DEF help please

I have a 2016 cascadia and I think I have a problem with my def. For the last 2 months my def tank stays full. I haven’t fueled def in more than 2 months now. Every time I try to fuel, the def tank is full. I have no check engine light and I have no code. In the last 2 months I did about 4 parked regen hoping def fluid would go down but the tank stayed full. No issues, no code, after each regen (45 min parked regen). I don’t know what’s wrong or what to fix. I think if it was a pump issue or 7th injector issue or clogged tank, I would get a check engine light. Truck runs fine so far, no issues. Any body ever experience this type of problem with def system? Please help.

 

I’m doing a parked regen once every 2 weeks. Yes a failed outlet Nox sensor makes sense. But wouldn’t it throw a code when it failed?

 

At some point I thought the chinese scan tool (NEXAS NL102 Plus) I’m using to do regens messed up my def system. Another somehow unusual thing too is I never replaced my dpf filter in more than 4 years. So far I put almost 500,000 miles on the truck and I never got the end of life code for dpf filter. So I never touched it. In other trucks I used to drive DPF filter was lasting 250,000 to 350,000 miles before end of life code. The only thing I’ve been replacing once a year, just for maintenance, is the def filter.

 

I don’t get emission inspection (or so I think). I mostly run FL to TX and back. I’ve decided to keep running until either I get a check engine light/code or get a mechanic with Detroit Diagnostic Link software to run a regen and check how the sensors act. If they are good or bad. This is one of those times when you think paying the $600/year subscription to the DDL software would be worth it. I think the idea of a failed outlet NOX sensor might be right. I would just wait for the code. The good news is NOX sensors are easy to replace, no need to visit the stealership.

it is just scary to keep running without fueling def. I was checking my receipts, last time I fueled DEF was 2/26 for $23.67. Between 2/26 to now, I only fueled for $1.14 and I probably ran more than 20,000 miles. Every time I tried to fuel, I’d get in maybe $.10 or $.20, the tank would be full. Will update if anything changes. Thank you for all your suggestions.