What causes this light to come on? Ever since I had new tires put on, I've gotten this light. The pressures are good, so I haven't worried about it for a while, but now I've heard that there are wheel speed sensors in the hubs /axles that are also important for the ABS. My question is where exactly these sensors are, do they transmit to the ABS and low tire pressures, and what type of reading should I get if I ohm them out with a meter? Oh, and how much are these baby's?

Some Tire Pressure Monitoring Systems (TPMS) are very sensitive. Specialized torque drivers are even supposed to be used to tighten valve stems and sensors properly. Discrepancies can trip the light to come on.

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As part of the Antilock Brake System, each wheel has its own notched iron ring and sensor. If you look behind each disc brake (or rear brake drum), you will see them. Wheel rotation speeds are sent to the computer for processing. The computer's algorithm evaluates rotation speeds and adjusts each wheel's brake pressure, if necessary, during braking.

First, the Tire Pressure Monitoring System (TPM) is completely separate from the Anitlock Brake System (ABS).
Now, the problem you're having could be either the tire pressure monitors need to be "relearned" to the vehicle's Body Control Module (BCM),
-or-
One or more of the actual tire pressure monitors that mount into the wheel has gotten damaged during the tire change.