What Is Fault Code SPN 1231 FMI 9? (Paccar, Detroit)

When you see one of the myriads of SPN codes pop up on your screen, you know it is not going to be good news. While some of the news from SPN codes is not that bad, generally, most of them are. Once you look up the code, it may ruin your day.

The SPN 1231 FMI 9 error code has to do with communication. It is a failure to communicate and the Motor Control Module (MCM) Stops Receiving Messages from the Aftertreatment Control Module (ACM). This may mean replacing some key parts.

To learn more about this code and what to do about it, just continue to read our article. It explores the topic so you have the best information possible. With that information, you should be able to handle the repair quickly and efficiently

What is The Diagnostic Code SPN 1231 FMI 9?


This code is dealing directly with the Motor Control Module (MCM) Stops

Receiving Messages from the Aftertreatment Control Module (ACM). There can be different sources for this code being set on your dashboard screen. Here are some of those sources:

- Over/under voltage supply to the ACM module

- Fault set during a reprogram event

- Loss of power to the ACM

- Loss of ground to the ACM

- Loss of ignition switch power to the ACM

- Open in the CAN-H/Can-L communication circuit to the ACM

That is the good news when you see this code. The bad news is that it can take up to 24 steps to fix this problem. You can read all those steps at this link. In some cases, it just may be a software issue and if you flash the ECM, update it, and you may be able to solve the problem.

Or, you should check your wire harness. If it has been rubbing on something above the transmission for some time, the wires or connectors may have become corroded.

That may take some time to track down, clean it up, and get everything reconnected again.

Freightliner SPN 1231 FMI 9


The above seems to be the main problem for the Freightliner series of trucks. It seems that the wires will rub and chaff a lot and that is due to the vibration of the vehicle. The vibration also helps dirt and dust to accumulate inside the harness.

When it does, then corrosion can start taking place and do its dirty work. With the vibration and collection of dirt, etc., water finds its way inside the harness and gets to the wires and connectors.

After a while, the corrosion will start. Then a secondary location for this corrosion can be found in the 21-pin connector. That is the problem. The other issue you will come across is that the SPN 1231 FMI 9 fault code will be set for the MCM unit.

You should get a second fault code for the CPC unit and it is not going to be the same numbers. It reads SPN 625 FMI and so far we have not come across any Cummins equivalent fault code.

Here is where some good news comes in and is appreciated. All you need to do is wrap some anti-chaffing tape around the wires and harness to stop the problem. Or you can replace the wire harness with a new one.

There is one thing to know about replacing the harness:

the current production and service ATS harness is P/N: A4721504320. It superseded former P/N: A4721503620 with approximate ATS S/N: 1240000769034 at the truck plants in July, 2017” (source)

If your vehicle is older, you will need those numbers to help you find the right replacement. Also, when you put the new harness in, make sure to wrap it with anti-chaffing tape so you do not have to do this repair any time soon.

We would advise going through the information at those links because they contain diagrams and images that will be very helpful to you.

SPN 1231 FMI 9 Paccar


This code may be the same as the Freightliner and Detroit explanations and descriptions. It is hard to say as Paccar has had the reputation of not being very helpful.

They do not like sharing their fault codes with anyone and that has caused a lot of problems for their customers who bought Paccar vehicles or engines. One reason for this lack of communication is that PAccar uses P and U then a number for their fault codes.

The U portion is for communication devices on your engine but it may not be for your specific model. The codes we have been able to find are for the Paccar MX13 EPA13. You can find those codes at this link.

That link has two more codes for the following Paccar engines or vehicles- the MX 13 EPA 10 & the MX 13 EPA 17. There are long lists but you won’t get much of an explanation for those codes on those webpages.

The customer service and technical help at Paccar have also been less than stellar. But this problem took place in 2015 and hopefully Paccar has changed its attitude and becomes more helpful.

So far, we have not found much information about Paccar or this code. It seems that they do not release a lot of information for the public to view. As of 2020 things have not changed and Paccar owners are very frustrated.

Some Final Words

If anti-chaffing tape is all you need, then that is good news indeed. Compared to some SPN 1231 FMI 9 repair costs, that is getting off very cheap. Some owners have paid over $15,000 to repair their Paccar so the tape is really a bargain.

Do some research to see if you can find Paccar information to make your repairs less costly and easier to do. Freightliner seems to have its act together and should save you a lot of time and money.

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