3 Most Common L5p Duramax Problems (Easy Solutions)

The most common problem with L5p Duramax I can tell from my personal experience is MAP sensor failure, Allison 1000 transmission issue, and fuel injector failure. If you face any of them while using l5p, have a little patience and a little trust in me, and I will tell you a quick fix for each problem.

For the MAP sensor issue, you have to clean it frequently and keep it tidy all the time. The Allison 1000 transmission is costly and very little you can do about it. The fuel injector failure may happen because of injector number 4 or a connector issue.

I further elaborated on the solution to the 3 most common l5p Duramax problems in this article. And a few FAQs that I frequently get asked by truckers. 

3 Most Common L5p Duramax Problems (Easy Solutions)

3 Most Common Problems with L5p Duramax

The latest version of the Duramax V8 diesel engine, the L5p Duramax, was introduced in the year 2017. It was introduced as an LML replacement. This fine engine, in my opinion, is the strongest pickup truck engine from GM.

You may face three commonly reported problems that are elaborated on below;

#1- MAP sensor failure

The MAP sensor, also known as the manifold absolute pressure sensor failure, is a common issue seen in l5p Duramax. Before I get into the problem, let me explain what a MAP sensor does first.

The MAP sensor sends the engine computer to send more fuel when air rushes into the intake manifold through the open throttle. The rushing air causes an airdrop in pressure. And when the throttle closes, air pressure rises, and MAP sense then signals the engine computer to reduce oil flow to the engine.

The l5p uses a manifold MAP sensor inside the intake manifold. This MAP sensor is responsible for sending air pressure readings and ECU vacuum to the engine computer, which determines how many fields should be sent for combustion.

The MAP sensor is crucial for determining when ignition would occur within the engine.

Like any other engine component, a MAP sensor can get blocked or sledged by oil, dirt, or grime. And when the MAP sensor gets slugged, it sends the wrong signal to the engine’s computer.

The wrong signal of air pressure readings and inaccurate vacuum signal effects the oil flow sent to the engine for combustion. Inaccurate oil flow sent to the engine will not only cause poor performance but also damage the engine greatly.

How To Know If l5p Has MAP sensor failure?

There are some symptoms that indicate a bad MAP sensor. Pay attention to the below symptoms and see if your l5p Duramax has them or not:

  • Engine misfires
  • Lack power
  • Lean/rich air to fuel ratio
  • Uneven idling
  • Bad performance
  • Engine hesitation, stalling, etc

#2- Allison 1000 Power Limitations

The l5p Duramax crate engine is a 6.6L design. To increase power if you go to transmission, you will face issues. This is not a fault of the engine, but if you try to modify it and add more horsepower, the transmission will struggle. Allison 1000 transmission on your l5p is the most powerful Duramax transmission you can have.

Now the issue is the stock transmission is not capable of handling 500whp. If you want to add even 600whp, the modification required to do the transmission will be quite costly.

#3- Fuel Injector Failure

Fuel injector failure is not that common, but it does happen occasionally. The connector for fuel injector number 4 is the known injector failure in l5p Duramax.

This injector “4” sometimes doesn’t function properly and sends code P0204 code to l5p. The l5p goes into limp mode after getting the code.

The engine will start to shake unnaturally, and a few engine lights may appear, like ABS and StabiliTrak.

Symptoms you should look for:

  • P0204, P020D, and P0300 engine codes
  • Limp mode
  • Lights turning on like- StabiliTrak, ABS
  • Engine misfire
  • Uneven idling
  • Bad performance
  • Shaky engine

Apart from these common three, there used to be another common issue of transmission cooler lines rubbing, which got fixed in 2017+ GM Duramax.

How do I Fix L5p Problems in the Duramax engine?

Now that we have talked about the issues, let’s talk about the fixes or solutions to those problems.

How to fix MAP sensor failure?

If you replace the MAP sensors, the problem is most probably going to repeat itself. To solve and prevent the issue, you have to keep your map sensor clean. If you don’t constantly clean it, even a new one will show the same problem.

A MAP spacer prevents soot build-up by spacing it away from the EGR stream.

So my solution or suggestion would be:

  1. Use MAF sensor cleaner to clean MAP sensor regularly.
  2. Consider installing a MAP spacer.

How to fix Allison 1000 issue?

Unfortunately, the cost here is difficult to cut down. Transmission to Alisson 1000 is costly, but I have a suggestion to handle this issue. That is if you don’t go overboard with upgrading l5p Duramax specs.

To balance it out, you can make other upgrades that work out cheaper in the long run. It includes a multi-disc torque converter, upgraded trans coolers, a hydraulic upgrade, and any friction material. If you hold back on these upgrades for a while, trust me, you won’t have any issues, and it will help you with your budget.

How to fix fuel injector failure?

It is possible that injector 4 may have an issue, but I have seen multiple times the connector is the real issue. So I recommend, from my experience, getting a quality connector first. For a quality connector, you can go with a gold-plated one or an LBZ connector.

Frequently asked questions (FAQs)

1. Is the LML better than the L5P?

L5p is a new version and replacement for LML. Even though l5p has a 6.6L design just like its predecessor, it still comes with multiple enhancements over the LML.

A few enhancements l5p has over LML are- a stronger cylinder block, a new camshaft design, better glow plugs, a superior engine oil cooler, and an enhanced EGR system, and all major items are covered below. There are no l5p injection pump problems to mention, all issues regarding the injection pump from previous engines are fixed in l5p.

2. Does the L5P have a lift pump?

Yes, l5p has a lift pump. For over 15 years, Duramax trucks didn’t have any lift pumps. Finally, the l5p, which is a replacement for LML, comes with a built-in lift pump from the factory.

3. Is the L5P reliable?

The l5p is the most reliable and considered the best post-emissions Duramax engine. It is a replacement for LML and has multiple upgrades to put itself above LML. As a new and upgraded version, l5p is very reliable.

Bad Fuel Injector Symptoms >> Check out the video below:


There is no doubt that l5p is the best and most reliable GM engine. I hope the 3 most common l5p Duramax problems I mentioned above and provided a solution that fits will help you in the long run. Just because I mentioned a few issues don’t mean it will happen to your engine. These issues, especially fuel injector ones, very rarely occur.

If I speak honestly, there is not much to complain about l5p. I would like to finish it off by saying, if you take care of your engine, it will take care of you. Even the best engine will show issues if not taken care of properly.

Check out these helpful Duramax-related posts below!

Who Worked on This post?



Anderson is an experienced auto mechanic with over 10 years in the industry. He is skilled in diagnosing and repairing a wide range of vehicles, from sedans to trucks and SUV

Syed Ahmed


Syed Ahmed has been working as an auto mechanic editor for the past five years. He has a background in mechanical engineering and a love for all things automotive

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