What Is Fault Code SPN 4374 FMI 0? (Detroit DD15)

When you own a diesel, one of the things you may notice is that diesel-powered vehicles may have more fault codes than gas-powered ones. One reason for this is that the diesel engine has to meet stricter EPA regulations than gas-powered engines do.

The SPN 4374 FMI fault code means you have an Under Pressurized Def System. You should see the MIL and CEL lights illuminated as well as experience a 25% derate if not taken care of right away. Plus, there are at least 5 sources for this fault code.

To learn more about this fault code just continue to read our article. It has the information you are looking for and places to get more information. Take a few minutes to see how this data can help you when you see this fault code on your dash screen.

Detroit DD15 SPN 4374 FMI 0


The long SPN version for this code is “Under Pressurized Def System: Aftertreatment 1 Diesel Exhaust Fluid Pump Motor Speed Data Valid But Above Normal Operational Range - Most Severe Level; Diagnosed Urea Pump Speed Signal Range Failed High”

The key words in this explanation will be the dreaded ‘most severe’ terms used in the middle of that explanation. This is not a code you want to be fooling with and should take care of it as soon as you can.

There are at least 5 possible sources for this code but the problem is not limited to the upcoming 5:

Restriction in the DEF line between the DEF tank and the DEF pump

Restriction in the DEF line between the DEF pump and the DEF dosing unit

Air in the DEF line between the DEF tank and the DEF pump

Restriction in the DEF pump

Failed DEF pump

Like other fault codes, you will have to connect your DiagnosticLink tool as well as turn the ignition to the on position to start working on this problem. Also, like other codes, you will need to clear any other fault code first before addressing this issue.

The codes you need to watch out for are- PN 4334/FMI 3, 4, SPN 4375/FMI 3, 4, or SPN 3361/FMI 3, 4, 5. There may be other fault codes, but these are the priority ones you should see in many cases.

The Repair Step To Watch Out For


There are up to 17 repair steps that you may need to go through to fully solve this problem and have your vehicle running like it used to. However, there is one specific step you should hope you never get to.

It is going to be an expensive step to take as you have to replace many parts. It is step #6 and here it is in all of its glory:

6. Using DEF test strip A0005850202, check for diesel fuel or oil contamination of the DEF fluid. Does the test strip indicate contamination?

a. Yes; replace the following components and refill the system with new DEF:

DEF tank

DEF tank header

DEF supply line from the DEF tank to the DEF pump

DEF line from the DEF pump to the DEF dosing unit

DEF return line from the DEF dosing unit to the DEF tank

DEF pump

DEF dosing unit

b. No; Go to step 7.

You will notice that you will have many opportunities to finish the repair before getting to this step. You can read all the steps at this link. The last step you should be aware of as well as it is the final one that has to be done to get you back on the road again:

17. Perform Selective Catalytic Reduction Airless Dosing System (SCR ADS) Self-Check Routine. Did the SCR ADS test pass?

a. Yes; clear the codes and release the vehicle.

b. No; replace the DEF pump. Refer to the section "Removal of the GHG14 Diesel Exhaust Fluid Pump Module"

Make sure to go through that data sheet carefully so you can be prepared for what you have to face when this code lights up.

What is The Normal DEF Pump Speed?


Before we tell you the normal pump speed for your DEF pump, we need to remind you that you will need a DiagnosticLink or similar tool to perform any commands related to this section of your engine.

If you do not have that tool, you can either buy one or go to a trusted mechanic who has one. The normal DEF Pump Speed ranges between 750 RPM and 1800 RPM.

If you get this reading and are well within these ranges then you move on to the next step in your repair. If your speed is outside of those boundaries, 1200 and 1800, then you need to call Detroit Diesel and get further instructions.

The phone number for their call center is 800-445-1980. It seems that you may or may not be able to fix this on your own. When checking your pump speed, if you have not encountered any problems up to this point including the speed rating test.

Then you are to do a Diesel Particulate Filter (DPF) Regeneration using the same diagnostic tool you used already. One thing you will find out if you do your own mechanical work is that many of the fault codes have similar troubleshooting steps. They are not all the same but similar in nature.

What is a High DEF Pump Supply Current?


This may not be the same for all diesel DEF systems but the voltage signal between the DEF system and the ECM is 24 Volts. This signal helps the ECM decide if the pump needs to be turned on or not.

But this is not the only voltage you need to be concerned about. The command signal should be at 20 volts and this signal is used to keep the pressure up in the system.

Then the speed sensor rating is between 4.6 to 4.85 volts. This signal is usually pulled to the ground by the pump. To measure these signals, here are the colors of wire that are used for the individual parts in the pump:

Green = on the feed wire

Yellow = on the command wire

Blue = pump Current

Red = the speed signal

There are some images that may help explain everything to you and there are explanations for those mages at this website. We suggest taking a look at those images so you can get an accurate idea of what is going on with the pump when it needs servicing.

The explanation goes into more detail than we can go into here. But if you are not an electronics expert find someone who is so they can explain it all to you in easier terms.

ATS Elect System Problem Detected SPN 4374


If you are not sure what the ATS elect system is, we will give you a short explanation right now. The letters stand for Automatic transfer station and it is a vital link to keep the power running to all modules.

It is connected to both the primary and secondary power sources and when it detects a problem with the primary source, it automatically switches the power supply to the secondary source.

That secondary source can be a UPS or an uninterrupted power supply. It will also start up backup generators until power is restored. When we checked the SPN and FMI codes to see if there was one that mentioned the ATS but they do not.

The closest ones we found were SPN 4374 FMI 11- Voltage Below Normal, Or Shorted To Low Source; Pnv Srl Loop Error Detection and SPn 4374 FMI 19- Def System Is Not Being Controlled As Expected

The only information we have found has already been quoted and linked to above. There is no reference to the ATS system in those repair steps. The best advice we found was for you to maintain your ATS system and the components connected to it to avoid any trouble with this system.

This system will connect automatically and should start your generator when the primary power source has failed. Upon receiving the signal, the generator will start and then send its power through the ATS instead of the primary circuit.

Be aware there are two types of ATS systems, one is automatic while the other has manual controls. The latter is also cheaper.

Some Final Words

Most of the 4374 SPN fault codes have to do with the DEF pump speed. Sometimes it will indicate an error in the electrical system but that warning is related to pump speed.

Without the power, you won’t have any speed. The biggest problem you will have is if there is any contamination in the DEF system. If there is then you are looking at paying a lot of money to replace all the DEF components. Not all SPN 4374 codes have this step included in their repair instructions.

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