Why is the tire pressure monitoring light on when the pressure is 2.5?

Spare tire

The spare tire pressure is low... I've been caught by this before.

If the tire pressure is around 2.5, it shouldn't normally trigger an alarm. I suggest checking the tire pressure monitoring system to see if there's any issue.

Generally, a car's tire pressure will only trigger an alert if it falls below 2.0. So, it seems like there might be an issue with your tire pressure sensor, and it needs to be checked.