With a fullsize tire as a spare, the TPMS ( tire pressure monitoring system) will still illuminate the "tire pressure lo" light.
The liberty "donut" spare tire is not TPMS monitored.
If you see this TPMS icon illuminated, it means your vehicle has a low tire. STOP, and check the inflation pressure in your tires. This warning light will come on if tire pressure in any of your tires falls more than 25% below the recommended inflation pressure (refer to your owners manual, or the tire inflation decal in the glovebox, driver door pillar or gas tank filler cap lid for tire inflation pressure recommendations).
The TPMS warning light should come on for about one second when the ignition key is first turned on for a bulb check. It should then go out if all of the tires are properly inflated. If it does not go out, or it comes on while driving, you have a low tire.
NOTE: If the TPMS warning light is flashing, it indicates a fault has been detected in the TPMS system that will require further diagnosis. Common faults include a bad tire pressure sensor, a defective keyless entry receiver module, a fault in the TPMS or body control module (BCM), or wiring problems in the keyless entry/TPMS/BCM circuit.
TIRE PRESSURE MONITOR SYSTEMS
This brings us to tire inflation monitoring systems. If motorists won't check their tires, vehicles can be equipped with their own tire pressure monitoring systems (TPMS). For model year 2008 and beyond, National Highway Traffic Safety Administration (NHTSA) regulations require all new cars and light trucks to be equipped with a tire pressure monitoring system. The TPMS system must warn the driver if the pressure inside any tire on the vehicle is 25 percent or more under the recommended inflation pressure.
Many late model cars and SUVs are factory-equipped with TPMS. Run-flat tires with a TPMS system have been used since 1989 on the Chevrolet Corvette. Other older run-flat/TPMS applications include the Plymouth Prowler, BMW Z8, Mercedes-Benz SL55 AMG, 2002 Infinity Q45 and 2004 all-wheel drive Toyota Sienna.
TPMS can also be found on the following vehicles that have conventional tires: 2003 Ford Explorer, Mountaineer, and Lincoln Navigator, 2000 Cadillac Seville & DeVille, 2003 Chrysler 300M, Town & Country minivan and Dodge Viper, and Jeep Grand Cherokee and Liberty. Import applications include 2001 and up Audi A8 and Quattro, 2001 and up BMW M3, 2003 and up BMW 3-Series, &-series and Z4 roadster, 2000 and up Nissan Quest minivan, 2003 and up Nissan 350Z and Murano SUV, and 2004 Toyota Sienna. For model year 2008 and newer vehicles, all passenger cars and light trucks that are sold in the U.S. must be equipped with TPMS.
DIRECT & INDIRECT TIRE PRESSURE MONITORS
There are essentially two basic ways to monitor tire pressure electronically. One is the direct method. A small pressure sensor is located inside each wheel. The sensor has a built-in transponder that broadcasts a radio signal to an external module. The module identifies the signal from each wheel and keeps an eye on pressure. If pressure drops below a predetermined threshold, the module turns on a light or displays a message to warn the driver.