What Is Fault Code SPN 3238 FMI 2? (Detroit, Freightliner)

When you start exploring the world of fault codes, you will find that different engine and other manufacturers have their own code lists. This just adds to the confusion especially when they do not line up with the SPN FMI codes. You need different lists to stay on top of your problems.

The Spn 3238 Fmi 2 problem code is about Nox Out Sensor Not Ready After Dewpoint Enable Check: Aftertreatment 1 Exhaust Dew Point Data Erratic, Intermittent Or Incorrect. The last 5 words are from the FDMI portion of the code.

To learn more about this code and what to do to fix it, just continue to read our article. It has the information you want to know about so that you can understand what is taking place and get to a mechanic as soon as possible.

What is Code SPN 3238 FMI 2?


This is what the code is saying when it is set on your dashboard: Nox Out Sensor Not Ready After Dewpoint Enable Check: Aftertreatment 1 Exhaust Dew Point Data Erratic, Intermittent Or Incorrect; Mu_nox_out_rc.

Unfortunately, the SPN FMI code list does not provide any information on the repair of this problem. It just tells you what the code is for.

The fix is not that complicated. When you monitor the following areas of your engine:

ASL102 engine speed

AS018 Selective Catalyst Reduction (SCR) inlet temperature

AS019 SCR outlet temperature

AS035 SCR inlet NOx sensor

AS036 SCR outlet NOx sensor

AS101 NOx conversion efficiency

You also need to do a parked regen and if the NOx conversion efficiency rises above 70% simply clear the codes and go on about your business. If it doesn’t get above 70% then you need to replace the SCR outlet NOx sensor.

That is not a bad repair to do as others are far worse and far more expensive. When you are doing the forced or parked regen, make sure not to do it in the area of combustible materials. The exhaust is very hot and a fire could possibly break out.

SPN 3238 FMI 2 Detroit


When you see this code pop up on your dash and you are driving a Detroit Diesel 15 the first thing you need to do is connect Diagnostic Link to the computer on your vehicle.

If the codes SPN 5357 FMI 16 or 18 are present as well, you need to fix and clear those codes first before working on this one. The repair is the same as described above.

When you are doing the replacement you are to follow the steps set out in the Removal of the EPA10 Selective Catalyst Reduction Outlet NOx Sensor" section. There are specific instructions to follow when working on this area of your engine.

Now if you replaced the sensor or sensors, and the vehicle is not taking the parked regen, you may do a forced regen. But, when you do that you may find that the exhaust manifold will get red hot.

If this happens to you, you could have a blockage in the DPF or the SCR. You may also have a bad turbo. If you are not getting to the proper exhaust temperature to do a parked regen, then you may have an exhaust leak or a faulty turbo.

If the problem persists and there is no real reason for the red-hot exhaust manifold, you will have to check out those parts first before searching for another problem.

Freightliner Cascadia Code SPN 3238 FMI 2


We found nothing different from the above information for this type of vehicle. In fact, the first subsection is for a Freightliner Cascadia and the fix is going to be the same for most vehicles in that category.

Just monitor those systems mentioned above and if they do not get above that 70% threshold, then replace the parts mentioned. As usual, do not do the parked regen anywhere near items that could catch on fire or explode.

This code does not seem to be a serious one as one owner was able to drive for 5 months with the code still set and the orange light staring at the driver. He also reported great fuel efficiency reaching almost 10 mpg in the mid-west region.

But we do not recommend that type of action for everyone. That may be an exception rather than the rule. When you see the code, just let your mechanic handle it as soon as you can.

Another person stated that the code will trigger a derate. But we have not seen any evidence to support that statement. If it does happen to you, you may have more problems than just that one code.

Some Final Words

Repair work is supposed to be made easier by the implementation of all these fault codes. The problem is that if you cannot afford the code reader, you may have to use a mechanic who owns one. That can get expensive.

With all the modern technological advances that have taken place over the years, it is difficult to be a DIY mechanic. You may have to rely on professional mechanics.

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