I am going to flat out guess that you have either a low tire, the obvious first thing to check, or, given the age of the vehicle and probably the TPMS on the tires being original, the battery in one of them is dead.

Or the control module cannot communicate with one or more of the sensors.

​​​​​​A full featured scanner should be able to tell you in what way the system is not functioning properly. Might simply need to be reinitialized or something like a battery is dead.

 

The display was sensing all four pressures fine for the last few days I've had the car, now it show three of the pressures with the fourth just showing ----. I'm going to replace the sensor and see if that fixes the problem.

 

So I just replaced the sensor, reinflated the tire and remounted it. Calibrated it with the car, and everything is great.