2021 GMC Sierra 1500 turn signal issue

[pie_slice]

I'm helping my brother out with his 2021 GMC Sierra 1500, and his turn signals have some issues!
his left turn signal started blinking super fast. The weird thing is, the rear taillight blinks quickly, but the front one doesn't even light up or blink at all.

We already checked the bulbs in the rear and front, they looked totally fine.

We even swapped the good right taillight into the left harness, and it still blinked rapidly.
Same deal with the front bulbs, he swapped them from the good side to the bad side, and no change, still blinking fast.

We even tried swapping out the turn signal relay with another one, and nope, it didn't fix it.

Anyone got any ideas about what we should check next?

 

[KrtSkrrrt]

Based on the troubleshooting you've already done, it sounds like the problem isn't with the bulbs themselves or the flasher relay.
Since the bulbs and relay are ruled out, I'd focus on checking the wiring harness.
It can be a bad ground or a wiring issue between the front bulb and the turn signal stalk.
Look for any signs of corrosion or a broken wire, especially around the front turn signal housing and where the harness connects to the main vehicle wiring.
It's also possible there's an issue with the turn signal switch itself, but I'd start with a thorough inspection of the wiring first, as that's a more common point of failure!

 

[tiger.star]

Check the wiring and connectors on that left side especially the grounds. A bad ground or corroded connector can kill the front signal and make the rear blink fast then maybe look for broken wires near the socket or in the harness where it flexes.

 

[Stryker]

Isn't it ridiculous that with all the tech in a 2021 Sierra, a simple turn signal can still act up like this, proving some GM electrical systems are more confusing than helpful?

 

[Rubbinrobbin]

Not sure if it's going to help, but here's a short story about a problem a friend of mine had. It was a 90s corolla, way less electronics, but it had the same kind of a problem. The bulbs were fine, but the front turn signal didn't flash, while the rear and side ones looks like they're on 10 espressos. It turned out that the connector on the front turn signal was rusty and after a quick clean, it returned to normal. I know yor car is 30 years newer, but sometimes the simplest explanations are the ones that turn out ot be true, so might want to check that out.