What Is Fault Code SPN 3556 FMI 18? (Cummins, Freightliner)

When you do your own research on fault codes, you will find some extra letters after the FMI portion of the code. These letters, EPA and GHG do have meaning when you see them on the data sheets when you are looking up specific codes.

The SPN 3556 code has to do with regen. No matter what FMI code follows it, you are looking at a problem with the regen temperature. No matter what, you will have to attach the Diagnosticlink code reader to start your repair process.

To learn more about this code, just continue to read our article. It explores the topic so you have the best information possible to make your repairs. Getting back on the road quickly and back to normal is the ultimate goal.

What is Fault Code SPN 3556?


This code is all about regen temperature. If you get SPN 3556 FMI 0 then that code means- Regen Temperature - Out Of Range High. If you get SPN 3556 FMNI 1 then the opposite will be true. That code simply means- Regen Temperature - Out Of Range Low.

There should be no impact on the engine performance if you do not get SPN 3556 FMI 0 GHG 14. If you get that code, then you can expect to have your engine derate 25%.

In all versions of this code, you should also see the MIL and CEL lights illuminate on your dash. For all codes the source for this code being set is:

This code sets when the exhaust temperature does not increase above the modeled threshold when the low temp regeneration is enabled." (source)

The good news is that these different codes only have 7 steps to resolve the problem if you do not do it in under 7 steps. The first two steps will always be the same, connect the Diagnostic link and turn the key to the on position.

If you are getting FMI codes, for example, 3 & 4 which are not on the official SPN list, then just read the FMI code list to see what the possible problem is. For Mack trucks, you may get #4 and that means your problem is- Voltage below normal or shorted low. FMI 3 is the opposite of that problem.

SPN 3556 FMI 18

This problem is said to be “Doc Outlet Temp Low (low Temp Regen)” however, nothing that looks simple turns out to be simple. In our research, we found that there were 2 listings for the code.

One had the letters EPA following the FMI code and the other had the letters GHG following the FMI code. Does this make a difference? It is possible that it does even though the problem is the same, comes from the same source, and has the same repair steps.

Now we will explain the difference between EPA and GHG later after we finish dealing with this code. There is a difference in those letters but those letters do not seem to affect the SPN code that much.

Step three in this repair is to fix all other codes first then tackle this one. Then you have to check for HC doser fault codes and if they are set, you have to handle those before going to repair this code.

The source for this problem is here “Low temperature regeneration enabled, 1100 to 2050 rpm, 50% to 100% engine load” Since the steps to fix this code are the same for the Freightliner trucks, we will highlight a couple of important steps in the next section.

Freightliner SPN 3556 FMI 1


This code also comes in two formats. One will be the EPA after the FMI code and the other has GHG after the FMI code. We do not see any difference between the codes being set, their source, or their repair from any other code or from each other.

When you have done your repairs, a parked regen is your verification process. But the important steps are #5 & 6 and they read:

5. Perform the low temperature ATD regen; Refer to section "EPA10 Perform Performance Check - Low Temperature

ATD". Go to step 6.

6. After the low temp ATD regeneration has run for 20 minutes, monitor the DOC outlet temperature sensor reading, Is the DOC outlet temperature sensor reading within 25°C (45°F) of the other exhaust temperature sensor readings?

a. Yes; Go to step 7.

b. No; replace the DOC outlet temperature sensor. Go to step Removal of the EPA10 Diesel Oxidation Catalyst Outlet Temperature Sensor. Verify repair.”

Step 7 only has you repairing any exhaust leaks that may be there and if there are not any exhaust leaks then you are to “No; replace the HC doser block. Refer to section "Removal of the Hydrocarbon Doser Block ". Verify repair”

SPN 3556 FMI 2

It may be difficult to find a lot of information on this specific code. It does seem to be one of the less common ones as it is not listed on the official SPN fault code list.

What we have been able to find out is that there is a leak in the 7th injector. Or it is indicating a leak in the 7th injector and could be a false reading. The AFT fuel pressure sensor may be faulty or it could be a defective 7th injector.

That injector could be stuck open and you may have to clean the injector to get it working again. After cleaning, you should try to do another regen to see if the code is cleared and the problem is resolved.

While there are 21 FDMI codes approx. most of them do not show up in any discussion or repair request. One example is SPN 3556 FMI 17. We only found one reference to that fault code.

It has to do with a spark plug but that may just be a side issue. The real problem may be soot build up in the inlet section of the DPF filter. However, this may be a CAT only fault code as this code appeared on a C-13 engine.

