Nobody Could Figure Out Why A GMC Kept Dying On the Highway… Until This Mechanic Took A Serious Deep-Dive Into The Diagnostics

Image Credit: Dave’s Auto Center / YouTube. An Arizona family’s GMC diesel truck kept shutting down without warning, leaving the owner’s daughter stranded on the highway. Even after two breakdowns and multiple visits to repair shops, nobody could explain what was causing the engine to die.

Several repair shops had examined the vehicle, but their solutions never addressed the underlying cause. Replacing a blown fuse got the engine running again, only for the problem to return without warning.

The owner, Michael, eventually sent the truck to Dave’s Auto Center in Utah, hoping for a definitive diagnosis. The intermittent nature of the fault meant the mechanics couldn’t simply rely on a warning light or a stored trouble code.

As documented on the Dave’s Auto Center YouTube channel, the investigation fell to a newly hired technician known as DQ. His persistence would uncover a problem hidden deep within the truck’s wiring, along with an unexpected connection to an earlier repair.

The GMC’s 2.8-liter Duramax diesel had approximately 134,000 miles on its odometer. Its 7.5-amp F9 fuse supplied the engine control module’s odd-cylinder injector-driver circuit, which proved central to the repeated shutdowns.

When the fuse blew, the engine lost power and became undrivable. Previous shops had replaced the fuse, while one reportedly suggested carrying spare fuses rather than identifying the fault.

DQ began by studying wiring diagrams and checking the injectors for abnormal resistance. All four tested within specification, leaving no obvious evidence of a defective injector.

He then monitored current draw with an amp clamp while driving the truck under load, including on steep mountain roads. The circuit never exceeded approximately 4.5 amps, well below the fuse’s 7.5-amp rating, suggesting the failure wasn’t caused by sustained excessive current draw.

Back at the workshop, DQ began twisting and moving sections of the engine wiring harness while monitoring the circuit. Suddenly, the signal disappeared and the fuse blew, reproducing the intermittent failure that had eluded several repair shops.

Further inspection revealed wiring insulation worn through where the harness contacted an aluminum air-conditioning line. Two damaged wires were exposed, including the circuit responsible for the injector fuse.

The second damaged wire carried an engine cooling-fan signal, potentially explaining an earlier fan-clutch replacement and a repeat repair under warranty just 10 miles later.

DQ repaired the damaged wiring, reinforced the harness, and added protection against further rubbing. Crucially, he accessed the problem through the wheel well, avoiding the expensive cab removal initially considered necessary.

A subsequent test drive and wiring checks showed no recurrence of the fault. After multiple unsuccessful repair attempts, Michael finally had a diagnosis backed by physical evidence rather than another replacement fuse.

Andre has worked as a writer and editor for multiple car and motorcycle publications over the last decade, but he has reverted to freelancing these days. He has accumulated a ton of seat time during his ridiculous road trips in highly unsuitable vehicles, and he’s built magazine-featured cars. He prefers it when his bikes and cars are fast and loud, but if he had to pick one, he’d go with loud.

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