A couple of years ago I had this warning with no CEL ( I had already been thru the failed heater prob, couldn't get one so took it to the dealer where they could get one next day, they told me they swapped out the reservoir but they just unplugged and swapped out he drop in part, I learned long ago to sharpie the parts before they SAY they replace them). I wasn't aware of shelf life considerations on the DEF. I googled the way to decipher the Peak DEF fluid code so I was sure to get the newest I could then siphoned out the existing fluid and replaced with new. warning went away and didn't return for a long time.
Fast forward to one month ago I get the poor def warning and the subsequent countdown, no code. I always get newest fluid I can, usually at local farm store. I topped it off with about 2 gallons of Peak DEF with Date code representing mid November, so really fresh. Warning went away immediately. 2 weeks later warning returned. I siphoned all of it out and replaced with the fresh and warning goes away immediately.
Now, 2 weeks later, warning is back, no CEL. we are at 98K on the odometer. Could my problem have any thing to do with the recent sensor and regen letters we received? I don't recall ever seeing any notification about a regen running, never inquired at dealer about one. I have had bad luck with dealer for problem diagnosis and paying for unnecessary parts before. I really hate to go there for anything. If I need to go I will find a different one.