Do they report differently when the battery dies? I had it go off last night, checked pressure and everything was within 2lbs of each other. Reinflated all to 36psi (after driving so tires were still warm). Drove 15mi to work and warning light is still on. Checked tire pressure and they are all still w/i 2lbs of each other, maybe 3.
It's possible I have a small leak, nail or screw or something. But I want to rule out a bad sensor and I'm not sure how offhand. EG., I think my wife's Subie has a different warning if a sensor is bad vs. actual low tire pressure.