60series 12.7 engine brake issues

Going downhill yesterday am the engine brake suddenly dropped out, sounded like it was only working on 2 cylinders, at the same time check engine light came on.
Stopped and listened to the top end with my stethoscope and dont hear anything that sounds like munching parts, shut it off restarted, no check engine light. Tried engine brake again, CEL back on. Shut off truck it resets again. I'm not anywhere I HAVE to have the brake working so I kept going. Have a delivery in the am, then home to get it looked at.
Any ideas? I had an injector harness and top set done last fall, I'm hoping it's just something easy?

 

It seemed like 2 quit at the same time, going downhill, check engine light came on immediately and stays on until you shut it down and restart. Doesnt come back on unless you try to use the engine brake again.

 

Ok, I'll be home later tomorrow after I drop this stuff, I'll make an appointment at my engine shop for Tuesday if he can. I need to get the software if I'm going to keep doing this so I can diagnose this stuff my self. Tho I did promise myself I'd stay in the shop working on my projects not the truck anymore.... dont seem to happen tho...

 

Hopefully that's all it is.