DDION

Perhaps this will help bring some clarity to the issue of resolving issues with DURAMAX protective speed restrictions due to DEF quality issues. Earlier this summer I had an DEF contamination issue with my 2020 GMC Sierra 3500 that resisted every fix posted on this forum and others. Draining the DEF tank to eliminate all contamination was an obvious first step, but not as straightforward as you might expect. There were several posts suggesting siphoning the tank by entering through the filler connection on the top of the tank because there is no drain built into the tanks to facilitate such an activity. That helped for a brief period of time only to have DEF quality issues continue to occur. Assuming I had not gotten all of the contaminated fluid out of the tank I went the extra mile and ordered an electric siphon pump and evacuated all of the fluid possible by going through the filler connection on the top of the tank. Again that didn’t resolve the issue and it persisted to the point the speed lock out was already limiting the truck to 55 mph. Frustrated with the issue I took the truck to the local GMC dealer for their intervention. About an hour into working on it they called to advise me the truck would require replacement of the entire system at a cost of $6,100. My answer was simply for them to button things up and return it to me. Resolving the issue would require an understanding of how the DEF system was designed to function particularly how the system detects DEF quality. What I found was that the majority of the parts in the system were static in their function such as the DEF injector and diesel particulate filter. The majority of the active elements in the system are actually found inside the DEF tank itself with the reductant control module mounted on the top of the tank. With respect to determination of DEF quality, the system uses two components to assess its state. There is an ultrasonic sensor built within the tank itself and the system also uses the lower NOX sensor after of the diesel particulate filter. It was obvious to me that there was still contamed fluid in the DEF tank after I had pumped it dry. When I removed the tank to attempt to empty it manually I found there was indeed a substantial amount of fluid still in the tank that I could not get out. There are obviously baffles within the tank that prevent extraction. A replacement tank was acquired and installed. That cleared all of the codes however the DEF quality indication on the dash remained as well as the speed reduction lock down though now it had reached the dreaded 5 mph limitation. I ran diagnostics and reset the ECM to factory defaults, disconnected the batteries, and even attempted to manually initiate a regeneration of the reductant system. Nothing cleared the DEF quality condition or speed lockdown. That focused my attention on the sensors upstream and downstream of the diesel particulate filter. There are two exhaust temp sensors and the lower NOX sensor. I replaced the two temperature sensors first to see if they would clear the condition. No luck. I then replaced the lower NOX sensor and evaluated the impact. The DEF quality indication remained even though no error codes were present. What I suspected was that a small amount of contamination still remained in the lines from the tank to the DEF injector so I elevated the engine RPMs to 2,000 for approximately 10 minutes. As I backed down the throttle the DEF quality indication cleared and the speed lockdown cleared. The net of it is this, if a DEF contamination exists the only way to assure no contaminates remain in the tank is to pull and replace the tank. If the DEF quality indication does not clear the lower NOX sensor will likely need to be replaced as that is the only other component in the system that influences the system determination of DEF quality. Hope this helps. It cost just under $1,000 to restore my truck. Not cheap but I didn’t have to part with another $5,100 to get my truck back in prime shape. Best of luck to you all.