What Is Code SPN 171 FMI 3? (Cummins, Freightliner, Detroit)

That is the way it is with SPN FMI codes. Some may have only one meaning but others, like SPN 171 code has over 10 different applications. It is one of the more populous fault codes on the SPN list.

The first meaning of this code is- “Ambient Air Temperature Circuit Failed High” and the second definition is the same except it leaves out the word ‘air’. The word ambient means ‘surrounding’ or ‘encircling’ and in this context, it usually refers to the outside air around your vehicle.

To learn more about this code and how it applies to your driving situation, just continue to read our article. It has the information you want to know about so when you see this code, you know what to do. Delaying any repair is not healthy for your engine, transmission, or vehicle.

A Mystery Solved


We have talked about the official SPN code list and how it is missing certain codes. That can be frustrating when you need to know what the code you are reading means. You may not have time to search for those missing codes.

It seems that the reason there are missing codes from the official SPN fault code list is that it only records the codes for three main manufacturers- Detroit, Freightliner, & Cummins.

Unfortunately, that missing information also includes FMI codes. Fortunately, SPN 171 error code has over 10 FMI codes listed and this is one of them. Before we get to what they mean, here is an image that lets you see where all the modules are located.

This should help you find the trouble spot quicker and get to the problem a lot faster.

Code SPN 171 FMI 3 (truck)

What is Freightliner Code SPN 171 FMI 3?

The SPN 171 code has 14 FMI codes listed. At least all the ones listed for those three manufacturers mentioned earlier.

The trouble with this list is that the larger numbers are placed first, with the smaller 3-digit codes listed after them. You have to scroll a lot to get to the 171 code. What the SPN 171 FMI 3 code means is as follows:

- Ambient Air Temperature Circuit Failed High

- Ambient Temperature Circuit Failed High

- Ambient Temperature Circuit Failed High: Ambient Air Temperature Voltage Above Normal, Or Shorted To High Source; Dm1 From Cpc Contains Spn 171 Fmi 3

- Outside Air Temp Sensor Voltage High

- Sensor Error Scr-system Environment Temperature Dpf-system Air Inlet Temperature, Signal Range Check High

What all this means is that you may have a short or an open circuit in the ambient temperature system. There are about 13 steps you have to take to make sure you resolve the problem.

You can get to those steps by clicking this link. The job of the ambient temperature sensor is to monitor the incoming air temperature to the turbocharger’s compressor inlet.

This sensor needs 5 volts of electricity to function and when it says it failed high, it means that more than 5 volts are coming to the sensor. When you see this code, you may have to replace the sensor, the CPC harness, or the CPC itself.

SPN 171 FMI 2


This specific code only comes with 2 possible problems. The first is- Ambient Air Temperature Not Plausible: Ambient Air Temperature Data Erratic, Intermittent Or Incorrect; Mu_isp_t_a_air_rat and the second is- Ambient Temperature Sensor Data Erratic.

These codes are for the ambient temp sensor and if you are not sure where it is located, its usual position is behind the right front bumper. This component is a 2-wire set up that sends signals to the ECM.

The first step in this repair would be to make sure the CPC parameter group PRG031 is correctly configured. If not then set the PRG031 Ambient Air Temp Sensor Enable is set to not available or not installed, whatever the words are that show up on your screen.

To fix this sensor, just follow these instructions:

2. Disconnect the ambient air temperature sensor.

3. Turn the ignition ON.

4. Measure the voltage between pins A and B on the harness side of the ambient air temperature

sensor connector. Is the voltage between 4.5 and 5.2 volts?

[a] Yes; replace the ambient air temperature sensor.

[b] No; restore voltage between CPC connector #3 pins 2 and 15 and pins A and B on the harness side of the ambient air temperature sensor connector. (source).

This does not seem to be a difficult fix and if you need a diagram, one is at the link at the bottom of the page after step 4.

SPN 171 FMI 5

With 14 FMI codes, one would expect to see #5 on the list. But for some reason, the people who compiled that list saw fit to leave that FMI 5 code off of it. This code may not be hard to figure out.

What we do know about SPN code 171 is that it deals with ambient temperature sensors. Then FMI 5 on its own stands for “Current below normal or open circuit”

This means you probably have a short in the system or an open circuit somewhere. Doing some testing with your multimeter should help you find the problem.

We cannot be sure in this case as no one is talking about this specific code. You may have to contact a qualified mechanic who can help you with repairs or at least some instructions to resolve this problem.

One point of interest is that while we said the usual spot is in the right front bumper. That may not be the case for all makes and models of trucks, etc. One company decided to place the sensor in the driver’s side mirror.

While that seems to be a simple and convenient location, the word is that it is not either.

SPN 171 FMI 13


This code also has 2 possibilities. The first is - Ambient Air Temperature Over Ptcan Is In State Sna: Fmm_as_et_test2 Condition Exists; Ambient Air Temperature Reported Part Of Cpc_c08 Is Equal To Signal Not Available.

The second will be- Ambient Air Temperature Signal Not Available Via Can. As you can figure out there is a loss of communication between the sensor and the ECM.

The fix for this problem seems to be to replace the sensor. It may have failed on you. Then after you replace the sensor you will need to clear the code. We wish we could be more informative but this is another code not many people are talking about.

The one mechanic who did, did not provide a lot of information. Check the ambient temperature sensor to make sure it has failed before you go out and buy a new one.

Or go get a second opinion as we have little to no confidence in the mechanic that gave that advice. You may experience some performance issues when you get this code on your dash.

SPN 171 FMI 19

This code means- Ambient Air Temperature Over Ptcan Is In State Err: Fmm_as_et_test2 Condition Exists; Ambient Air Temperature Reported Part Of Cpc_c08 Is Equal To Error.

There is only one definition associated with this error code so it stands to reason it will be easy to diagnose and repair. It may be as the ECM is getting a false signal.

According to one mechanic, the repair steps are the same as listed in SPN 171 FMI 2 above. At least from steps 2 to 4.

Where is The Ambient Air Temperature Sensor?


So far, we have come across three different locations for this component. The location may depend on the brand that built your vehicle. For example for Freightliner, the sensor is located under the right front bumper.

For a Peterbilt truck, the sensor is supposed to be installed in the driver’s side mirror. This is one where it may be really difficult to remove and replace. The third location we have come across did not state which make or model the vehicle was.

All the information said was that it was located near the radiator. It was supposed to be in front of the radiator or in front of the engine compartment. If you are going to replace it, the cost will vary depending on who made it.

Also, the cost will depend on the brand and model of vehicle you are putting it in. The overall cost is between $10 and $100.

Some Final Words

Normally, it seems that the sensor is failing on you but that may not always be the case. There are different possibilities including not upgrading your system. Read the data sheets we linked to above to get more information on the parts that may need repairing or replacing.

The fix is not that complicated and with the right tools, you should be able to make the repair in no time.

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