scr warmup

[JimT]

Back when I had a functional system I was experiencing the issue of frequent SCR warmups. Nearly any time the engine dropped back down to idle for more then (i'm guessing here) about 20-30 seconds it would kick into warmup mode. Initially I thought it was DPF regens (which I was having frequently because my DOC and DPF were shot) but after connecting Insite a few times I was able to see that it was actually SCR warmup. This led me to try replacing my DEF doser. That didn't help. My next suspect was the Outlet NOx sensor -- in range but higher than expected NOx values can trigger the SCR warmup at idle. Never got around to testing that as my DOC/DPF became non-functional about the same time and were subsequently permanently retired. I read many sources, both forums and official bulletins, that pointed to recurring failures of the Outlet NOx sensor, especially on the earlier 2010-2013 systems with the long winding undercarriage exhausts. Something about the placement of the sensor, they had a campaign or something about installing a splash shield. Also, it was common for people to find that when the sensor failed it would report inaccurate but still in-bound values making it hard to spot.

Anyway, this was late 2019 to early 2020 when I was researching this, so everything above is pulled from my memory. My point here is that something is signalling to the ECM that it needs to enter SCR warmup. It could be anything, but I would start by really looking closely at all the sensors. Hook up Insite, pull up data monitor with the relevant items and go for a drive. Look for any abnormalities, even those that might still be valid values. Also, inspect the SCR decomp tube and the DEF doser valve. Look for leaks and/or DEF buildup. Also, if temps coming OUT of the DPF are too low that could be cause the issue too. That could be caused by poorly functioning DOC, exhaust leaks, leaking DPF doser / 7th injector, or malfunctioning EGR valve. Basically anything that could lead to lower DPF temps.

Something else to consider, some of the values are actually calculated by the ECM instead of measured by a sensor. It's possible that the calculated value is wrong due to other sensors or assumptions that the ECM is making which, for whatever reason, are inaccurate. For instance avalid but inaccurate IMAP reading could throw off ECM calculations about what it expects the engine to produce during combustion, then later downstream when measurements are taken they don't fall into the range the ECM expects. This is another reason why regular EGR tuneups are so important. There is a delicate balance that must be maintained from air and fuel intake all the way to the end of the exhaust pipe. An issue anywhere in the process could disrupt this balance of circular dependencies.