2016 Silverado High Country Dmax here. I have the same DEF fault "P21DD DEF heater." I work a lot of hours, and time off to get to the dealer is tough. Here's what I've been doing as a band-aid:
I have the Android app called "Torque" (I have the paid version) and a cheap ebay OBD2 Bluetooth adaptor. Through the app you can search and reset fault codes, among a ton of other super cool monitoring modes. I have reset the P21DD fault and it stays gone for a week or better. I haven't noticed any driveability difference, and I know I'll have to get it fixed before fall, but this is an effective stop gap.
Here's the adaptor:
https://www.ebay.com/itm/BAFX-Produc...-/251488093341
The app is incredible, you can monitor almost every bit of information in your drivetrain. Throttle, boost, coolant temp, DPF temp, EGT temps, oil temp, rail pressures, and a whole bunch more... Then you choose displays, and build essentially a screen full of instant real-time data to watch while towing. It's really great tool to have!
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