I have a '99 Avalon with both the MIL and Trac Off light on. The MIL came on first with codes P0125 and P1133 pending. After some research I believe this was caused by a bad A/F sensor on B1/S1. I have not replaced the sensor yet. I checked the codes a second time before the Trac Off light ON and the additional codes were P1133, P1135, P1135 Pending. After about 200 - 300 miles the Trac Off light came on. I was not driving the car at the time, but from what I can tell it happened during/after a two times spinning the tires from a stop in the rain. I just scanned for codes and found no additional codes compared to the second scan above.