Keeping it full is your problem. I bought a new aerial fire truck at work (over $1M), and it was the first the department had with DEF. They "topped it off" 2-3 times a WEEK, so the level sensor never moved. Threw a code for a defective sensor and the entire tank had to be replaced under warranty. 6 weeks later, same thing. That's when we found the root cause. It's one of the many safeties required by the government to make sure you don't mess with the system. let it go down to AT LEAST 1/2 is what Cummins told us...