What Is Fault Code SPN 2631 FMI 1? (Detroit, Freightliner) (2024)

No matter what type of engine you own, there seems to be a fault code for every problem imaginable under the sun. The problem is finding the right list that contains all those codes so you know what is wrong.

This code means the Engine is in Smoke Mode Control for an Excess Amountof Time Due to Inadequate Clean Airflow into the Engine and you should see the MIL and CEL dash lamps illuminated when it is set. Plus, expect a 25% engine derate.

To learn more about this code just continue to read our article. It explores the topic so you have the information you need to find the right repair for the problem. Even simple codes like this have many steps to completely solve the problem.

Fault Code SPN 2631

What is SPN 2631 FMI 1

What Is Fault Code SPN 2631 FMI 1? (Detroit, Freightliner) (1)

In simple terms, you have low airflow in your engine somewhere. This problem may be due to a leak or a blockage somewhere in your air intake system.

The two places to check first will be the air filter and the air inlet pipe to the turbocharger. If the air filter is dirty, you either need to use high-pressure air to clean it or simply replace the filter.

If the air inlet has a leak or is plugged, then you need to make the necessary repairs which can include replacing the piping. Those are just the first two fixes that may solve the issue.

If they are not the source of the problem, you have about 22 more steps to go through until you solve the problem. If there are other fault codes present when you see this one, those need to be addressed, fixed, and cleared first before tackling this issue.

Another key part that may be causing the problem will be the MCM. This is where you have to use the barometric pressure scale. This reading is important and if your barometric reading in the MCM is not within 6 kPa (1 psi) of the local barometric pressure reading, then you need to replace the MCM.

Barometric pressure readings are important for the intake manifold as well. Your reading needs to be within 10.3 kPa (1.5 psi). If it isn’t, then you have to replace the intake manifold pressure sensor.

There are more steps to follow if these are not the problem, as that is found in step #10. Finding the right source can be time-consuming and frustrating when working on modern engines.

Those steps can be found at this link on the NHTSA gov data sheet.

Freightliner Code SPN 2631 FMI 1

What Is Fault Code SPN 2631 FMI 1? (Detroit, Freightliner) (2)

Some of the steps in the previous section will apply here but may not be in the same order as what will follow in this section. While the different fault lists may have different code numbers, often the repair is the same.

In this case, check the prop valve. It needs to be installed and configured correctly for the system to work and you can clear the code. If it is not installed or configured correctly, that is your first repair.

At the same time, you should be inspecting the turbo inlet seal, intake manifold, and EGR hoses and clamps to make sure there are no leaks or other damage to those parts. You may have to perform a leak test to make sure those parts have no problems.

The advice for this CAC inspection and leak test is to follow the instructions in the OEM literature. If the parts pass the test, then you move on to the next step but if they fail, then you need to make the appropriate repairs.

You may have to perform an air mass adaption to solve the issue. If that doesn’t work keep moving to the next step till you are done. You will find more steps at this link in the bold section. Unfortunately, the mechanic doesn’t know that paragraphs are their friend.

Detroit Code SPN 2631 FMI 1

What Is Fault Code SPN 2631 FMI 1? (Detroit, Freightliner) (3)

This code’s definition or description is the same as for any other engine in this article. It indicates a low air flow problem and there is a troubleshooting section found at this link to help guide you through the repair.

That link takes you to the Detroit manual, section 97.1.1 and the heading is low air flow. The first few instructions are the same as we reported above. This is a common fault code and the repair or sources for it will be common and not unique no matter which motor you own.

Here are a couple of steps we have not quoted and you can see why making these repairs is so time-consuming and frustrating:

6. Check EGR valve operation. Using Activate Outputs Service Routine, command PWM1 to 50% while monitoring EGR actual position. Does EGR actual position read between 47-54%?

Yes, go to the next step.

No, go to step 8.

7. Command PWM1 to 90% while monitoring EGR actual position. Does the EGR actual position read between 87-94%?

Yes, go to step 12.

No, go to the next step.

You have lots of steps to follow to make sure you have repaired the problem. This is not a difficult problem to solve but it does take time to get to the source of the initial options are okay.

The difference between the Detroit manual troubleshooting list and the Freightliner troubleshooting list is about 5 steps. The Detroit Diesel manual has about 5 fewer steps to take to resolve the problem.

Some Final Words

While the lists may be different, the problems they cover seem to be unlimited. The current fault code lists cover just about any problem engine and other builders could think of.

Besides having so many fault codes, the biggest problem is that these lists are not complete and many omit the code you may be looking for. That is part of the time-consuming work of repairing modern engines.

It is also part of the frustrating aspect of modern engine repair.

What Is Fault Code SPN 2631 FMI 1? (Detroit, Freightliner) (2024)

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