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JimT

(01-22-2021 )Wildcat Wrote: Hi yall. New here. I'm a company driver for now. I drive a 2016 Volvo with a D13. I've been in it a week and a bit now. It has turned on a check engine light with a code for low NOx conversion efficiency. It has forced me to do a parked regen twice in the short time I have been in it. Also, it threatens every day or two to derate to 5 mph within <xx minutes, regen required.

I believe it derates power to 70-80 percent at this time because i can tell i have less power as well.. But before I get to where I can pull over and request parked regen it changes its mind, lights stop blinking and power comes back. On a side note it seems to only happen when its cold,, 35 or below..
What could the problem be? I have no idea on the history of this truck. It has 426,000mi on the clock now.

So, I have no experience with working on a D13, but I did drive one in a 2015 Volvo for a few years. However what follows is more blanket info that probably applies to any engine/truck.

Lets start with the NOx issue. NOx conversion is all about the SCR can/module. Exhaust gas comes out from the engine, through the turbo, past the 1st NOx sensor (Inlet NOx Sensor), through the DOC/DPF, through the Decomp tube where DEF is sprayed in, through the SCR can/catalyst and out the tail pipe passing a 2nd NOx sensor (Outlet NOx Sensor) as it goes. The ECM uses it's programming and sensor readings, along with measurements from the 1st NOx sensor to determine how much DEF is needed to reduce NOx to the desired levels. It's complicated and takes into account regen activity that might be producing additional NOx.

Anyway, your truck is complaining about low NOx conversion efficiency, which means Outlet NOx is not low enough compared to the Inlet NOx measurements. There could be a number of reasons for this. Poor or nonfunctioning DOC/DPF will affect the calculations, though most likely resulting in lower NOx production. Next, DEF spray could be outside of the normal resulting is an unexpected conversion result. SCR catalyst could be less effective, though unlike the DOC this isn't dependent on a special coating so usually only a problem due to damage. Finally there's the NOx sensor readings.

OK, foundation laid, here's my thoughts:

1) Faulty Outlet NOx sensor.

The ISX/cummins aftertreatment systems have had repeated problems with the Outlet NOx sensor going bad, throwing bad readings. They introduced a shield to help protect it from road spray and corrosion, but it's still a common fail point. When they go bad it's internal and they often still provide valid readings sometimes resulting in a situation like you mentioned. It's also been noted that cold temperatures make it worse (because even under normal circumstances it needs to warm up to a certain temp before it reads correctly).

2) Bad or clogged DEF valve.

These valves clog up with dried DEF or other contaminants that make it past the filter. Or they just fail to spray properly. Either way if the spray angle, pattern and volume are wrong then the NOx conversion will suffer. Also, there's a condition refereed to as an SCR or DEF volcano where dried DEF builds in the Decomp tube and causes an exhaust restriction. Dried DEF melts around 300-400 degrees (if my memory is correct) so normal operating exhaust temps should self clean any build-up and active/parked regen temps should always clean it up.

3) Exhaust leak(s)

Exhaust leaks between the turbo outlet and the tail pipe can lead to all sorts of performance problems in the aftertreatment system. Usually has a bigger impact on the DOC/DPF but, depending where the leak is, it could affect SCR conversion efficiency too.

There could be some other problems, including engine related issues, but based on your description they probably don't apply here.

I'd say most likely cause is #1, the Outlet NOx sensor (or the wires for it). Since it seems the default internal troubleshooting step for the ECM is to perform a parked regen, this should fix the issues you listed. If that's not it then additional troubleshooting needs to be done with #2 being the next likely problem area.

Again, this info is based on what I've learned trying to understand and troubleshoot my own Aftertreatment system and from what I read it's pretty much universal in design. In fact many OEM's actually use the Aftertreatment modules designed/produced by Cummins (especially post 2010) (largely with parts built by Bosch) so in many cases they are functionally identical. More recently some OEM's have built their own aftertreatment modules, but I'd be willing to bet the underlying parts and design is nearly the same.