Chevrolet / GMC 2500HD Duramax (LML / L5P)
Inspection Guide
Strong HD diesels backed by the bulletproof Allison — but the 2011–2016 LML carries the catastrophic CP4 fuel pump. The 2017+ L5P fixed it.
GM's 6.6 Duramax HD trucks are towing favorites, and the Allison transmission behind them is a genuine strength. The make-or-break detail is the fuel system: the 2011–2016 LML uses the CP4.2 pump that can grenade and contaminate the entire system, while the 2017-and-up L5P moved away from it. Identify the engine before you do anything else.
A CP4 failure sends metal through the fuel system, requiring a full and very expensive replacement.
CHECK On LML trucks, inspect for fuel-system debris and ask about a CP4 bypass/disaster-prevention kit.
After-treatment faults and clogging are common, and deletes create legal and resale problems.
CHECK Scan the after-treatment, verify regen, and confirm hardware is intact and unmodified.
The Allison is robust but suffers behind aggressive tuning and hard towing.
CHECK Check fluid condition and road test for slip or harsh shifts under load.
Heavy front ends wear tie rods and steering components.
CHECK Inspect steering linkage and road test for wander or wobble.
- Identify LML vs L5P — the CP4 risk is the whole story
- Verify emissions hardware intact (no illegal deletes)
- Check Allison fluid and shift quality
- Inspect front end and steering
- Look for hard-tow and plow stress
Bidirectional OBD-II scan tool, Paint depth gauge, Tire tread depth gauge & DOT date decoder, Brake pad & rotor calipers / measurement tools, Battery & charging system tester, Compression and cylinder leak-down testers, Digital borescope, Floor jack and jack stands …
See the full Pre-Purchase Vehicle Inspection tool kit
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Pre-tagged for the Chevrolet / GMC 2500HD Duramax (LML / L5P) (2011–2023).