The L5P Duramax diesel engine, introduced in 2017, marked a significant step forward for General Motors’ HD truck lineup. With 445 horsepower and 910 lb-ft of torque, it quickly earned a reputation for power and performance. But no engine is without its faults. If you’re considering buying a vehicle with an L5P Duramax, or already own one, understanding its most common issues is key to getting the most out of your investment.

A Quick Overview of the L5P Duramax

The L5P is the fifth generation of the Duramax diesel V8, succeeding the LML. It was first introduced in 2017 (the first year of L5P availability) and came with a complete redesign. Unlike previous iterations that were co-developed with Isuzu, the L5P was a GM-led project, allowing more control over emissions, performance, and reliability.

Owners praise the L5P for:

1. Excellent towing capability

2. Refined driving experience

3. Improved fuel efficiency (for a heavy-duty diesel)

4. Fewer emissions issues than earlier models

But despite the improvements, a few persistent Duramax problems still exist.

The Most Common Problem: Fuel System Failures

Among all the issues reported by L5P owners, fuel system failures—especially related to the fuel injectors and the CP4.2 injection pump—are the most frequently cited and costly.

Understanding the CP4.2 Pump Issue

While the L5P Duramax features many upgrades over its predecessors, it retained the Bosch CP4.2 high-pressure fuel pump. This pump has become infamous in the diesel world, not just in Duramax engines, but also in Ford and Ram diesel trucks.

The CP4.2 pump is known to fail due to poor lubrication, especially when air or water enters the fuel system. When it fails, it can send metal shavings throughout the entire fuel system, often requiring:

1. New injectors

2. Fuel rails

3. High- and low-pressure lines

4. Even the fuel tank and filters

This type of failure often costs $8,000–$10,000 in repairs if not caught early or covered under warranty.

Why Does This Happen?

The CP4.2 was originally designed for European diesel fuel, which contains more lubricity than U.S. diesel. Without additives or higher-quality fuel, the risk of pump failure increases. It’s a design flaw many hope GM will address in future Duramax generations.

Other Notable L5P Duramax Problems

While the CP4.2 pump is the most notorious issue, there are other problems L5P owners may experience, including:

1. EGR and DPF Troubles

Despite improvements, emissions systems like the Exhaust Gas Recirculation (EGR) valve and Diesel Particulate Filter (DPF) can still clog or fail, especially in trucks that do a lot of short trips or idle frequently.

2. Transmission Concerns

Though the L5P is paired with the sturdy Allison 1000 and later the 10-speed Allison in newer models, some owners have experienced hard shifting or torque converter issues, especially under heavy towing loads.

3. Sensor and Electrical Glitches

Modern diesels are packed with electronics, and the L5P is no exception. Some owners report issues with MAP sensors, DEF sensors, or electrical harness failures over time.

L5P Duramax Life Expectancy

When properly maintained, the L5P Duramax engine can last well beyond 250,000 miles—and many reach 300,000 to 400,000 miles with few issues. Using quality fuel, changing filters regularly, and avoiding excessive idling all contribute to a longer lifespan.

Adding fuel additives or upgrading to a CP3 pump (a common modification) can also help prevent catastrophic failure from the CP4.2.

Learn more: What is the Best Duramax Engine?

First Year of L5P: How Did It Hold Up?

The 2017 model marked the first year of L5P production. While the redesign brought praise for power and drivability, early production models did experience more of the aforementioned fuel pump and sensor issues.

Many of these problems were addressed in later years with updated software and warranty coverage. Still, if you're buying a 2017–2018 model, make sure the truck has a clean service history and possibly an extended warranty.

Preventive Tips for L5P Owners

1. Use diesel fuel additives to improve lubrication

2. Stick to a strict maintenance schedule

3. Monitor your fuel system for signs of contamination

4. Consider an aftermarket lift pump and filter system

5. Have the truck scanned regularly for diagnostic trouble codes

FAQs

Q1: How long will an L5P Duramax last?
With regular maintenance, the L5P Duramax life expectancy is typically 250,000–400,000 miles.

Q2: What is the most expensive L5P repair?
Fuel system replacement due to CP4.2 failure is often the most expensive, costing up to $10,000.