2020-2023 GMC Duramax L5P Parts & Upgrades

What the 2020 redesign changed on the Sierra HD

The 2020 Sierra HD is a new truck on GM's T1XX heavy-duty platform: 10.4 inches longer and 1.4 inches wider than the 2019, a heavier boxed ladder frame, a 28-inch cooling fan, a prop shaft 30 percent larger in diameter, an 11.5-inch rear ring gear on the 2500HD and a 12.0-inch on the 3500HD, a 2.4-quart transfer case, a frame-rail cartridge fuel filter with a water-in-fuel sensor and drain, and a relocated DEF fill. The engine did not change. It is still the Denso-era 6.6L: common-rail injection to 29,000 psi, a three-plunger HP4 high-pressure pump, G4S solenoid injectors capable of seven events per cycle, and one BorgWarner variable-geometry turbo that doubles as the exhaust brake. What changed was the Allison 1000 six-speed becoming the GM-built 10L1000 ten-speed, which runs DEXRON ULV and shares no parts with the older unit, plus a 52 percent jump in maximum trailer weight. Shop accordingly: engine long-block parts usually cross back to 2017, while transmission, frame, airbox, belt and exhaust parts belong to this body only. The wider Duramax catalog holds the parts that span generations.

Where 2020-2023 Sierra HDs give trouble

Owner complaints on this body cluster in the transmission and in sensors. In the NHTSA complaint data for these trucks, transmission ranks first and the electrical system second.

• Torque-converter clutch shudder in the ten-speed, felt as a rumble at light throttle and steady speed, has been reported from low mileage. Valve-body pressure-regulator and feed-limit valves also wear their aluminum bores, which shows up as delayed engagement, a harsh second-to-third upshift or a flare, some of it under 40,000 miles.
• Twisted or kinked factory transmission cooler lines are documented on these HD trucks and cause above-normal fluid temperature, harsh shifts, surge and shudder; a thermal bypass valve stuck closed will overheat the fluid on a modest grade. The thin stamped pan is also reported to deform with heat cycles and seep.
• The intake-tract MAP sensor fouls with soot from EGR flow and skews its readings, giving rough idle, hesitation, low power and limp mode.
• Number four injector connector failures mimic an injector fault, bringing P0204, P020D, P0300, limp mode and ABS or StabiliTrak lights. Owner reports put them anywhere from roughly 30,000 to 144,000 miles.
• DEF and NOx sensor and DEF-heater faults are frequent and usually sensor-level rather than a failed catalyst, but they still start a derate countdown, so do not leave one sitting.
• Variable-geometry turbo complaints, meaning lazy spool and boost that does not track the command, are often intake restriction or a charge-air leak instead. Check both before condemning a turbo, and clean the cooling stack, because a face full of bugs produces the same soft-under-load feel.
• Recall campaigns on this body are worth a VIN check: hood-latch striker wires on 2020 trucks, brake pressure modulator valve bolts on 2021-2022 2500 and 3500 models, a spare-tire and ABS issue on 2020-2021 2500s, and a power tailgate release switch that can short from water intrusion on 2020-2024 trucks.

Service parts vs. real upgrades

A Sierra HD that tows for a living does not need more power; it needs cooling and clean fuel. Keep the filter schedule, exercise the water-in-fuel drain, use DEXRON ULV only, and shorten fluid and filter to the 30,000 to 45,000-mile range if you tow heavy. Replace thermostats and the upper and lower hoses with the first coolant service. If the truck already wears a lift and larger tires, regearing to restore the factory effective ratio is the recommended answer to converter and clutch stress, and pitman and idler arm support brackets stop the linkage wander that extra leverage creates. From there the upgrades that earn their keep are a deep aluminum pan with a magnetic drain and temperature port, an auxiliary cooler, a billet multi-disc converter or a reinforced valve body on tuned or max-tow trucks, and a drop-in replacement turbo or a 64/67 or 67/67 unit when you want airflow and lower exhaust temperature without fabrication. On intake and exhaust we stay inside the Executive Order list: a CARB EO-numbered Ram-Air intake, and a DPF-back system with every factory emissions device left in place. On a dually the dually-specific Banks system opens up far more flow area and moves the outlet to the side, which matters because tailpipe temperature during a regeneration can reach roughly 1,100 degrees and has scorched trailer tongues and jacks. For response and modest output the Derringer High Output carries CARB EO D-161-167 and includes the iDash monitor; the locked E41 ECM means any custom calibration needs an unlock tool, and the calibration itself has to be EO-covered to belong on a registered truck.

