P20EE Code and DEF Issues: Is it a bad injector or a clogged cat?

[CrankMaster]

My 2015 is throwing P20EE (SCR Efficiency Below Threshold). The truck runs fine, but I can’t get rid of the light. I’ve cleaned the DEF injector, but it keeps coming back. Is my SCR catalyst cooked, or is there a sensor I’m missing?

 

[RigHeavy]

Don’t swap the SCR catalyst yet! If cleaning the injector didn't work, check these three first:

1. NOx Sensors: 80% of the time, it’s a 'lazy' outlet sensor sending skewed data to the ECM.
2. DEF Concentration: Use a refractometer to verify it's at 32.5%. Stale fluid from the pump is a common silent killer.
3. High-Load Run: Hook up a trailer and hit the highway for an hour. The SCR needs extreme heat to burn off internal DEF crystallization that city driving can't touch.

If your NOx readings and fluid quality are both spec, only then is it time to look at a new catalyst.

 

[ForgeSteel]

What kind of engine oil are you running? If you're burning a little oil and using a non-low-ash formula, the phosphorus and sulfur can 'poison' the face of the SCR catalyst over time, creating a film that stops the chemical reaction. Also, try switching fuel stations. High-sulfur fuel is rare in the US now, but bad batches do happen and they will temporarily tank your SCR efficiency. Run a tank of fresh, high-quality diesel and a good cleaner before you pull the trigger on a delete or a $3k replacement.