Bad DEF quality and Nox sensor ? Check engine lights but they come and go

seriously, take it to another shop.

we use the bulk containers of DEF as we get onsite fueling. if the fuel truck has his supply of DEF, they fill up our tanks, so that too is bulk.

 

Time for a new mechanic. Codes NEVER tell you to replace anything. A computer can only tell you what's going on in a circuit, which includes, sensors/switches, wiring and plugs. There is a diagnostic procedure to follow to identify the problem. An intermittent problem like yours is the toughest to pinpoint. The exact fault code (SPN and FMI) will help, also any thing else you can tell us. Is there anything pattern or conditions that may be associated with it? Like does it do it more in rainy or damp conditions vs. dry weather? Cold/hot? Loaded/empty? Bumy vs. smooth roads? Anything you can say about the fault is another clue that may help.

 

What year is your truck. I have a ghg14, 2016 Cascadia.
On this model of emissions there is a little tiny def fluid filter that is in the def line right where it goes into the one-box. It can get a build up of white slime that inhibits the amount of def going into the chamber and give these sort of codes. Called a def Doser inlet screen.
Just a thought and easy to check out.
https://ddcsn-ddc.freightliner.com/cps/rde/xbcr/ddcsn/608-14.pdf

 

You can avoid the white slime by keeping your DEF tank full.

Reduced tank levels cause crystalized buildup in the tank that falls off and produces the slime you describe...

 

There's nothing "best" or "cheap" about this. You've changed parts and still have a problem. Blindly throwing parts at it is the worst and often most expensive way to fix things, the exact opposite of what your mechanic told you.

What we need to help you now is the SPN and FMI of your fault code. It looks to me like you do not have a NOx sensor fault at all. NOx Conversion efficiency is more of a logic fault, not a sensor problem. From the limited information I have, I can guess that your NOx sensors are working correctly, but it is not sensing enough of a NOx reduction. Meaning you have a problem in the SCR system. Could be many things, most common would be a clogged DEF doser. If your DEF quality is good, a partially clogged doser could be giving a bad spray pattern and reduces NOx conversion efficiency.

 

If it is a DD15 Detroit, can be as bad as a worn out SCR oxidation coating of the catalyst. If that were the case the whole one-box would have to be changed.