no tail light but brake and blinker work

04 Avalanche, 4WD ,
Noticed the other day my left tail light was not functioning. Changed the bulbs. Still no tail lights, however the brake and blinker do work back there. Fuse under the hood is good. Any suggestions? Could it be the sensor?

No sensor should keep the tail/running lamps from functioning. Since the brake and turn lamps operate, there is at least a ground at the harness at the left rear lamp housing.

I would try a second bulb, or verify that your bulb works in another location.