Tire pressure monitor shows horizontal lines

I agree, but with just a dummy light and without an actual reading to compare to my gauge, I have no way of knowing if the sensors are reading wrong or if the truck is just alerting early.

 

i had heard that you get the warning in low 20's psi which fits your "significantly under-inflated" description above ...
but others are saying they had the warning at 28 PSI ... i need to do a test one of these days!

there maybe variations in TPMS sensors as they come into market or Toyota can change the spec for different years! maybe the sensors were not very accurate to begin with and now they are using better sensors and are displaying the real psi in newer models. to cut cost & to compensate for more expensive sensors, Toyota may drop another interior light ... lol

i assume it also depends on when the TPMS is reset!
remember that you have to "reset" your baseline pressure. basically have to press & hold the TPMS reset button after tire pressures have been adjusted (read the manual) and that can result in variation in tpms warning from person to person for example if u reset at 35 psi vs. 30 or 28 psi.

 

I'll have to look into that.