I released the air on each tire to see when the light would come on. It came on at 25psi. I did it one at a time to see if all would come on at 25psi and they did. My 8 gallon air compressor made the test go a little faster. I didn't think to do the spare to see if the light would come on though. HMMMMM......Think I'll go do it now....I'll post once I get done.