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

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.