Talk to any diesel mechanic over forty and they'll tell you the same thing: the Bosch CP3 was the gold standard, full stop. It made power, it lived forever, and it didn't take the fuel system with it when it finally retired. Then Bosch rolled out the CP4, manufacturers slapped it on every late-model light-duty diesel, and the industry inherited a known time bomb. Today, owners of 6.7 Power Strokes, late-model Duramaxes, and 5th-gen Cummins are paying for that decision in five-figure repair bills.

There's a better way: get rid of the CP4 entirely and put the legendary CP3 (or its modern replacement, the S&S DCR) back in its place. This is the comprehensive look at the CP4 vs CP3 conversion debate — what each pump is, why it matters, what a conversion involves, and why the smart money makes the swap. If you'd rather skip ahead and just have us spec and install it, request a quote.

What is the CP3, and why does the diesel world love it?

The Bosch CP3 is the high-pressure injection pump that fed most light-duty common-rail diesels through the 2000s — including the legendary 5.9 Cummins, the LB7/LLY/LBZ/LMM Duramax, and the 6.0/6.4 Power Stroke. Decades of real-world abuse have proven it to be one of the most durable high-pressure diesel pumps ever made. Why owners worship it:

• Stout internals — three pistons (vs the CP4's two) and a rotating-element design that's far more tolerant of less-than-perfect fuel.
• Forgiving of U.S. ultra-low-sulfur diesel — the CP3 doesn't care about lubricity nearly as much as its successor.
• Plenty of fueling headroom — even a single stock CP3 can support tuned trucks well into serious power figures.
• When it does fail, it fails gracefully — typically you lose pressure and limp home; you don't grenade your injectors.

Translation: the CP3 is the kind of pump you bolt on, forget about, and replace once at 250,000 miles when you finally feel like updating it. The diesel community didn't pick the CP3 as the gold standard by accident.

What is the CP4, and why is it a problem?

The Bosch CP4.2 replaced the CP3 starting around 2011 on light-duty diesels. Bosch's engineering goal was reasonable — lighter, more efficient, lower parasitic loss. The execution, in the real world, has been a disaster. We covered the whole ugly story in our CP4 disaster prevention guide; the short version:

• Two pistons riding a cam via a roller "follower." When the follower rotates out of position, it's metal-on-metal at extreme pressure.
• Lubrication-sensitive design. U.S. ULSD has lower lubricity than the fuel Bosch validated against. Add a bad tank or a dry start and the pump can grenade.
• Catastrophic failure mode. When the CP4 lets go, it sends metal through the entire fuel system — injectors, rails, lines, sensors. Repair bills routinely hit $8,000–$12,000+.
• Class-action lawsuits against Ford and GM exist precisely because this isn't a hypothetical problem — it's a known one.

Manufacturers haven't redesigned the pump. They haven't recalled it. Which means the only real fix is on you.

The CP4 vs CP3 cheat sheet

It's not subtle. The CP3 wins on every meaningful axis except weight and parasitic loss — and neither of those matter when you're paying for an emergency tow because your CP4 just took out a $5,000 set of injectors.

What a CP3 conversion actually involves

A CP3 conversion removes the CP4 from your engine and replaces it with a CP3 (or, in the modern era, with a CP3-style replacement like the S&S DCR) along with the supporting hardware to make the swap clean. Components in a typical conversion kit:

• The new CP3 / DCR pump — built for your platform and power goals.
• An adapter plate or mounting hardware to interface the CP3 with the engine that originally shipped with a CP4.
• Updated drive hardware (pulleys, gears, or related parts depending on the platform).
• High-pressure lines or fittings sized to the new pump's outputs.
• Tuning support — most conversion kits require a calibration update so the ECU manages the new pump correctly.

This is real engineering — not a "swap the pump and go" job. Done right by a reputable manufacturer, it's a bulletproof permanent fix. Done wrong, it can create more problems than it solves. That's why we stock the proven kits from companies that have been doing this for years.

CP3 vs DCR: same idea, modern execution

If you've started shopping conversions, you'll see two flavors: classic CP3 swaps and the newer DCR (Disaster-Cardinal-Resistant) pump conversions. Both solve the same problem; here's how they differ:

Classic CP3 conversion

Bolts an actual Bosch CP3 (often a remanufactured unit, sometimes new) into your CP4-equipped engine via an adapter kit. It's the original solution and it's been done thousands of times. Reliable, well-documented, parts are easy to source.

