1. How much DEF should I be using. Used about 2.5 gallons in 700 miles. I think this is high based on what I have read but not sure where to look for a problem.
2. I had a P21dd - reductant heater 1 current low code. Cleared it and has not returned but only been 1 day. Not able to find much about what part would be the issue if this code returns. Looked at the gmdirect parts breakdown and it does not list the heater.

Suggestions or ideas?