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Similar problem on our 2018 3701lk w 2015 GMC 3500hd. First the brake wiring was melted so got that fixed but problem persisted. It became finger pointing between GMC & trailer repair shop. Finally ended up at an electronic specialist shop who spent 3 days trying to track it down on the truck. The trailer brakes go first thru a trailer brake module and then a chassis module. They finally replaced the trailer brake module and that fixed the problem.
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