left rear brake turn signal interior lamp flashing?

2017 GMC SIERRA
123,400 MILES • 6.6L • V8 • TURBO • 4WD • AUTOMATIC
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2017GMCDENALI2500HD
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  • 3 POSTS
2017 sierra denali 2500hd diesel clignotant arrière gauche et lumière a frein de fonctionne pas ainsi que les lumière intérieur en ouvrant les porte et si je pèse sur le frein et que j'active les clignotant gauche les lumière intérieur suis les clignotement du flasher jai 0 code les relets et les fusible sont toute bon
Dec 30, 2025 at 4:17 PM

What to check next

Diagnostic guidance. Suggested checks and possible repairs are separate from the owner's reported results.

Have the affected lamp connection, circuit supplies and grounds tested while reproducing the interaction between the brake, signal and interior lamps. Confirm the actual lamp equipment before ordering an assembly or module.

Guidance references and applicability

2CarPros: Original discussion 1711739 https://2carpros.com/questions/left-rear-brake-turn-signal-interior-lamp-flashing-49393188 Source for reported history and outcome. Contributor suggestions are not independently confirmed diagnoses. Attachments were not verified. Fluke: Diagnosing voltage drops https://www.fluke.com/en/learn/blog/automotive/electrical-automotive-troubleshooting General support for checking supplies and grounds under load. Follow the matching wiring diagram and test limits; this does not identify a failed component.

Read the replies · Discussion summary and reported outcome

AI-ASSISTED GUIDANCE · REVIEWED BY 2CARPROS
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STEVE W.
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  • 15,399 POSTS
The description sounds like either a bad ground at the left tail light module or a bad module. Being a Denali it should have the LED taillights that have a logic circuit in them which can short if water gets into the connector and corrodes. I would pull the tail light out and look at the connection. Does the reverse lamp work on that side? If yes the problem isn't as likely to be the ground as it uses the same ground. To remove the lamp, remove the two screws and pull straight back on the lamp, they can take a bit of force to remove. Then unplug the harness connector. Now see if the issue has gone away, if it has it's most likely another module failure. Just be setting down when you price the replacement module.
Dec 31, 2025 at 4:27 AM
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2017GMCDENALI2500HD
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Yes, the reversing lights and park lights work; only the brake lights and turn signals don't work, and the dome lights flash at the same time as the turn signals.
Dec 31, 2025 at 10:49 AM
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STEVE W.
  • ✓Experienced Mechanic
  • 15,399 POSTS
Yeah that sounds like the wiring isn't the issue but the light module itself, not uncommon unfortunately. Not sure who thought it was a good idea to put control chips in the tail lights.
Jan 1, 2026 at 5:04 AM
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Discussion summary

left rear brake turn signal interior lamp flashing?

The owner reported that the left-rear brake and turn-signal functions did not work and the interior lamps flashed with the left signal when the brake was pressed. Reverse and parking lamps still operated. The owner reported no codes and described fuses and relays as good.

Reported outcome: No module fault or successful repair was confirmed.

Working reverse and parking lamps did not provide a documented loaded test of every affected circuit. The contributor's assumption about LED equipment was not independently verified.

Read the original discussion

AI-ASSISTED SUMMARY · REVIEWED BY 2CARPROS
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