I recently had the common P21DD code come on for the first time at approx 36K miles. My dealer is telling me the DEF heaters are on a nationwide back order. Code was reset and came back about a week later, which I figured it would. I reset it through my CTS2. Hasn't returned (yet)? Anybody heard of this current back order (I know this has happened in the past)...or know of an ETA of the DEF heaters to dealers?