DEF Quality Sensor Bypass and Location: Cummins, Freightliner
If you need one, it is going to be expensive to replace. The cost of the DEF system is high no matter which part of the system you are working on. The trick is finding a new one so you can replace it. These are large devices, so finding it should not be hard when it comes time to make the replacement.
This sensor should be inside the DEF tank as it has 3 jobs to do. It monitors the following items-- tank level, tank temperature, and DEF quality. Do not confuse this with the NOx sensor that monitors NOx levels in your exhaust system.
To learn more about this topic, just continue to read our article. It has the information you want to know about so you can make any repairs necessary or bypass the system. DEF just keeps getting more complicated as time goes on.
DEF Quality Sensor
How Does The DEF Quality Sensor Work?
This device works with radio signals. There are 4 wires connecting it to the ECM. Those wires are power, ground, CAN hi, and CAN low. What happens is that these wires let the DEF quality sensor communicate with the ECM.
Then the quality portion of this sensor sends out radio signals inside the tank. After that, the sensor times those signals to see how long they return to the receiver. From that data, it calculates DEF quality.
This data is then verified by the inlet and outlet NOx sensors and that data is reviewed by the ECM to determine QLS accuracy. The NOx conversion data is the information used to verify the DEF quality sensor data.
It sounds complicated but it is a very straight forward system that has its own checks and balances. Because DEF can be converted to ammonia gas in the right conditions, you may have trouble with this sensor if the DEF is not kept at the right temperature.
Why do DEF Quality Sensors Fail?
There are many reasons why this device fails. One of the reasons is that the electrical charge may not be where it is supposed to be. The charge for the CAN high and low equals 5 VDC and it is 2.6 VDC for the high and 2.4 VDC for the low. If that is not there your sensor may fail.
Another reason would be that the cooling system is continuously warming the DEF. This is done when the coolant valve stays open. As urea warms, it converts to ammonia gas and since ammonia gas is very corrosive, it damages the electronics inside the sensor.
After that, the sensor fails. But not only do you have to replace the sensor but you are required to replace the coolant valve at the same time. This will prevent future problems until that new valve breaks down later on.
At this time you should check the coolant flow to make sure it is operating normally.
How To Test a DEF Quality Sensor
There are at least 4 different tests you can conduct. These tests target specific issues like crystallization, dirt or metal contamination, urea concentration, and other fluids. The most complicated test will be for crystallization and those steps are as follows:
- look at the exhaust piping and DEF injectors for signs of crystallization
- use ICP Spectrometry or XRF Spectrometry to test the DEF for minerals
- send in a sample to a qualified testing lab.
For dirt and metals, you just need to look at the DEF to see if those components are present. DEF is clear and the change in that color quality tells you that you have a contamination or age problem.
Then to test urea concentration you will need a handheld DEF refractometer and the concentration level should be between 31.8% – 33.2% (the target is 32.5%).
Finally, for other fluids, just smell the DEF to see if diesel is present. Use the urea concentration test to check for coolant contamination and look at the fluid. If there are layers, then you have oil in the tank.
How To Bypass a DEF Quality Sensor
The news is that the EPA has approved a bypass code or change in ECM software to bypass the quality DEF sensor. This news was released somewhere around Sept. 2021 and so far we have not heard if that has been changed or not.
The reason for this software patch permission is that the microchip supply has been depleted and getting microchips is very difficult these days. Or at least for the past year.
This is the good news about this bypass patch. There is one bit of bad news you need to know about. To receive this patch, you have to first qualify to get it. Not everyone is going to get it.
There is more bad news, unfortunately. You have to own a Cummins engine that is affected by this sensor. The first step in qualifying is that your DEF light has already been turned on. If it hasn’t you do not qualify.
There are more steps to qualify for this software patch you should contact your dealer or Cummins for specific information. Or click here to read more about this topic.
Cummins DEF Quality Sensor Bypass
This is a complicated issue. Cummins was slow in creating a bypass and we reported the software patch above. However, because all the engine makers, the EPA, and other organizations were dragging their feet over this issue, RV owners took matters into their own hands.
Their internet activities drew the attention of some very technology-minded people who took it upon themselves to solve this problem. The solution turned into a DEF simulator that plugs into your RV.
Once plugged in, the simulator tells the ECM that the DEF is working fine and the former component won’t de-rate your vehicle. You still have to keep your DEF system full and check it manually but that is nothing compared to being de-rated and sitting at the side of the road.
The good news here is that this simulator does not cost a lot to get. Also, it took Cummins about 4 days after the creation of this simulator for them to come up with their patch. Just one more word on this topic. From the developers of the simulator:
“ “Modifying emissions systems to defeat their purpose is not legal. The DSS [DEF Sensor Simulator] is NOT a means to defeat the DEF system, a DEF system ‘Delete’ or in any way provide a means to discontinue use of the emissions system on your diesel-powered equipment.” The developers are not offering any legal advice, as they say, this should be viewed as a temporary fix. They strongly recommend that users get a new sensor when available or to take advantage of the Cummins solution as soon as possible.” (source)
Paccar DEF Quality Sensor Bypass
This is what we have been able to find out:
“PACCAR has been working closely with government agencies to address issues related to the ongoing global shortage of computer chips and the impact that has had on the availability of certain replacement parts. One part, in particular, the Urea Quality Sensor (UQS), is having a significant impact on the model year 2016 through 2021 engines since the failure of this part leads to the vehicle is inoperable. In addition to working tirelessly on securing replacement parts, PACCAR is working with the agencies to provide a temporary software solution that would enable our trucks to operate with a failed UQS while the industry works to produce replacement parts to sustain a recall program. The agencies have reviewed the proposed approach and have agreed to this two-step program going forward as quickly as possible.
