How much air pressure in 03 Liberty?

Tire pressure should be within 5 psi of the tires recommended max pressure but never higher. All tires have there max pressure stamped on the sidewall of the tire. Never go by the vehicles recommendation for tire pressure, always go by the tire. Some tires have different pressure recommendations than others. 35psi is most common- but some car tires rec. 44psi or more.

What you are seeing is a low tire pressure light and it will illuminate when the air pressure in any of the tires is above or below the factory recommended pressure. It can also illuminate when there is a sensor that has gone bad or is defective, this usually happens after you have had new tires installed, if the technician damaged one. I would recommend that you first look in the drivers side door jam of the vehicle to see what the recommended tire pressure is and then check all of your tires to see if the pressure incorrect, and if so then fill or let air out of the tire to the recommended psi.After you have done this, drive the vehicle and the light will automatically turn off. If this does not work, then I would recommend taking the vehicle to Toyota and they will be able to pin point which TPS (tire pressure sensor) is not reading correctly and you can then have it replaced. I hope that you have found this information useful as that is my goal.

As with Most Vehicles this is an Indication of 1 or more tires needing attended to. Generally speaking it is warning of a Low Tire Pressure.
Check (or have Checked) all the Tires for the specified Tire pressure and inflate to "Recommended" PSI as found on the Tires, (raised type lettering on Tire) closest to Rim/ Wheel.

If it is Low Pressure, with Economics as they are, have tires inflated to Maximum load can increase gas mileage

Low tire pressure warning. Fill your tires with proper air pressure.

That is the tire pressure sensor light. It will come on if one or more of the tires are over-inflated or under-inflated. It will also come on if there is a signal problem with one or more of the sensors.

this indicator is telling u that u have low tire pressure, each wheel has a tire pressure monitor including the spare, air up the tires and the spare to the spec on the driver door frame or door itself. if u want an owners manual fee download go to the link below.
http://www.jeep.com/en/owners/manuals/index.html

Hello partner. The difference in air pressure variations from a 16',17' or 20" tires is a non-factor. Any size tire will go up about 1psi per 10 degrees ambient temperature. The recommended pressure for an 03 Grand Cherokee is 33psi. So if it's a rainy 55 degrees in the great northwest when you set your pressures the temperature on your trip to Death Valley would have to climb to about 175 degrees. On a different note maybe your alarm just isn't calibrated correctly.

Tire pressure always changes...
when you drive your vehicle for a long period of time the tires' temperature goes up along with the
pressure in your tires, the molecules in the air inside your tires get excited (hot)
when your vehicle is immobile for a while your tires' temperature goes down along with the
pressure in your tires, the molecules in the air inside your tires get compacted (cold)
This is the best time to get a true PSI reading from your tires....
the recommended tire pressure on most tires should be set at 35PSI
But there might be a problem with your sensors, I work with TOYOTA their sensors are set up to
where when the PSI on your tires drops below the 35PSI, the tire pressure light would then
come on.
You should also check if your spare tire comes equipped with pressure sensors, which in most cases
they do, depending on the type of tire the PSI on spare tires will usually range from 35PSI- 80PSI
you can check your owners manual for this, or simply read the recommended PSI written on the
tire.

Is the spare tire at least 31 lbs of air in it?

You can find the recommended pressure molded into the sidewall of the tire.