Table of Contents

What GM changed from the LML

GM's own new-features bulletin (16-NA-362) calls the 2017 L5P a clean-sheet redesign that shares only the bore and stroke of the previous engine. The numbers: 445 hp at 2,800 rpm and 910 lb-ft at 1,600 rpm, up from the LML's 397 hp and 765 lb-ft. A few changes matter for reliability:

  • A new block and rotating assembly. Cast-iron block, four-bolt cross-bolted main caps, a forged crankshaft, and larger piston-cooling oil jets.
  • A bigger oil cooler. GM lists the integrated oil cooler at 50 percent more capacity than the previous engine.
  • An in-tank electric fuel pump. The primary tank holds a three-phase electric pump that runs whenever the engine is running. The LML had no factory lift pump.
  • A new fuel filter. GM says the water-separating filter media can extend filter life by up to 33 percent over earlier designs.
  • Downstream regen fuel. Like the LML, the L5P injects regeneration fuel into the exhaust, not the cylinders, so fuel does not dilute the engine oil.

The high-pressure pump changed too. Drivingline reports that the L5P uses a Denso HP4 in place of the Bosch CP4.2 on the LML, fed by the in-tank pump, and that it has proven more reliable than the CP4.2. GM's bulletin does not name the supplier, so treat the brand as Drivingline's report, not GM's. For the LML side of that story, see our guide to which Duramax years to avoid.

Problem 1: Injector connector faults and misfires

This is the L5P misfire issue GM has published a bulletin for.

What GM says. Service Bulletin 23-NA-027 (April 2023) covers 2017 to 2023 Silverado and Sierra 2500HD and 3500HD trucks with the L5P. The complaint is poor engine performance and misfires. Codes can include P0201 through P0208, P0300, and P0301 through P0308. GM's stated cause is engine vibration wearing the contact between the injector connector terminal and the high-pressure injector.

What the repair involves. GM's instruction is to check the connectors before replacing an injector:

  1. Probe-test every injector connector.
  2. Look for fretting or black powder on the connector or its seal.
  3. Replace any connector that fails the test or the visual check.
  4. Apply dielectric grease to the connectors before installing them.

The point for owners: a misfire code on an L5P does not automatically mean a bad injector. A failed connector can look the same on a scan tool, and a connector is a much cheaper part than an injector.

Which cylinder? Parts retailers and forums often name cylinder 4 and code P0204. GM's bulletin covers every cylinder, so we do not single one out.

Problem 2: Fuel contamination and DEF in the diesel tank

This one is not a design flaw, but GM calls it out because of how much damage it does.

What GM says. Service Bulletin 18-NA-361 (December 2022) covers 2017 to 2023 L5P trucks, among other GM diesels. GM states that when engineering analyzed warranty return parts, many fuel pump and fuel injection component failures showed evidence of DEF contamination in the fuel. GM also notes that misfueling or fuel contamination is not a product issue and should not be charged to warranty.

Symptoms and codes. The bulletin lists no start, a lit MIL, and codes that include P0087, P0171, P0172, P026D, P0300, P228A through P228D, P2264 (water in fuel sensor), and others.

What the repair involves. If contamination is found, GM says not to replace the fuel system as described in bulletin 16-NA-102, which is the LML/LGH pump procedure. The truck needs the fuel system flushed first, then diagnostics continue. GM also tells technicians to clean the outside of the fuel filter assembly before removing it and to clean the water separator reservoir before reinstalling the filter.

What owners can do:

  • Check which fluid you are filling, every time, especially when you carry DEF in containers.
  • If DEF or any wrong fluid goes into the diesel tank, tell the shop before diagnosis starts. GM's procedure is to flush the fuel system first.
  • Change the fuel filter on schedule. GM's own text says water in the fuel prevents the system from cooling and lubricating its parts.
  • Buy used L5Ps with fuel filter records, not just oil change records.

