Anti Lock shows up on 99 Galant

[99_Galant]

Before I had a flat tire the Anti-Lock wouldn't appear on my dash kit :). Yesterday, I had a flat tire in my rear driver side. Today, I removed the flat tire and I install the spare tire. However, when I turn on my car, the Anti (orange) Lock would appear. Everything seems to be fine. Could someone explain to me why my Anti Lock keeps showing? thanks

[RedGalant2k1]

It will come on from either a fault in the system or a measurement of RPM difference from side to side of the car, especially so when a spare is on the car.