lamp failure sensor

The lights out sensor for a '97 Avalon is in the trunk on the left wall (from the perspective of standing behind the car looking forward) between the trunk trim and sheet metal so you will have to remove the interior side trim to get at the sensor. You did not mention what year Avalon you have, but the sensor is likely to be in the same place. The sensor does not need to be reset when you change bulbs.

The two most likely reasons that the warning light on the dash is still illuminated is that there is another bulb burnt out or the incorrect replacement bulb was used.

Before going through the trouble of getting at the sensor, re-check both license plate bulbs, and all of the rear lights, including tail lights, brake lights, turn signal bulbs, backup light bulbs, and center high mounted stop lamp bulb.

the bulbs are "working" ... but if one (or more) of the bulbs is "on its last legs" (discolored), that can be the cause. I'd advise replacing ALL the bulbs (including the high mounted center brake light) ... relatively cheap fix to avoid future frustrations with the warning light!

I believe that the lamp warning sensor works by detecting an open circuit and is not capable of detecting whether a bulb is on its "last legs." The OP probably has another bulb burnt out or a problem with wiring.

I had that problem and it traced back to shorts in the wiring harness in the vicinity of the left trunk lid hinge. Once those shorts were corrected, the indicators shut off.

============= Same here, however, the wires were not broken or shorted, some of the strands were broken. The electronic sensor is grounded through the various bulb filaments and will detect differences in resistance short (no pun intended) of an entirely open or shorted circuit.