PACCAR is nearing completion on these field software solutions and should have early versions for some specific model years approved and released in the coming weeks, with the first version available mid-September with others following in quick succession. As parts become available, we will release details on the recall to update each truck.” (source #76 on page 8 of that thread).
In other words, the EPA software patch is applicable to Paccar as well.
Where is The DEF Quality Sensor Located?
This sensor is located inside the DEF tank. It is directly connected to the ECM and sends data to it in real time so all calculations are made right away. We explained how this works above.
It is not a complicated system but it can be a very frustrating one. If the sensor tells the ECM that the quality is bad, and the Nox sensors agree, then the ECM will illuminate the DEF quality warning light and take you to de-rate mode not long after.
The problem that comes in is that the sensor is sort of fragile and can fail on you, sending the wrong message to the ECM. To repair this part, you are looking at around paying #300 to $350 just for the sensor.
Labor is going to be more expensive and some people just do not have the budget for that repair. While people do run into this problem, not everyone does and their DEF sensor lasts a long time.
DEF Quality Sensor Peterbilt
You can get one of these through Peterbilt’s online store. If the microchip issue has been resolved, then you should be able to find a replacement just about anywhere.
This sensor costs $330 and it is made by Paccar. However, the Peterbilt online store is saying that this part is out of stock and it may not be so easy to get. You may have to check other sources or contact Paccar directly to see what they can do for you.
We did find a Kalmar Ottawa DEF quality sensor at advancedtruckparts.com but it costs $2750. Unfortunately, their listing states that they are low on available models. You need to rush to get one if you want to pay that much money.
You will be competing with every other RV owner and trucker who is having the same problem. There may be tens of thousands of these owners out there looking for the same part.
If you can’t find one, use the software patch if you qualify.
Freightliner DEF Quality Sensor
The same company as mentioned in the previous section would carry this part for you. However, they are out of stock at the moment. That may be a good thing as that part costs almost $1000.
The same company sells a Paccar model for $1000 but the company says it is low stock which means you better hurry if you want one. Another company KUS, says they sell a DEF sender with a quality sensor but won’t give a price at this time.
The reason for that is that they are closed till Jan. 8, 2023. At this time of year, it may be difficult to find this part due to the microchip issue and availability. There is a backlog of orders for companies to fill.
With that in mind, you should check to see if you qualify for the patch. It is a temporary fix till everything returns to normal. Or check out that simulator and see if that works for you.
DEF Quality Sensor Kenworth T680
This part seems to be available. It is on sale at Vander Haag’s online store. It retails there for $424. That price is for a used model and it seems that all they are selling are used sensors.
It seems that eBay also has used T680 models for sale as well. We did not check the price at that marketplace. If they are in stock and are new models, the only place to find this part will be the Kenworth and Peterbilt dealers.
The bad news is that they are on back order and it may take months to get a new one. If you can get one, look to pay around $500 to $800 or more for it. This part fails easily but can be expensive to replace.
DEF Quality Sensor International
If you hurry and it may be too late by the time you read this, you can get one of these at First truck parts.com. We say it will be too late because there is only 1 left at the time of this writing.
The part, if you can get it, sells for $2339, and those that do go there may be put on back order. It is hard to say. As of July 2021, this part has been on back order and there are companies waiting for this part to arrive.
The International version may be built by Navistar and many trucks and RVs are placed on the critical back order list. Whether that list has cleared up by now or not has not been mentioned.
If the sensor is made by Cummins or Paccar, then the patch should get you through this crisis.
DEF Quality Sensor Problems
The biggest problem these sensors have is the heat control valve. This part gets stuck open and continues to let the coolant through to heat the DEF. The reason this system is designed in this way is because DEF can freeze fairly easily.
However, if the DEF is constantly heated, then the urea changes to ammonia gas which then corrodes everything it touches. When that happens, the sensor fails.
If you want a list of both Cummins and Paccar error codes, then click on this link. There is a short list of codes and what they mean on that website. There is at least one code that states the root cause is not known.
If you get that code then you need to talk to a dealer about it.
DEF Quality Sensor Abnormal Rate Of Change
The code you will see will be SPN 3364 FMI 10 or Cummins 4277. What that code says is After treatment Diesel Exhaust Fluid Quality - Abnormal Rate of
Change and all you should get is a yellow warning light.
This code may refer to the supply line and you need to make sure the lines are not crossed. That the instructions found on the data sheet for this problem. You just want to make sure that the supply routes are connected correctly and routed properly.
You can read the data sheet at this link.
Some Final Words
Given the current microchip shortage, just about every engine product requiring that part is back ordered and may still take weeks to get it. We have not seen anything yet that says this problem has been solved.
If you need to get back on the road again, then contact Cummins to see if you qualify for that patch or look into getting the simulator. We will not tell you to get that solution or recommend it but it is there if you need it.
Hopefully, this problem will be resolved soon.
