Mass air flow sensor-How do you tell if it's bad-2001 le pathfinder 3.5 litr code 172-171

Unless you have a scanner that'll do data or have a tester mass airflow meter you can't tell. Is it running badly??

No it runs tip top

mass airflow failure should make it run rough unless it's totally failed. start it and once it's warm unplug the mass airflow while it's running and see if it stalls or stumbles. that'll tell you if it's running without it or not. if it's failed it won't change when you unplug it cause it'll already be running without it. if it's good it'll stall or stumble when u unplug it.