Lexus GS Undervoltage Code

Hmm, electrical gremlins are the worst! You say the alternator 'seems' fine, but have you actually put a meter on it to see what it's putting out under load? And what about the battery itself? How old is it? Could just be a tired battery struggling to hold a charge after all these years.

Good shout. I haven't load-tested the alternator, just checked voltage at rest. Battery is maybe 3 years old. I know it's possible it's the battery, but could a bad battery really cause all these other system failures? Seems weird that a low battery would cause flickering lights rather than no lights... Is that logical? Thinking more about a possible voltage drop...

Yeah, a bad battery can definitely cause all sorts of weirdness. Modern cars rely on stable voltage, and if the battery is failing, it can cause all sorts of seemingly unrelated issues. While it could be something else, the symptoms really point to a weak battery. At 3 years old, it's definitely in the range where it could be failing. Best to get it properly tested, or better yet, bring it to a workshop.

Appreciate the insight! Ended up being the battery – heavily discharged, couldn't hold a proper charge. Replaced it for 290€ and all the problems are gone. Crazy how much havoc a simple battery can wreak!