Is the scr constantly active?

Your likely very right. To be clear im talking about the can on the bottom. Always heard it called the scr or soot can.

 

Both Pete's and KW depending on the configuration either or could be on top or bottom. In order of flow from the engine the component order is- DOC- DPF- Mixer or Decomp( this is where the DEF doser mounts or injects)- SCR - tailpipe/stack.

Typical high mount or undercab exhaust plumbing the DPF will be on top. Low mount exhaust and DPF will be on the bottom. This is for the typical passenger step mounting for the switchback design.

EPA 17 and newer all components are together into one single "can" assy. The main reason for this was to prevent heat loss across the system which in turn make it more efficient- Less frequent regens- faster warm up- ect.......

 

When your DPF differential pressure reaches a threshold of pressure too high after after a completed regen FC 5383 will log a CEL for ash load percent high. When troubleshooting I always review the regeneration history which includes the last 10 completed regenerations. I look at the time between regens and the DPF delta pressure across the history. EPA 13 ISX normally run around 25 engine hours between timed regens. Anything less than this is a concern. Faults will log if the regens get too short and you will be seeing the HEST lamp a lot. DPF delta pressure should be low. I start recommending cleaning or filter replacement with consistent numbers above 3 or 4 Inhg average. Newer products have increased timed regen engine hour timed intervals to 50 and now EPA 17 and newer is up to 100. Sporadic or rapid fluctuating pressures from one regen to another suspect a bias DPF pressure sensor or intermittent wiring issues to the sensor.

 

No apologies necessary.......I am off work for a stint, recovering from neck surgery. My boredom gives plenty of time to respond and help, which I enjoy. First time in my life I have been forced to sit still, trying not to go insane............