Sorry to disappoint you but...
Nitrogen Tire Inflation: Nitrogen Tire Inflation vs. Temperature Change

My light came on once. I added a little air to all 4 tires since none seemed to read below 35 psi. The light went off after I got on the road a short distance.

yep nitrogen is over rated...they work better than regular air for tracked cars as temperatures can go up quite a bit, but for street cars, just use air. keep adding about 2psi extra until you're at winter temp.

As previously indicated you have to drive a short distance to reset the light, usually less than a mile. The light will come on at 29 psi from experience.

...and at about 39 or 40 if overinflated

really? cause even 35psi cold can easily go over 40psi in summer temps... :confused:

Yes around 40 will trigger for over inflation.

You set the 35 on cold tires not the coldest night :) Should only go up 2 or 3 psi on normal driving. For track you have to start below 30 in order to stay below 40.

that's werd. im at 39psi at 50F (garage) where it would drop about 1.3psi when i pull the car out in 35-40F. while driving moderately quick, i dont get the light. psi is over 40.

How do you know it's over 40? It's almost impossible to generate tire heat in 35-40 degree weather.

Yeah, it's true. Just happened the other day coming back from the dealership. All 4 tires were at 40 psi after a brief time on the road.

My Mazda will go off at 40 too, but the rec. pressure on it is 32 all around.

Cause i measured it. :icon17: the drive started out at those temps in the morning, by the time i got back ambient was 55F. Tire was set 39psi in my 50F garage. Also i went out for a drive today at 60F 41.7psi when i got back. No light. :p

Im pretty sure one would need to be considerably higher psi than 40psi to get the light.

Huh, looks like someone pulled out my tpms back light bulb. :icon14: :D

Your sensors are broken :p