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.