out the tail pipe. Thus the higher HC number. Which in your case was too hight.

But, was that because he accelerated too quickly, meaning falling outside the parameters of the test? That I couldn't tell you because I don't know the test procedure in AZ.

Here in NY, the down-state (think NYC and surrounding heavily populated areas) emissions test has the tester work through a sequence of set speeds and has specific window of time to reach each one. The computer monitors the speed on a dyno while the tail pipe sniffer monitors the emissions. In the end a set of numbers appears for the max and min (and also a pass & fail). If the test falls outside of the necessary speed envelop during the test, the computer invalidates the test and has the mechanic start again.

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Old 04-22-2012, 02:51 PM
Registered User
Join Date: Jul 2011
Posts: 90
Wow. What complicated emissions testing you have.
Our UK W124, petrol powered, has just been emissions tested under the new and tightened up Euro regulations.
On the pre cat car the CO was 0.21% (max allowed 3.5%) and HC 127 ppm (max is 200 ppm, I think). Nox is not measured and the emissions test is only measured at natural idle and fast idle (approximating 70 mph in engine speed). Last year the CO was 18% at idle , but was tweaked down with a mixture adjustment.

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