SPN 3556 FMI 5 Cummins


The Cummins equivalent code is 1977 and you should see an amber light illuminate when this code is set. You will find the code on page 9 of the complete Cummins code list we published in another article.

What the code sheet says is- Aftertreatment Hydrocarbon Doser Aftertreatment Doser Circuit - Current below normal or open Circuit.

Those words tell you that you may have a loose wire, corroded connection or damaged wires, or other components. You would have to do a visual inspection to see where the problem lies.

The location of the hydrocarbon doser is on top of the exhaust pipe right behind the turbocharger.

SPN 3556 FMI 7

The description for this code is very specific and it says:

If the fuel lines are not routed correctly the AFI Fuel Supply line can rub through from the fuel system return line and cause regen issues or an inability to perform regens. Due to the ECM sensing a pressure issue in the line, it will stop sending fuel to the AFI from the DSI so you will not see a fuel leak in the area. What you will see is a regen issue and after treatment temperatures not reaching the correct temp and eventually, the ECM will abort the OBFCT.” (source)

The repair is to inspect the line and take the right steps to keep it from rubbing against any other part in the area. That link will provide some excellent photos for you to look at so you know where to look and what needs to be done.

The Difference Between GHG And EPA


We mentioned these letters a couple of times earlier and you may not think they have any significance. However, those letters do mean something and while the repairs and the sources are the same, the impact on the environment is a bit different.

What these letters stand for are- GHG- greenhouse gases & EPA- smog-forming pollutants. While it is hard to see the difference there seems to be one. Here is what is involved when you see the letters on a data sheet:

1. GHG

- Greenhouse gases (GHGs) are emitted from the tailpipes of cars and trucks that combust fuel.

- Once GHGs are released, they can stay in the atmosphere for 100 years or more.

- GHGs act like a blanket around Earth, trapping energy in the atmosphere and causing it to warm. This can change Earth's climate, raise sea levels, and result in dangerous effects on human health and welfare, and on ecosystems.

2. EPA

- Cars and trucks that combust fuel also emit smog forming emissions, such as nitrogen oxide, non-methane organic gases, carbon monoxide, particulate matter, and formaldehyde.

- These emissions are usually trapped close to the ground and can form a brownish haze that pollutes our air, particularly over cities in the summertime.

- Smog can make it difficult for some people to breathe, triggering lung diseases such as asthma, emphysema, and chronic bronchitis, which may lead to premature death.

Both sets of letters have to do with different gases and other pollutants which is why there are so many new parts on modern engines that were not there 40 to 50 years ago,

The History Of Each Standard

1. EPA

This came first with the passage of the Clean Air Act (CAA) in 1970 and for 20 years this act focused solely on nitrogen oxide. Then in 1990, the Act was amended to include the following pollutants:

- Non-methane organic gases (NMOG)

- Carbon monoxide (CO)

- Particulate matter (PM)

- Formaldehyde (HCHO)

For a while, there was a double standard with cars and light trucks having to meet different regulations. But that changed in 2000 when Tier 2 regulations were imposed. Right now Tier 3 is being phased in.

2. GHG

The EPA governs both sets of letters and in 2010 the new greenhouse gas regulations were created. There was a 2-year grace period and in 2012 auto and truck makers had to meet those new regulations.

However, in 2012 the GHG regulations were revised and they were revised once again but more recently:

In December 2021, EPA finalized revised national greenhouse gas (GHG) emissions standards for passenger cars and light trucks for Model Years 2023- 2026. The final standards would achieve significant GHG emissions reductions along with reductions in other criteria pollutants.”

This is why you will be seeing the letters EPA and GHG on the different data sheets. There are different fault code numbers attached to both letters, for example, EPA 10 or GHG 14, but so far we have not found any explanation for those numbers.

Most recent cars and trucks are to meet Tier 3 regulations while older vehicles can meet either Tier 1 or Tier 2 regulations. It all depends on their age. Tier 3 was implemented in 2017.

The classification for different cars depends on which one the automaker chooses to certify their car. These classifications are called ‘bins’ and if the car is certified for Bin 50 then its emissions cannot exceed the standard set for that bin.

You can get more details at this link and there will be a couple of tables showing the standards for the individual bins. All of our information for this topic came from that link as well.

Some Final Words

While the letters EPA and GHG are attached to this fault code, the repairs are very straight forward and easy to follow. Those letters do not affect the source of the problem nor do they affect what kind of repairs you need to do.

When you are done making those repairs, then your emissions should meet the EPA and GHG standards for your vehicle. Everything gets more complicated as time goes on. This takes the fun out of engine repair.

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