P20EE00 DEF ISSUE

A pic of the flex pipe installed would help. The DEF injector is usually right before/at the inlet of the SCR. So I'm wondering if that build up is coolant and not DEF. Coolant will ruin the SCR. What is your coolant level and have you been adding? The code is for SCR NOx catalyst efficiency below threshold. The materials in the SCR combined with DEF are what reduces NOx. The NOx level is not being reduced enough between the SCR inlet and outlet. How many miles on the truck? SCR's will eventually go bad like a catalytic converter on a car, basically the same thing. But you have other problems with the build-up. A flow test of the DEF injector should be performed.

 

If you had the software then you would do a flow/volume test and spray pattern with the injector removed. If not, I would first check the wiring and connections to the DEF pump and injector/doser. And check the filter and make sure the lines are clear. I'm guessing that the injector is not spraying and just dumping DEF. You could try replacing the injector/doser. Keep in mind that the doser is operated by a module but there are no other codes. SCR problems usually start in the 700-800k mile range so yours should be good but you won't know until you correct the crystallization/deposits problem. Look closely where the doser is attached and check for cracks anywhere near that area.

 

Usually only gonna happen if you have a leak somewhere. Have you replaced the bellows pipe? How do the surfaces look where the gaskets seal? Do you have the DPF that's stacked behind the step?

 

Have you had it apart for cleaning? I've seen quite a few where guys screw up the gasket putting the dpf back together, or the gasket falls inside the housing and there's no gasket at all.

It usually happens on the back side where it's hard to see. Symptoms will be similiar to yours.
I've had one bellows pipe so full of crystalized def I had to cut it off.

 

How long have you had the truck? I would have started there. I would expect a flow issue, and those aren't hard to disassemble.