Besides ourselves, we have found only one other person who has asked this question. Cummins and Ford had FAQ web pages and this question was not on either. But do not worry, we did find an answer.
The operating pressure for the DEF system may be different for different vehicles. On the Cummins discussion forum, the normal operating temperature was said to be 194. But it also fluctuated from that down to 175, among other temperature levels.
To learn more about this topic just continue to read our article. It explores the topic to get you the best information possible which may not be much. It seems that out of all the diesel owners out there with DEF systems in their vehicles, no one has bothered to ask this question.
As far as we can tell, the normal operating pressure is somewhere between 173 and 194 and could possibly go up to 200. The problem with not being able to nail this down more firmly is that there is no information about this level.
We know there is a normal operating temperature as there are many SPN and FMI fault codes flagging this problem. One example would be SPN 4374 FMI 0 & 1 and those codes read:
“Under Pressurized Diesel Exhaust Fluid (DEF) System” & “Over Pressurized Diesel Exhaust Fluid (DEF) System” and when you go through the data sheets, there are repair instructions but no information on what level of pressure you should have in your DEF lines.
You can read the data sheet for those two fault codes at this link to verify what we told you. Fault code SPN 4334 deals with DEF pressure in an indirect way and it is defined as:
“SPN 4334 is a diagnostic trouble code for Aftertreatment 1 Diesel Exhaust Fluid Doser” The FMI codes 2, 3, 4, 16, & 18 all direct you to DEF pressure.
Yet, not one instruction mentioned the normal operating pressure level. That means that until we see or hear differently, we will state that the normal operating pressure is between 175 and 200.
In some vehicles, the problem may be a leak somewhere in the system. You would have to use a scanning tool to perform a leak test and you should do this test three times.
If you do not find any leaks and don’t forget to do a visual inspection just in case, then check the pump. The pump can easily go bad on you and will not be able to build enough pressure in the system for the DEF to work.
If the pump is bad and you replaced it but still get a low or no pressure fault code, then the problem may be loose connections or a kink in the line somewhere.
Another problem would be SPN 4334 which directs you to a malfunction with the DEF doser. If this part is failing it could cause low or above normal pressure in the DEF system. That would be a part you would have to replace.
Another problem may be that the pump filter dried out and would need to be saturated again. Or it is clogged and in need of cleaning. Other parts that can fail and help create the problem would be the original UREA pump, the O-ring on the filter, as well as the filter itself.
What is discouraging is that the person reporting this particular solution only said, his truck went back to normal and did not say what his normal was. They were reporting figures of building to 145 and then dropping to 132 or 100.
We actually found several codes that would indicate a problem with the DEF pressure. We have linked to the two main ones already and those are the common codes for this problem.
One we did not include was SPN 4334 FMI 7- the same SPN code but a different FMI number. For this code alone, there are a maximum of 18 steps to be taken unless you solve the problem earlier in the process.
You can read those steps at this link and the good thing about this data sheet is that it provides very good diagrams. Some are large enough to actually see what they are talking about.
Other codes that seem to be related to this problem are SPN 1682, 3574, and 3596. They seem to be identifying debris as the problem for any low pressure you may be experiencing in your DEF system. The root cause for these codes could be:
“- EF system is contaminated with debris.
- In some instances, DEF tanks are cleaned and DEF dosing units or intake lines are flushed or even replaced, but these actions are not performed simultaneously.
Subsequent testing or operation between component cleaning causes cross contamination of previously cleaned components and repeat fault code occurrences.” (Source)
There are 6 pages of information to help you clear the debris and resolve the problem. Once those instructions are followed and the problem is solved, then your DEF pressure should return to normal.
That data sheet also has some good diagrams illustrating where the problem may be and what the parts look like.
The data sheet for the SPN 4334 fault code only has this to say under its test the pump section:
“Test the DEF pump: Confirm that the DEF pump is functioning correctly and delivering the appropriate pressure. Follow the manufacturer’s testing procedures or use a diagnostic tool to verify its operation.”
The SPN 4374 data sheet had this to say about the pump:
“10. Remove DEF pressure and suction lines from the DEF pump. Inspect the line for restriction.
a. If line restriction is found, replace the line. Refer to the section "Removal of the GHG14 Diesel Exhaust Fluid Lines".
Go to step 11.
b. If no restriction is found, reinstall lines. Go to step 12.
11. Inspect the suction filter (screen) in the DEF pump.
a. If the restriction is found, replace the suction filter. Refer to the section "Removal of the GHG14 Diesel Exhaust Fluid Pump Module Filter Screen". Go to step 12.
b. If no restriction is found, replace Pressure Relief Valve (PRV). Refer to the section "Removal of the Diesel Exhaust Fluid Pump Pressure Relief Valve". Go to step 12.
12. Perform SCR ADS Self-Check Routine to prime the system. Did SCR ADS Self-Check Routine complete successfully?
a. Yes; clear codes and release vehicle.
b. No; Go to step 13.”
The best instructions that we found were to put the DEF tank on a bench, fill it with water and then connect 12 volts to it. The water should be pumped out if the pump is working.
You can do this test with the tank still in your car, just remove the right hoses to see if fluid is being pumped out.
We may have our dilemma answered. It seems that the DEF pump pressure should be at 127- 130 psi for some vehicles. The above numbers are not invalid yet as there is still more to find out about this system and the vehicles using it.
