I‘d think the weather from the last 2 weeks would of been enough for people to know if they have this issue or not, unless they live in FL.

 

The Emission Reduction Fluid Tank heater may be faulty on some 2020 Silverado 1500, Silverado 2500HD/3500HD, Sierra 1500, Sierra 2500HD/3500HD; 2021 Tahoe, Suburban, Yukon and Escalade models equipped with the 6.6L Duramax diesel engine (RPO L5P) or 3.0L Duramax diesel engine (RPO LM2). If the Emission Reduction Fluid Tank heater is not operating properly, there may be a Service Emission System/Service Exhaust Fluid System message on the Driver Information Center along with an illuminated Check Engine MIL. DTCs P20BB (Reductant Heater 1 Control Circuit Low Voltage) and P20C3 (Reductant Heater 3 Control Circuit Low Voltage Test Status) also may be set in the Engine Control Module (ECM).
The faulty Emission Reduction Fluid Tank heater may be caused by the tank heating elements being exposed to Diesel Exhaust Fluid (DEF) and shorting to the pump ground. Within the Emission Reduction Fluid Tank is the Emission Reductant Fluid Supply Pump Module, which contains an electrically operating reductant pump; reductant level, quality, and temperature sensors; and reductant system heaters. (Fig. 16) The reductant control module is mounted outside the tank.

Fig. 16

To determine if there is a short to ground, measure resistance at the Emission Reductant Fluid Supply Pump Module (Fig. 17), which would indicate that the heater has been exposed to DEF.

Fig. 17

TIP: Be sure that the DEF tank is at least half full of DEF before performing the resistance measurement.
With the Emission Reduction Fluid Tank wiring harness disconnected, measure resistance from pin 9 to pin 14 using a Digital Multimeter. Next, measure resistance from pin 10 to pin 14. The illustration (Fig. 18) shows pin 9 (#1), pin 10 (#2), and pin 14 (#3).

Fig. 18

If either resistance reading is less than 10M ohm, the heater is faulty and the Emission Reduction Fluid Tank should be replaced. If both resistance readings are greater than 10M ohm, continue with diagnostics according to the appropriate Service Information.

Cold Weather Operation
The two reductant (DEF) heaters are designed to thaw DEF that has become frozen in temperatures below 32°F (0°C) in the Emission Reduction Fluid Tank and supply line to the DEF injector. The reductant control module monitors the reductant temperature sensor located within the Emission Reduction Fluid Tank in order to determine if reductant temperature is below its freeze point. If the control module determines that the reductant may be frozen, it energizes the reductant heaters. Reductant pump operation is disabled for a calibrated amount of time to allow the heaters time to thaw the frozen DEF.
For more information, refer to Bulletin #21-NA-002.

 

Is this a recall or TSB. or a just FYI problem? BTW I do live in Fl !

 

OP, good post explaining how the DEF system is supposed to work, I don't know about a recall or TSB but if the DEF freezes and doesn't thaw as designed you will probably get a code that will limit your driving distance because of no DEF injection. This has been a problem since DEF was first introduced in the L5P, I am not sure if GM has redesigned this component or merely replaced the failed ones?
There were reports of some full DEF tanks cracking as well so it may be better to run the DEF at 1/2 full or below in the really cold weather to allow some room for expansion?

 

Fwiw, DEF was introduced in 2011 with the LML...

It's not 'new ' with the 2017+ L5P's

 

Cracked by expansion cold weather, yes