According to the Factory Service Manual, the TPM is triggered at 26 psi or below...however, there is a note regarding changes (be they higher or lower?) of more than 1 psi between revolutions will also trigger a TPM error....hmmmmmm...might be that a sudden change of differential between readings could cause the light to be triggered, it is got both an absolute (nnn indicates low pressure AKA 26 psi) and a relative (differential between last reading and current) threshold for trigger...just a guess...ct