How to troubleshoot flat tire light on but no tire is flat on a 1998 Toyota Sienna?

Assuming you reset the system you may have a bad ABS sensor. Try unplugging each one individually and see if the light goes out.

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I recently had my 1998 Sienna serviced and this "flat tire" light/sensor came on several minutes after I drove away from the service garage. I checked all 4 tires and the pressures were correct. The service personnel manager thinks it may be because the car was slightly off-kilter on the hoist. Apparently this light comes on after detection of uneven height of the car; hence if not level on the hoist, the light will come on. The 1998 Toyota Sienna car manual does not mention that there is a "Reset button" located on the front dash at the level of the driver's left knee. To Reset, push in this button: the "flat tire" light will flash, then after a few more moments the light will go out.