Problem 3: DEF injector code P1048 on early builds

GM bulletin PIP5594 (October 2018) covers code P1048, Reductant Injector High Control Circuit Low Voltage, on 2017 to 2019 Silverado 2500/3500 and Sierra 2500/3500 trucks built before a specific VIN cutoff. GM's stated cause is a possible mis-build of the DEF injector. The fix depends on the injector serial number: a serial starting with "G", or J067B04875 or lower, on an early-build truck gets a replacement injector. Anything outside that range goes to normal diagnosis.

If you see P1048 on a 2017 to 2019 truck, ask the dealer to check the DEF injector serial number first. It may already be a known part.

Problem 4: Fuel rail pressure sensor connector

The same kind of connector wear shows up on the fuel pressure side. GM Service Bulletin 20-NA-197 covers 2017 to 2021 L5P trucks with the MIL on and fuel system codes that include P0087, P0089, P0191, P026C, P026D, P1089, P163A, P163B, P2293, and P228A through P228D. GM's stated cause is corrosion or damage at the fuel pressure sensor electrical connector.

The procedure is to inspect the connector, put dielectric grease on a terminal test tool (J-35616-16), and drag it across the terminals up to 10 times to find corrosion or fretting. If a terminal is suspect, GM says to pull it and look for corrosion, damage, or dielectric grease turning black, then replace the connector. GM lists 0.9 hours of labor.

The point for owners is the same as Problem 1. A low rail pressure code does not automatically mean a failed high-pressure pump. The connector is the cheaper thing to check first.

Problem 5: Cold-weather codes (MAP sensor ice and fuel gelling)

Two GM bulletins cover winter drivability on the L5P.

  • MAP / TMAP sensor ice (20-NA-043). GM says ice can form on the sensor during a 1 to 2 hour drive in below-freezing temperatures. That can cause a reduced engine power message and codes P0106 and P140C. Once the truck is stopped and idling the ice melts, and the next key cycle clears the power message, but the MIL stays on. GM recommends a winter cover on 2020 trucks below 24°F (-5°C).
  • Soot on the sensor is not a failure. GM says not to clean the sensor with chemical cleaners, water, or compressed air, and not to try to wipe soot off, because that can damage the thermistor. If the sensor is clean and dry, GM says there is no need to replace it.
  • Fuel gelling (18-NA-103). GM says some diesel blends gel in cold weather and can block the in-tank fuel pump pickup. Symptoms are a whining fuel pump, hard start or stalling, and codes P0087, P018B, P228A, P228B, and P2635. GM's fix is to warm the truck so the fuel is above 32°F (0°C), confirm it starts and the whine is gone, and drain the water separator. GM says not to replace the in-tank fuel pump for this concern. If you use a winter additive, GM says it must not contain metal-based additives, alcohol, or other water emulsifiers.

Problem 6: Block heater cord recalls (2017 to 2019)

If your truck has the optional engine block heater, check recall status.

  • NHTSA recall 19V-328 covered 2017 to 2019 Silverado and Sierra 2500/3500 trucks with the L5P and the optional block heater cord, built July 18, 2016 through April 18, 2019. GM found that a short circuit could develop in the heater cable or in the terminals connecting it to the heater.
  • NHTSA recall 21V-496 followed in 2021 and covers 338,020 vehicles. GM reported 122 related complaints and 17 reports of potential fires on vehicles that had already received the first remedy. The new remedy replaces the heater and cord, with a redesigned element attachment that uses a solder connection.

Enter your VIN at nhtsa.gov/recalls to see whether a campaign is open on your truck, and ask a GM dealer whether the remedy has been done.

Problem 7: Turbo actuator parts restriction (2017)

GM bulletin PIP5439A placed a parts restriction on the 2017 L5P turbocharger (part numbers 12679694 and 12683985) and actuator assembly (12665348). Dealers had to complete a diagnostic questionnaire before getting replacement parts. GM's wording is that the restriction helps engineering gather product feedback. It is not a recall and not a statement that these turbos fail. If you are buying a 2017 truck and the turbo or actuator has been replaced, ask for the paperwork.

