The light has been coming on for the last couple of weeks. I can reset the light by pushing the dash button. On a 700 mile trip last weekend, I probably reset it half a dozen times. The tires are all at 30 PSIg. Two tire gauges agree. I don't know how the system works. I don't think there is a sensor in each wheel like some later cars have. I suspect it works off of ABS sensors and wheel revolutions. The ABS light is not on, nor has it been.
The right rear tire has been replaced with one of the same size, but a different brand. This issue started a month after that. Can that tire have a different revolution length per rotation and cause this issue?