S&S DCR pump conversion

S&S Diesel Motorsport's modern take: a purpose-built CP3-style pump engineered specifically for the conversion application. The DCR is built to survive the lubrication conditions that murder the CP4, supports more flow than a stock CP3 for built trucks, and is often the cleaner installation on newer platforms. See the S&S collection for current offerings.

For most owners, the answer between the two comes down to platform, intended power level, and what's currently available for your year. Either route gets you off the CP4 — that's what matters.

Who should do a CP3 conversion (and who can get away with a prevention kit)?

Not everyone needs the full conversion. The decision tree:

• Stock daily driver, plan to keep it forever: Start with an S&S disaster prevention kit. Cheap insurance that contains a CP4 failure if it happens.
• Tow rig or work truck that depends on uptime: Lean toward the full CP3/DCR conversion. The day a prevention kit limits damage is still a day your truck is down. The conversion eliminates the failure mode entirely.
• Tuned truck or planned build: Go straight to the full conversion. You'll outgrow a stock CP4 anyway, and you'll want the fueling capacity that CP3 / DCR brings.
• Race or competition truck: Conversion is non-negotiable. You're not running a CP4 in a competition rig.

Not sure where your truck falls? Send us your specs and goals and we'll make the call with you.

What does a CP3 conversion cost vs. ignoring the CP4?

This is where the math gets persuasive:

There's no scenario where "ignore it" wins. Even if you never have a CP4 failure (some owners genuinely don't), the resale value of a truck with a CP3 conversion is meaningfully higher — and the peace of mind alone is worth real money.

Install: this is engine work

A CP3 conversion is a multi-hour shop job. The high-level process:

• Remove intake / valley components to access the CP4 (on the 6.7 PSD specifically — Duramax and Cummins are more accessible).
• Drop the CP4, clean the area thoroughly.
• Install the new pump and mounting hardware per the conversion kit instructions.
• Plumb the high-pressure lines to the new pump outputs.
• Update the tune to manage the new pump correctly.
• Bleed the fuel system, prime, and cold-start carefully. Test under load.

This is well within reach of an experienced diesel DIYer with a service manual and a weekend, but it's exactly the kind of job most owners are smarter to hand to a shop. Our techs do these regularly. Get a quote and we'll bundle the conversion with any other reliability work the truck needs.

What to bolt on while the system's apart

A CP3 conversion means the fuel system is open. That's the right moment to round out the rest of the job. The high-value pairings:

• A FASS lift pump — clean, pressurized supply for the new pump. See our FASS guide.
• Fresh injectors from our injector lineup if yours have miles on them.
• Updated high-pressure lines and a fresh filter setup.
• Tuning matched to the new pump's capabilities.

Building a full 6.7 Cummins? Cross-reference our first 5 upgrades guide and our CP4 disaster prevention deep dive.

The supply story: why a lift pump matters even with a CP3

Here's a mistake we see all the time: an owner does the CP3 conversion, breathes a sigh of relief, and figures the fuel system is bulletproof now. The CP3 is famously durable — but it's not invincible, and it still gets fed whatever the supply system delivers.

Even with a CP3 (or DCR) in place, you still want:

• A real lift pump — a FASS Titanium Signature Series is the obvious pick. Clean, air-free, pressurized supply keeps the high-pressure pump and injectors happy.
• Quality filtration — fresh primary and secondary filters on a regular service interval.
• A water-separator with a working drain — water is the enemy of every pump and injector.
• Lubricity additive in every tank — cheap insurance for any high-pressure pump.

A CP3 fed garbage will still wear out faster than a CP3 fed clean fuel. The conversion is half of the story; the supply side is the other half. Do both.

Common myths about the CP3 conversion (debunked)

The CP3 conversion topic is old enough that a lot of mythology has built up around it. The takes that aren't quite right:

"A CP3 conversion automatically makes you a deletes truck."

No — the CP3 conversion is an internal fuel-system modification, not an emissions modification. Your DPF, EGR, and DEF systems are independent of which high-pressure pump is bolted on. You can run a CP3 conversion and remain emissions-compliant.

"A CP3 conversion will fail your inspection."