A 2024 note

Our guide covers 2017 to 2023, but if you own a 2024, GM's voluntary emission recall N232415860 covers certain 2024 Silverado and Sierra 2500HD and 3500HD trucks with the L5P. The ECM may not correctly adjust for normal injector flow variation, which can cause a rough idle or higher fuel use. The fix is a dealer software update using injector flow data, at no charge.

The 2024 trucks also had a non-engine safety recall. NHTSA recall 23V-549 covers 3,893 early-build 2024 Silverado and Sierra 2500/3500 trucks, built June 7 to June 24, 2023. GM says the steering gear shaft may contain a supplier manufacturing defect that could fracture. Dealers inspect the steering gear and replace it if necessary. Run your VIN if you own or are buying an early 2024.

Before you buy a used L5P: checklist

  • Confirm the model year and engine code (L5P pickup vs L5D vs chassis cab).
  • Run the VIN at nhtsa.gov/recalls for open campaigns, especially block heater recalls on 2017 to 2019 trucks.
  • Ask for fuel filter service history.
  • Scan for stored codes: P0201 to P0208, P0300, P0087, P0191, P2264, P1048, and P0106. Ask which ones were diagnosed and what was replaced.
  • On a MAP sensor with soot on it, do not treat soot as a failure and do not spray it.
  • Check the fuel filler area and ask whether the truck has ever been misfueled.
  • Ask whether the truck has been tuned or had emissions hardware removed. A prior owner's changes affect what you can legally do with it.
  • Confirm warranty status with a GM dealer using the VIN.

Emissions hardware and compliance

Some L5P owners ask about removing the DPF, EGR, or DEF system after repeated emissions faults. Some emissions-related products are intended only for off-road, race, or non-public-road use where permitted. An off-road label does not change the legal status of an EPA-certified vehicle. Check federal, state, and local laws before purchase, and the customer is responsible for legal compliance. Professional installation is recommended where appropriate.

If you are working through a repeated fault, diagnose first. Several documented L5P issues above are connectors, an injector serial number, fuel quality, or cold-weather icing, not the filter itself. Our L5P Duramax delete kit buyer's guide explains fitment, what is included, and what tuning may be required. For ECM calibration options, read our guide to the best tuner for the L5P Duramax.

Important fitment note: Match parts to your exact truck, not just the engine badge. Check year, cab and chassis type, and bed length before ordering, and confirm cab and chassis trucks with support first.

Frequently asked questions

What causes misfires on an L5P Duramax?

GM bulletin 23-NA-027 names wear between the injector connector terminal and the injector, on 2017 to 2023 trucks. Check the connectors before replacing an injector.

Does the L5P have the CP4 pump problem?

No. Drivingline reports that the L5P uses a Denso HP4 pump instead of the LML's Bosch CP4.2, and GM's bulletin shows an in-tank electric fuel pump feeding the engine.

Can DEF in the diesel tank damage an L5P?

Yes. GM says many fuel pump and injection component failures showed evidence of DEF contamination, and that misfueling is not charged to warranty.

Is the L5P a good truck to buy used?

The documented issues are limited and specific, and most have a clear check. Run the VIN for recalls, ask for fuel filter records, and scan for codes before you buy.

Do I need a tuner for L5P emissions parts?

Some emissions-related exhaust modifications may require tuning to address check-engine lights, diagnostic codes, or engine calibration. Confirm the full setup before the first bolt comes off.

Next step

Not sure what your L5P needs? Send your year, engine code, cab style, bed length, and current setup, and EngineGo support will help you confirm fitment before you order. You can also browse the L5P Duramax delete kit collection for off-road, race, or non-public-road use where permitted.

Check fitment before ordering. Upgrade with confidence.