If you own a 2011–2016 Duramax LML, there’s a good chance you’ve heard the phrase “CP4 failure” tossed around in truck forums, Facebook groups, or even at the parts counter at your local diesel shop. It’s not just internet hype — the factory CP4.2 high-pressure fuel pump in these trucks has a well-documented track record of failing, and when it does, the damage can be catastrophic.

We’re talking thousands of dollars in repairs, weeks of downtime, and in some cases, trucks being parked for good.

That’s where the S&S Diesel Motorsport 50 State CP3 Conversion Kit comes in. Designed to replace the troublesome CP4 with the rock-solid Bosch CP3 pump, this kit not only restores peace of mind but also delivers proven reliability — all while staying emissions-compliant and legal in every U.S. state.

Let’s break down why this upgrade is more than just a performance mod — it’s insurance for your Duramax.

Understanding the CP4 vs. CP3 Debate

When GM introduced the LML Duramax in 2011, it came equipped with the Bosch CP4.2 high-pressure fuel pump. On paper, it was a step forward — smaller, lighter, and designed to meet stricter emissions standards by producing the same pressure with less fuel.

The problem? In the real world, the CP4 pump has a nasty habit of self-destructing. The internal cam and roller system in the CP4 rely heavily on constant lubrication from diesel fuel. Even a tiny amount of air, water, or contamination can cause metal-to-metal contact. When that happens, the pump can literally grind itself apart from the inside.

The result: tiny metal shavings get sent through the entire high-pressure fuel system — injectors, rails, lines — destroying everything in their path.

The CP3, used in earlier Duramax engines and still respected today, takes a completely different approach. Instead of a two-piston design like the CP4, the CP3 uses a three-piston setup that provides:

• More consistent fuel delivery
• Better lubrication
• Greater tolerance for small contaminants in fuel
• Long-term reliability under high load and high mileage

It’s not a performance fad — the CP3 has been proven in heavy-duty diesel applications for nearly two decades.

The True Cost of a CP4 Failure

If your CP4 pump decides to give up the ghost, it won’t do so quietly.

Some common warning signs include:

• Sudden loss of power
• Hard starting or no start at all
• Excessive smoke from the exhaust
• Metal flakes in the fuel filter

By the time you notice these symptoms, it’s often too late — the pump has already sent shrapnel through your system. And because these are high-pressure common-rail systems, you can’t just swap the pump and call it good.

A full CP4 failure repair usually means replacing:

• The high-pressure pump itself
• All eight injectors
• Both fuel rails
• All high-pressure lines
• The fuel tank and filters

It’s not unusual for dealerships and diesel shops to hand out estimates in the $6,000–$10,000+ range for this job. That’s a big bill for something that could have been prevented.

How the S&S Diesel Motorsport 50 State CP3 Conversion Kit Solves the Problem

S&S Diesel Motorsport is one of the most trusted names in common-rail diesel fuel systems, and their 50 State CP3 Conversion Kit is designed as a complete, bolt-on solution to the CP4 problem.

Here’s why it stands out:

CARB-Legal in All 50 States

• Certified under Executive Order D-756 from the California Air Resources Board.
• Fully emissions-compliant, so you don’t have to worry about inspections or smog checks.

No Tuning Required

• Unlike some other conversions, this kit works with your factory ECU calibration.
• No expensive reflash or custom tuning needed.

Factory Emissions Equipment Retained

• EGR, DPF, and SCR systems remain in place and functional.
• Keeps your truck street-legal without sacrificing reliability.

Complete Hardware Package

• OE-quality quick-disconnect fuel supply lines.
• Braided fuel lines for durability and heat resistance.
• High-pressure fuel line to connect pump to rail.
• Injection-molded ECU mount for a clean, factory-like installation.

This is not a “cobble it together” kind of mod — the S&S kit is engineered for a seamless swap that looks and functions like OEM equipment.

S&S Diesel 50 State CP3 Conversion Kit w/ Pump | 2011–2016 GM 6.6L Duramax LML

$2,600.00

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Installation Overview

The CP3 conversion isn’t as simple as bolting on an intake or swapping a fuel filter, but it’s a straightforward job for a skilled diesel mechanic or an experienced DIYer with the right tools.

High-level steps include:

1. Removing the factory CP4 pump and related hardware.
2. Installing the Bosch CP3 pump in its place.
3. Routing and connecting the new fuel supply and high-pressure lines.
4. Mounting the ECU using the included injection-molded bracket.
5. Priming the fuel system and checking for leaks.

On average, a professional shop can complete the job in 6–8 hours. DIY installs may take longer, especially if you’re working with basic tools.

Performance and Reliability Benefits

While the main reason most owners make the switch is reliability, there are performance perks to the CP3 as well.

• Steady Fuel Delivery: CP3 pumps maintain consistent rail pressure even under heavy towing loads or during extended high-RPM runs.
• Stronger Under Modding: If you plan on tuning or upgrading injectors later, the CP3 can handle higher horsepower demands without breaking a sweat.
• Longevity: Many CP3 pumps last 200,000+ miles without failure when paired with proper filtration and clean fuel.

It’s the kind of upgrade you forget about — because it just works.

Cost vs. Risk: Why the Upgrade Pays for Itself

Let’s do the math:

• CP3 Conversion Kit with Pump: ~$2,600
• Average CP4 Failure Repair Bill: $6,000–$10,000+

Even if you never push your truck hard, the CP4’s failure history makes it a gamble. Installing a CP3 kit is essentially an insurance policy that protects your fuel system, your wallet, and your truck’s uptime.

Factor in the downtime of waiting for parts and the possibility of being stranded far from home, and the CP3 swap starts to look like a bargain.

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Final Thoughts

If you own an LML Duramax, the CP4 pump is a known weak link — and hoping it never fails isn’t a great strategy. The S&S Diesel Motorsport 50 State CP3 Conversion Kit is a direct, emissions-compliant, and tuning-free fix that replaces that weak link with one of the most proven fuel pumps in diesel history.

It’s not just a performance upgrade — it’s peace of mind.

Don’t wait for the CP4 to take out your fuel system. Protect your Duramax with a CP3 conversion before it’s too late.