But for the website, we found this information at, 127 to 130 is normal. We linked to that website because it has a lot of valuable images from the different diagnostic tools the person used.
It is worth reading to get other important information. It also provides the solution that the website owner did to finally resolve his problem of low or high DEF pressure.
Another website discussion forum had one post about DEF pump pressure. The range for normal pressure is 120 to 140 psi and 130 psi is in an ideal spot. This would negate the numbers we gave above and those numbers would represent DEF pressure too high.
One thing to be wary about, sometimes your screens may show the pump pressure at 934 psi. It has happened so there will be a malfunction somewhere if you get a reading that is basically off the charts.
There is still a possibility that the initial figures may be correct for some vehicles. We have not heard anything or read any information where the pressure is the same for every vehicle with a DEF system.
The first symptom you will see will probably be the check engine light or CEL. This will illuminate when you have a problem with the DEF injector. Another symptom will be that the remote engine starter will not work.
That is a common symptom any time the check engine light turns on. A final symptom will be limp mode. It is possible that the limp mode will not activate before you get home but that is a gamble and not a sure thing.
One owner had to have the catalytic converter replaced along with the external and internal DEF injectors. The NOx sensor or valve may need to be replaced as well. Or you may be told that the DPF pressure sensor may need to be removed and a new one put in.
There are a lot of possibilities for this problem that causes it as well as fixes it. The good news in all of this is that these clogged or faulty injectors may not be the result of contamination in the DEF system.
If it was, the repair bill may be extremely high. Instead, the clog came from the exhaust stream. The bad news is that there is nothing to prevent the exhaust stream from clogging the injectors in the near future. After fixing it you may run into the same problem down the road and in a few months.
This image should show you exactly where this pressure sensor is located:
Now that you know where it is, there are some facts you should know about. First, this part does not fail very often. Second, if it fails, you may not get a fault code.
Three, the way to tell if it is failing or not is by looking at your DEF levels. If it is going down too fast or going down very, very slowly, then you most likely have a bad DEF pressure sensor.
The image came from this link, and that link also has a diagram for the location of the Diesel Exhaust Fluid Level Sensor and other parts in that system.
One word about that link is that it comes from a FORD website and the location may not apply to all brands of trucks, etc. Your owner’s manual should provide a diagram of the location of this and other essential parts. Make sure to look at it and see if it is there.
These problems could directly or indirectly affect the DEF pressure in your system.
1. Improper handling and storage of DEF- this is a major concern that happens more frequently than you may have thought. DEF has to be stored carefully and it must not come in contact with direct sunlight.
Plus, that storage area should be on the cool side not the warm side of things. Keep the bottle sealed until you need it to refill your DEF tank. Outside air is also not good for this fluid.
Then keep it from freezing. You want the fluid to remain a liquid at all times so do not store it if the room is going to go below the freezing point for water.
2. Contaminated DEF- this happens far more often than one would expect. We have read stories where someone pulls into a gas station and is in automatic mode. They do not look at what they are doing and just open the fill cap and put the nozzle inside.
Unfortunately, the fill cap that was opened was the DEF and the nozzle was for diesel fuel. The repair cost over $5000. Even though its ingredients are very simple, the DEF is very sensitive to any contamination.
Dust falling in through the open cap can also contaminate this fluid. Be extra careful at all times.
3. Mileage problems- this will not affect DEF pressure but eliminate it from the equation. It seems that this fluid impacts your fuel efficiency and not for the better. Many owners have reported getting fewer miles per gallon when they use the DEF system.
When they delete their DEF system, they have reported a 30% increase in their mileage per gallon. This may not work for everyone but it is a possibility to get better gas mileage by deleting the DEF and ignoring it.
4. Crystallization- when you over-fill your DEF tank you create the environment for this process to take place. This can cause over dosing, stopping the DEF from Hydrolyzing correctly.
When this happens, small crystals may form in the injector nozzle or the exhaust. This crystallization can be a source of DEF pressure issues. It is also caused by a failing pump, plugged DEF lines or even using hard water.
In all of these cases, your DEF pressure can get too low or it can get too high. Watch how much DEF you put in your system.
5. Dilution- when people top up their DEF tank with water, that extra liquid will dilute the fluid. When you do this you can cause crystalization, as discussed previously, or it could make the fluid ineffective.
If it is ineffective, you may see a drop in your DEF pressure. It is best to just use new DEF when you want to top up the tank and make sure you do not run afoul of those DEF level sensors.
When you top up, make sure to use top-quality DEF so you can avoid low-quality DEF fault codes.
6. Freezing- when in storage you do not want to have the fluid freeze. But if it does, you simply thaw it out and it should still be okay. But double-check that to make sure.
However, if your truck or RV is out in the cold and not in use, frozen DEF can damage your SCR system. If it does, you will be paying a lot of money for repairs. Try to keep the fluid from freezing to protect your exhaust systems.
It is said that the SPN and FMI fault codes target the DEF and exhaust systems. The majority of the codes are involved with those two systems. That means there may be more fault codes related to DEF pressure than we reported.
To avoid seeing any of those codes, make sure to treat your DEF system right and use top-quality fluid and watch the pressure. If the pressure drops or goes too high, you know you have other problems in the system.