Is the TPMS warning light always an indicator of low tire pressure?

Yes, if functioning correctly, the tire pressure warning light will always serve as an indicator that the pressure in one or more tires is at least 25% below factory recommended specifications. 

If the warning light flashes before it is continuously on and the pressure is out to be within the appropriate range, contact your dealer or local service bay for a tire pressure monitoring system inspection.

If the TPMS warning light is on and remains solid and the pressure in each tire is within the appropriate range, it could be possible that one or more of your tire pressure sensors needs to be replaced.