SPN 4334 FMI 4 Cummins: What Is Code 4334 On Freightliner?
The official SPN FMI code list covers the big three diesel engine makers. Freightliner, Cummins, and Detroit Diesel. If you get the same code on all three engines, you have the same problem. The only question is, are you skilled enough to fix the problem?
The SPN4334 FMI 4 fault code stands for Def Pressure Sensor Circuit Failed Low. This is an electrical fault code and it is telling you that there is a short between the Diesel Exhaust Fluid (DEF) Pressure Sensor Circuit and the ground.
To learn more about this code and its variety of FMI options, just continue to read our article. It explores the issue so you have the information you need to make the repair or take it to a technician. Knowing how to fix the problem is the other half of the battle.
What is Code 4334 on Freightliner?
This code stands for- Def Pressure Sensor Circuit Failed Low: Aftertreatment 1 Diesel Exhaust Fluid Doser Absolute Pressure Voltage Below Normal, Or Shorted To Low Source; Urea Pressure Signal Range Failed Low.
In layman’s terms, you have a short in the system somewhere and you should take care of it before it turns into a bigger problem. That explanation is for FMI 4. When you see the SPN 4334 fault code, you know you have a problem somewhere in your DEF system.
In looking at the different FMI codes attached to this SPN fault code, you have up to 12 different problems that are associated with the DEF system. According to the EPA 2010 rules, if you see FMI 4 on your screen, your CEL and MIL warning lights should be illuminated.
Plus, you should expect to feel a 25% derate in your engine’s performance and power. This is not going to be a code you can ignore as according to some data sheets, you will experience some sort of derate with some of the FMI code options.
The following sections will give you an idea of what you can expect when you see the SPN 4334 fault code illuminated on your dash or code reader.
SPN 4334 FMI 4
The more specific definition for this code is the Aftertreatment Control Module (ACM) detects a short to ground on the Diesel Exhaust Fluid (DEF) Pressure Sensor Circuit.
When that code is illuminated, you will see the two lights we just talk about light up. Plus you should get ready for a derate. That derate is said to only reach 25% with this FMI code.
The good news is that there are only 6 steps to resolve this issue. Those steps are as follows:
1. Connect DiagnosticLink™. Go to step 2.
2. Turn the ignition ON (Key ON, Engine OFF). Go to step 3.
3. Check for multiple codes. Are codes SPN 4335/FMI 4 or SPN 3510/FMI 4 also present?
a. Yes; repair the short to ground on pin 84 of the ACM. Verify repair.
b. No; Go to step 4.
4. Disconnect the DEF pressure sensor electrical connector. Go to step 5.
5. Disconnect the 120-pin ACM connector. Go to step 6.
6. Measure the resistance between pin 3 of the DEF harness side connector and the ground. Is the resistance less than 1k ohms.
a. Yes; repair the wire between pin 1 of the DEF pressure sensor and pin 100 of the ACM 120-pin connector. Verify repair.
b. No; replace the DEF pressure sensor. Refer to the section "Removal of the Diesel Exhaust Fluid Pressure Sensor". Verify repair. (source)
This code seems to be an easy fix although it comes with a possibility of a harsh punishment.
SPN 4334 FMI 7
We will be quoting the official SPN FMI code list today as that will give you the most definitive definition for these different codes. In this case, this code means:
“Def Pressure Low: Aftertreatment 1 Diesel Exhaust Fluid Doser Absolute Pressure Mechanical System Not Responding Or Out Of Adjustment;” The source for this problem comes from the following:
“This code comes when the diffuser screen in the metering block on the side of the 1 box is clogged with DEF crystals.” (source). The solution comes from the same source and it is:
“Make sure the pressure limiting valve is delivering about 75 lbs of air to the metering valve. once that is confirmed, remove the metering block and remove the 3 screws for the screen cover and remove the screen. soak it in hot water and blow it clear. reassemble and re-check air pressures. they should be around 78 supply and greater than 19 through the screen.”
That should be all there is to this fix. But according to the NHTSA data sheet, there are far more steps involved than mentioned in that short answer. You are going to have up to 18 steps to take before you finally solve the issue.
The good thing about these data sheets is that they normally come with some great diagrams that are clearly labeled and all the parts are clear and not fuzzy.
SPN 4334 FMI 3
The information for this code variation is from both the official SPN FMI code list and the link that is in the section talking about FMI 4. The NHTSA does put more than one fault code on its data sheets from time to time.
The definition for this code from the official list is “Def Pressure Sensor Circuit Failed High: Aftertreatment 1 Diesel Exhaust Fluid Doser Absolute Pressure Voltage Above Normal, Or Shorted To High Source; Urea Pressure Signal Range Failed High”
The better definition from the data sheet is “Aftertreatment Control Module (ACM) detects a short to voltage or open on the Diesel Exhaust Fluid (DEF) Pressure Sensor Circuit.”
When you see this code, you will see the MIL and the CEL light turn on as well as the possibility of a 25% derate. Like FMI 4, you will want to take care of this problem as quickly as possible.
