What Is Code SPN 1548 FMI 5? (Freightliner HVAC Duct Temperature)

That is the trouble with the modern error or fault code system. There are far too many codes and when you add in the FMI subcode, the number balloons even more. Finding the right answer is not always going to be easy especially when the codes are not placed in numerical order.

When you see this code, it is supposed to be for the HVAC-Front Control Unit. What it is telling you is that there is a short to battery or open circuit problem with the COTC sensor. When you check the SPN code list it says- HVAC Duct Temperature.

To learn more about this code and how it applies to your vehicle, just continue to read our article. It has the information you want to know about so you can get to the heart of the problem quickly. Once you know the source of your troubles, you can find a solution a lot better.

What is Code SPN 1548 FMI 5?


This code is telling you that there is a problem with your HVAC or air conditioning system. It could be the result of a short or open circuit connected to your COTC sensor and that may be the first place to look.

At the same time look at your wiring harness and connector pins to see if they are connected properly and are without any damage. When you inspect these parts, you should be thorough as corrosion and other surface damage that causes an open circuit are not always easy to see.

COTC stands for constant outlet temperature control which is supposed to discharge the air out of the ducts. What this system does is bring in the air from another source, and then run it through the evaporator coils if you want cool temperatures.

If you want hot air, then the control system routes the air through the heater core to heat it up for you. Then, as usual, make sure to clear away the other fault codes if there are any before you solve this one.

The fix may be simple if it is the parts that were described earlier in this section. But get a good technician to double-check any work you do.

SPN 1548 FMI 5 Freightliner


We have talked about the SPN side of this fault code, and you will need to know what the FMI side of the code is referring to. The SPN fault code is for the HVAC Duct System and that system, as you know is more electrical. It handles the cooling and heating of the air in your cab or sleeper.

The FMI side of the code is the specific problem, usually, and tells you exactly where to look. In this case, FMI 5 is saying that the Current Below Normal Or Open Circuit.

If it is just below normal then you may have a deeper problem like the evaporator or the heater core. But if it is just an open circuit, then you need to check the sensor, the wiring harness, the connections, or all three first.

It is not hard to find an open circuit but if the wiring harness wires are in top shape, and the connections are clean, then you know it has something to do with a faulty sensor.

But if the sensor checks out and is operating correctly, then you may need a skilled technician to diagnose further to see what the problem is. You can check the two filters and the brushless motor that are used to clean and move the air to your cab or sleeper.

SPN 1548 HVAC Duct Temperature


This is a step-by-step fix and some people jump steps when they do not have to. For example, one owner changed out the duct sensors, two of them, but the code kept appearing.

If you do that and get the same result, the code is actually referring to the evaporator temperature sensor. That may be the part that is bad. This sensor is found on the evaporator.

If the code is still showing, then the problem may be with the amount of voltage the sensor is receiving. It operates on the 5-volt scale and if it isn’t receiving 5 volts, it will trigger this code.

If it isn’t receiving the 5-volt electrical supply, then you may have a problem with the module or the wiring to the module. That means you may have to replace them or repair those parts.

If the code is still triggered, then you may have a problem with a broken tab in the harness connector. The broken tab will let the wire or connector fall out which will trigger the code.

Fix the tab and the code should go away. As we said, this is a step-by-step process and when you replace one part and the code is still there, you need to move on to the next likely source until you finally find the problem.

SPN 1548 FMI 4


You already know what the SPN 1548 fault code refers to, the HVAC duct system, now you need to know what the FMI 4 stands for. This specific subcode refers to FMI 4 - Voltage Below Normal, Or Shorted To Low Source.

That is another electrical problem that may or may not be within your skill set to work on. You should test the circuit to see if 5 volts is making it to the parts that require 5 volts.

You may have a short somewhere in the wiring that is connected to the low source. That is the place for you to start. Check your wires to make sure none have frayed, bent, or broken, or even crossed with another wire.

Also, make sure to use a multimeter to check the voltage. 4.99 seems to be an acceptable reading. There are not a lot of people talking about the FMI 4 fault code at this time.

It may be a rare code that most people will not see or they just know how to fix it and do not talk about it on the different websites. Even Detroit Diesel has not put out a data sheet on this code that we were able to find.

If you do get this code and can’t fix or find it, then go to a mechanic who specializes in vehicle electrical systems. They should be able to take care of it for you in a hurry.

How do You Reset The Heater Control on a Freightliner?


The process or procedure for this is not that hard. There is a very good and short YouTube video on this topic which we will quote from. If you need to see images to make sure you get it right, then click on this link.

The first thing to do is to set your three dials to the last option on the left or right-hand side of the dials. That would be 0 for the first dial, all the way to extreme heat on the middle dial, and to the picture of the person with the arrow pointing to its chest on the third.

It is the last position on the third dial going to the left. In the center of those dials, you will find a button on each one. You press and hold the buttons on the first two dials, coming from the left, for 5 seconds.

There will be two lights that start blinking after that. Release the buttons and wait for the blinking lights to go out. When they do, the HVAC system should be reset.

This was for the 2013 Freightliner Cascadia so check your owner’s manual to see what the process is for your Freightliner vehicle. It may be the same as this one but things change over time.

Outside Air Temperature Sensor Resistance

One mechanic talked about the proper resistance readings for the COTC sensor. These readings do have some diagnostic importance so we decided to place the table here so it is handy and easy to read

Temperaure F Temperature C Resistance minimum Resistance Maximum (ohms)
-40 -40 97894 103950
14 -10 16285 16849
23 -5 12485 12839
32 0 9656 9890
41 5 7480 7708
50 10 5852 6054
68 20 3659 3815
77 25 2922 3060
95 35 1905 2003
104 40 1551 1637
113 45 1270 1344

** information is taken from this web page- https://f01.justanswer.com/HeavyEquipmentTech/1012096c-28ab-477a-a7d9-5d5e08085990_cotc_resistance_values.png

Some Final Words

This fault code is not a mechanical problem although some mechanics are involved. Make sure to have a multi-meter handy to check the voltage if needed. But before you use the meter, make sure to check your wires, connectors, and similar parts to see if they are the problem.

Those tabs in the harness can be sneaky and break without revealing their position. Make sure to be thorough in your investigation so that you do not miss any broken tabs.

We suggest checking those first so you do not replace any parts unnecessarily

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