What we carry

PPE has the deepest shelf here, from high-efficiency and oversized oil filters and billet filter sockets to forged XR tie rods, a drilled steering assembly kit and a fuel-coolant pump relocation kit. Wehrli Custom Fabrication covers the fabricated side: thermostat housing, upper coolant pipe kit, billet exhaust manifold and stainless up-pipe kit, OEM and VGT turbo gasket kits, low coolant level sensor and a lift-pump relocation bracket. Hot Shot's Secret and Lucas Oil supply CK-4 engine oil plus the additive and coolant shelf, ACDelco and Alliant Power the OE service items such as glow plugs, oil filters and coated brake rotors, and FASS the extreme water-separator elements. MBRP brings mufflers and resonators, Banks Power the Ram-Air intake and Monster-Ram inlet elbow, and BD Diesel with Mishimoto the turbo accessories and cooling parts. For the chassis there is BDS Suspension for lift kits and heavy-duty tie rods, Air Lift for air springs, and B&W Trailer Hitches with Big Hitch Products on the hitch side. Edge Products handles gauges with monitors, EGT kits, pressure sensors and pillar mounts, while Fleece Performance Engineering, Merchant Automotive and Calibrated Power cover coolant block-off plates, HP4 pump gasket kits and Stealth drop-in turbos. Platform-specific shop tools are here as well.

Ordering by configuration

Sierra HD parts split by cab length, bed, single or dual rear wheels, and by make. Front-end and lighting parts are GMC-specific, and lamp type varies by trim across Pro, SLE, SLT, AT4 and Denali, so order by RPO or VIN rather than by year alone; the 2024 refresh changed the fascia and lamps again. Dually fenders are 14.8 inches wider than single-rear-wheel, which changes bed accessories and mud flaps, and the dually takes an 8x210 wheel instead of 8x180. Suspension is listed for this body: three-inch kits bring upper control arms, indexed torsion keys, front shock spacers, differential relocation and a rear block, while a 6.5-inch system runs about eight hours, needs GM's torsion-bar unloading tool and comes in with-overload and without-overload versions, duallys taking the overload version. At 6.5 inches the factory 17 and 18-inch wheels contact the tie rods, so you move to 20s, which clear a 35x12.50. Tall lifts on a two-piece rear driveshaft need a carrier-bearing drop, and a dual-Cardan front shaft settles 4WD highway vibration. Tow ratings are quoted by configuration, not by engine, and gooseneck balls that drop into the bed pucks require the factory fifth-wheel and gooseneck prep package.

Frequently asked questions

How much will my Sierra HD tow?

That depends on cab, bed, drive and rear axle far more than on the engine, which is identical across the range. A 2500HD diesel tows up to about 18,500 pounds conventional and in the 18,200 to 18,600-pound range on a fifth wheel, while a 3500HD dually configured for it reaches 35,500 pounds for 2020 and 36,000 pounds for 2021-2023 on a gooseneck. GCWR runs from 26,000 pounds on a 2500 to above 40,000 on a 3500 dually. Use the ratings for your exact truck, and plan the hitch around them.

Is there a compliant tuner for a 2020-2023 Sierra HD?

Yes. The factory E41 ECM is locked, so the usual route is an OBD-II module rather than a flash. The Banks Derringer High Output carries CARB Executive Order D-161-167, is legal in all 50 states, adds output across six levels, includes the iDash monitor, needs no ECM reflash, and its TransCommand logic watches the ten-speed. A throttle-response controller is a separate piece. If a shop does custom ECM calibration instead, it needs an unlock tool, and the calibration itself must be EO-covered to stay street legal.

What fluid does the ten-speed take, and how often?

DEXRON ULV, not the DEXRON VI the older Allison 1000 used, and the factory service interval is 45,000 miles. If you tow heavy or sit in Florida heat, 30,000 to 45,000 miles with a fresh filter is the safer plan, and it is worth inspecting the cooler lines for kinks while the truck is in the air. Watching trans temperature is the single most useful habit on this platform; the cluster shows it, and a monitor that logs it tells you more on a grade.

Will 2020-2023 parts fit a 2024 Sierra HD?

Often not. The 2024 truck is a refresh with an uprated engine: a different turbo, tapered-bowl pistons, higher-flow injectors, a higher-pressure pump, thicker fuel rails, revised heads and a new ECM generation. Banks lists the 2020-2023 Ram-Air intake as not fitting 2024 and up, gauge pod mounts are listed separately for the refreshed interior, and front styling changed. A few parts do span both, so check the listing against your model year rather than assuming.

My truck set an injector code and dropped into limp mode. Is the injector bad?

Not necessarily, and on the L5P it usually is not. The number four injector connector is a known failure point and throws the same P0204, P020D and P0300 codes an injector would, so the first move is to inspect it and fit a gold-plated or LBZ-style replacement connector. A sooted MAP sensor can also produce misfire-like running complaints. Rail pressure codes such as P0087, P0088 and P1089 point at fuel supply and contamination, not at the turbo, even though P1089 gets misread that way.

Not sure what fits? Call the shop at the number in the header, or use the fitment checker on any product page. We install everything we sell.