Ok so I need some help trouble shooting the def system on my 2013 chevy duramax. I have been getting the CEL code P0420 and P20EE on and off for a several months now. Meaning the light would come on for one or the other code for about a few days then go away and not come back on for a month or so then come back on. A few months ago go I received the P01F0 code with the P20EE code. So I changed the thermostats and all was good for about a month then rcieved the P2BAD code for a week or so then went off. It came on again a couple of weeks ago and did not go off this time. After about a week or so it's also threw code P20EE for a few days before P249D came on. My truck is now limited to 65 mph speed for another 60 miles until it will reduce to 55 MPH. I only have a code reader so unable to do any diagnostics on my own. Looking for help on what the issue may be and if can be repaired without tech tool.