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JFN1, ASE Certified Technician
Category: GMC
Satisfied Customers: 5694
Experience:  GM Master Technician
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Signal lights stop blinking & goes solid when i apply brake.

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Second opinion] Hey my signal lights stop blinking & goes solid when i apply brake. Brake lights flash when signal lights flash
JA: Have you checked the brake fluid? How's the level and when was it last changed?
Customer: Will this cause signal light failure when brakes appllied?
JA: What is the model/year of your GM?
Customer: 04 yukon xl
JA: Are you fixing your Yukon XL yourself? What have you tried so far?
Customer: Checking fuses
JA: Anything else you want the mechanic to know before I connect you?
Customer: My hazards goes solid also when brake applied

This is typical of a poor ground to the lighting circuit. Just where the ground has failed is something you're going to have to work on, but when a ground circuit fails, the electricity will often find an alternative path, which is through another light bulb, and this causes that bulb to glow as the power is running through it.

I would start at the rear lights themselves. Run a redundant ground to the circuit and see if the condition suddenly goes away. If so, you'll know you need to correct the ground circuit issue.

Customer: replied 7 months ago.
Will i see the glowing bulb? & if so thats the bad ground?

You mentioned that the brake lights flash when the signals flash. That was my clue that you have a ground issue. You still need to find out where the faulty ground is, probably where it attaches to frame near the rear bumper underneath, but if you run a redundant ground to that circuit using a jumper wire, that will prove the theory and you'll know you're on the right track.

JFN1 and other GMC Specialists are ready to help you
Customer: replied 7 months ago.
Thanks its getn dark here so il continue tmrw.