Failed Smog Test High NOX at 15 MPH but passes at 25 MPH?

1997 TOYOTA 4RUNNER
156,000 MILES • 6 CYL • 4WD • AUTOMATIC
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ROYBOY59
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I failed the smog test.

The NOX reads high at 15 MPH but passes at 25 MPH. No check engine light. I replaced the spark plugs and air filter before I took to Smog test.

I was told to clean the Mass Air Flow Sensor. I have never cleaned before.

Does this sound correct or are there other problems to look for?

Last year the check engine light came on but went out after two days.
Aug 23, 2010 at 9:22 AM
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RASMATAZ
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A problem with the EGR valve not opening will increase the Nox-oxides of nitrogen
Aug 23, 2010 at 9:27 AM
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ROYBOY59
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This Vehicle does not have an EGR valve.
Aug 23, 2010 at 9:55 AM
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RASMATAZ
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Knox is caused by high temperature in the cylinder vehicle does not have EGR that’s what it uses to cool the cylinders down the only other thing is lots of carbon buildup in the compression is higher than normal
Aug 23, 2010 at 4:21 PM