You can't just look at the light.
Need to pull all sockets out and look at the bulbs when on.
Iffy sockets or ground(s) can make the bulbs to light but not the right way and thermal flasher won't have enough load etc.
See my Cave, Sneak Path
Dr. Ian Malcolm: Yeah, but your scientists were so preoccupied with whether or not they could, they didn't stop to think if they should.
The Ogre's Fiero Cave