had low def, engine was at derate, dealership told me that 4 out of 6 cylinders are not giving enoug

I had the similar code on my Shaker back at pumpkin. Clogged def filter.

 

SPN 605 says refridgerant high pressure switch. Your a/c .

 

If the DEF yellow light is something is wrong with the DEF system. Not the square light. Their is a yellow round light. Just like the low fuel light but is means something is wrong with the DEF system. You can fill the DEF tank and the round yellow light will stay on.

Once that round light come on you can only drive like 500 miles, then you get the 25% derate and limit to 55MPH or something like that.

 

As stated above by other posters, what tests were performed to come to the conclusion that 4/6 cylinders are not up to snuff?

 

Don’t be upset with people cracking a joke.
If you need advice you really have to go into some details.
What does it mean 4 cylinders not giving enough power? Low compression? Incomplete combustion? How did the dealer determined that?