Unlike FMI 4, there are up to 11 steps to completely clear this code, which we will not quote here. For both FMI 3 and 4, you should expect to see additional codes set as well. You will need to connect a code reader to see all the codes that have been set.
The additional codes you may see are SPN 4335 FMI 3 & 4 and SPN 3510 FMI 3 & 4. You will need to clear those codes before fixing the original 4334 codes. The FMI 3 & 4 will correspond with the SPN 4334 FMI 3 or 4 codes, whichever is set on your ACM.
SPN 4334 FMI 18
All the official list says about this code is that the “Reductant Pressure System - Reductant Pressure Too Low” Right now we have not come across too much information on this problem.
But because it is a low-pressure fault code, you may have kinks in your system or other restrictions hindering the flow of DEF, etc. Or you may have experienced a problem with your line heater and some of the fluid has frozen inside its line.
This information was given to a trucker who had this code reappear after making several expensive fixes. Cummins told the mechanic to change the pump, filters, & decomposition tube at a cost of $4800 to the owner.
When the code reappeared, the dozer injector was replaced at a charge of $2156. Then the code reappears after this fix and the NOx sensors are said by Cummins to be bad. That cost the owner another $1917 and still, the code keeps reappearing.
This is why we tell you to check the simple things first. They are cheaper to replace or repair and you may fix the problem before Cummins and other mechanics start throwing expensive repairs your way.
Here is the sad end to that tale. Other owners were experiencing the same fault code and problem and were spending big bucks as well. They asked the original poster if he ever found the problem. His answer was, no, they never did find the cause for this code being set.
The good news is that there may be a fix. Here is the solution from two owners who had the same problem:
“ I searched the wiring harness from the DEF DOSER I found that the wiring harness because it’s so long Cummins had to loop a portion of the wiring harness and it was laying on top of the PTO where it flattened and actually chaffed the wires.” and
“had the same code come up on my 2023, truck running fine, thought this thru walked to the truck, removed the DEF filler cap, stuck my pointer finger, and pushed the flap open, this released the pressure, started up the truck, codes cleared. got lucky”
Try those two fixes to see if you can avoid expensive repairs when they are not needed.
SPN 4334 FMI 2
From the official list linked above- “Def Pressure Sensor - Drift, Def Supply Module Pressure, Plausibility Error”. Here is what the data sheet has to say:
“Aftertreatment Diesel Exhaust Fluid Pressure Sensor - Data Erratic, Intermittent, or Incorrect. The diesel exhaust fluid pressure sensor has reported a reading too high or low for the operating conditions.
Diesel exhaust fluid injection into the aftertreatment system is disabled.
The aftertreatment diesel exhaust fluid pressure sensor is internal to the aftertreatment diesel exhaust fluid dosing unit and can not be serviced separately.”
The possible sources for this problem are:
Leaking diesel exhaust fluid lines
Malfunctioned or malfunctioning diesel exhaust fluid dosing unit
Blocked or restricted diesel exhaust fluid dosing unit supply line fitting
Blocked or restricted diesel exhaust fluid filters
Malfunctioning diesel exhaust fluid pressure sensor
Malfunctioning diesel exhaust fluid controller.
Low diesel exhaust fluid tank level
Blocked, restricted, or frozen diesel exhaust fluid dosing unit supply line
Blocked or restricted diesel exhaust fluid tank filter
Malfunctioning diesel exhaust fluid dosing unit
Contaminated diesel exhaust fluid
Signal wire shorted to sensor supply or battery voltage
Low diesel exhaust fluid tank level
Blocked, restricted, or frozen diesel exhaust fluid dosing unit supply line
Blocked or restricted diesel exhaust fluid tank filter
Malfunctioning diesel exhaust fluid dosing unit
Contaminated diesel exhaust fluid
Signal wire shorted to sensor supply or battery voltage (source)
Unfortunately, the mechanic who supplied this information did not stick around and offer possible repair steps to take. We have not found the data sheet he quoted from as yet.
What we have been able to find is:
“ I would do your power and grounds checks at the engine ecm and the batteries, as well as checking ignition power… I would back-probe your power and ground as well as the ignition wire and see if that is dropping out. It sounds like you are losing communication like the ECU is dropping out. The faults you have will not cause this”(source)
Where is The DEF Pressure Sensor Located?
It is not going to be easy to find. The DEF pressure sensor is located on the diesel exhaust metering unit. The best thing we could do for you is to link to a diagram that provides the location for you.
This website has 8 pages of diagrams and labels and the DEF metering unit is on page 5 and is #7. The identification of the location is on page 4 about halfway down the list.
Some Additional Words
Once you know what the codes mean, they do not seem as scary. Some of the SPN 4334 fault code problems can be an easy fix. But not always. You would have to know your limitations to judge if you are up to the repair tasks or not.
When you see this code, you may be in for a derate or you may never find the actual source of the problem. It takes lots of research at times to get to a solution.
