GMC Sierra 1500
3.0L inline-6 diesel, aluminum block, single turbo — the only I6 diesel in a US half-ton. LM2 (2020-2022): crankshaft reluctor wheel fails, engine cranks without starting. Repair requires cab-off, 40+ labor hours. LZ0 (2023+) got steel pistons, revised turbo, 305 hp / 495 lb-ft. Oil pump sits at rear of the block: transmission-out job for a $30 part. Highway 26-28 mpg empty, 12-14 mpg towing.
3.0 Duramax Sierra — diesel torque, Denali quiet
The diesel suits the Sierra's premium positioning: quiet, torquey, 27 mpg highway. Denali + diesel + highway miles is the ideal use case. Same LM2 avoidance rule applies — LZ0 (2023+) or nothing. DEF system will cost money eventually. For the long-haul luxury buyer.
Engine Weaknesses 3
Crankshaft reluctor wheel fails — engine cranks without starting. Repair requires cab removal (40h labor).
Symptoms: Engine cranks without starting, especially cold, intermittent
Seal leaks on early LM2 (2020-2022). Oil spots under the vehicle.
Symptoms: Oil spots under vehicle, dropping oil level
"Service DEF System" warning, limp mode, no-start in extreme cases. Injector, sensor or pump defective.
Symptoms: Service DEF warning, power reduction, limp mode
Vehicle Weaknesses 6
Corroded sub-frame ground strap causes simultaneous power steering and brake assist failures. Wiring harness chafing on ECM causes limp mode. Same platform issue as Silverado T1XX.
10L80 transmission develops harsh shifts and shudder. Multiple class actions filed. GM fluid flush (Mobil 1 LV ATF HP) helps temporarily. Severe cases require torque converter or complete transmission replacement under warranty extension.
Factory wax coating peels off frames within months of purchase. Visible surface rust on new trucks. GM issued service bulletin acknowledging the issue. Same problem across all T1XX platform trucks (Sierra, Silverado).
Shift to Park warning appears with transmission already in Park. Key gets stuck, battery drains overnight. Caused by worn shift cable bushing or electronic gear selector contact issue.
Sierra-exclusive MultiPro 6-way tailgate has actuator latch failures (micro-switch sends false signal), hinge binding, and step deployment issues. Load stop does not lock open. Exposed mechanisms vulnerable to dirt, ice, and salt. PIT5895A TSB for 2022 refresh disable function. Warranty covers most failures but repeat visits common.
Power running boards fail to deploy, retract immediately, or deploy randomly when parked. Motor failure on one side common from ~60k miles. GM service update: reprogram K4 Assist Step Control Module (RPO B3L/BRS). Physical motor failure: $190 OEM part, $850 dealer installed.