The conversion itself is invisible to most inspection processes. Local regs vary; check yours.

"You'll lose horsepower compared to the CP4."

Stock-vs-stock, the CP3 supports very similar fueling to the CP4 — and the modern DCR exceeds it. You're trading fragility for durability and not giving up power to do it.

"It's not worth doing on a low-mileage truck."

Low-mileage is exactly when it's worth doing. You're maximizing the time you'll spend with the conversion in place and minimizing your exposure to a CP4 failure during the truck's ownership.

"All conversion kits are the same."

They are emphatically not. Quality varies, fitment varies, included hardware varies, tuning support varies. Stick to reputable manufacturers — the S&S Diesel and Industrial Injection kits we stock — and you'll be fine.

What about emissions and the legal landscape?

The CP4 conversion is a permanent, well-documented fix that's been done thousands of times across multiple states and use cases. The legal landscape varies — California's CARB regulations are stricter than most other states, and federal EPA rules apply to certain modifications. The conversion itself is internal and doesn't touch emissions hardware, but as with any work on a late-model emissions-controlled truck, follow your local rules. We're happy to walk you through what's appropriate for your truck and your state. Send us your situation.

Frequently asked questions

What is the difference between a CP3 and a CP4?

The CP3 is a three-piston Bosch high-pressure pump used on most pre-2011 light-duty diesels — bulletproof, forgiving of fuel quality, fails gracefully. The CP4 is a two-piston successor used on 2011+ diesels — lighter and more efficient on paper, but with a catastrophic failure mode that destroys the fuel system.

Why do CP3 conversions exist?

Because the CP4 design has been failing at unacceptable rates, and owners have rallied around the proven CP3 (or modern DCR) as the permanent fix. There's no manufacturer recall, so the aftermarket built the solution.

Is a CP3 conversion legal?

It's an internal fuel-system modification, not an emissions modification — the CP3 conversion doesn't change your DPF, EGR, or DEF systems. As with any engine work on a late-model truck, follow local laws and check with your shop about applicable regulations in your area.

Will a CP3 conversion void my warranty?

Magnuson-Moss prevents a manufacturer from voiding your entire warranty over an aftermarket part — they can only deny a specific claim if they can prove the modification caused that specific failure. For most owners, the CP3 conversion is done after warranty concerns are no longer the priority.

Can I do a CP3 conversion myself?

An experienced diesel DIYer with the right service info and a free weekend, yes. Most owners are smarter to have a shop do it — and to bundle it with related fuel-system work while everything's apart. Get a quote.

How much horsepower can a CP3 / DCR support?

A single CP3 supports well into competitive horsepower figures; the modern DCR even more. For most street trucks, the conversion is permanent overkill in the best way.

How to know when it's time to convert

For some owners the answer is "yesterday." For others it's a planning decision worth thinking through. The triggers we tell customers to watch for:

• You're approaching or past the mileage where the CP4 typically shows trouble. If you're sitting on 80k+ miles on a CP4-equipped truck with no protection in place, you're playing odds you don't want to play.
• You're about to add power. Tuning and bigger injectors push the CP4 harder. If a tune or fuel upgrade is in the plan, the conversion belongs in the same shop visit.
• You're already in for major fuel-system work. Replacing an injector? Doing CP4-related preventative work? Open the wallet a little wider and just do the conversion now while everything's apart.
• You depend on the truck. Work trucks, tow rigs, anything where unplanned downtime is a real problem — the conversion is cheap relative to that downtime risk.
• You plan to keep the truck a long time. The longer the ownership window, the more obvious the conversion math becomes.

The opposite case: a low-mileage truck with a near-term sale plan, where the next owner can deal with their own CP4 decision. Even then, the conversion adds resale value — but if you're flipping soon, the prevention kit is probably the right balance.

Get off the CP4 once and for all

The CP4 has been the most-discussed problem in the late-model diesel world for a decade, and the solution has been sitting right there the whole time. The CP3 was right when Bosch built it; the modern DCR makes it better. Either route gets you off the time bomb and onto a fuel system that'll outlast the truck.

Shop the S&S Diesel collection at DNR Customs for the kit that fits your platform, browse our broader fuel system selection for the supporting parts, and when you're ready to make the swap, request a quote — we'll get you converted right the first time, every time.