Paccar MX13 epa 17

[Ninja]

Hello guys I have a paccar mx13 in the shop right now throwing a code P3818 SCR catalyst conversion efficiency - reduction efficiency too low.
I usually work on Cummins engines and have trouble shooting material for that but on these engines i dont really have anything like that or to much experience on them. I was wondering if you guys have had this problem and how you guys went ahead fixed it. Any help is appreciated thank you.