Posted this in the ML subgroup but no response, so I will post it here for help.

Wife's 2012 ML350 Bluetec popped a check engine light. Code is P20BD which is which is for "Redundant heater "B" (DEF) circuit low or no voltage. In this case it is for the DEF heater. I can clear the code and religiously about 30 miles later it will come on, so it is a hard failure.

Sooooo. it appears that the DEF tank heater took a dump. Before I throw $400+ on a new heater unit, does anybody have any good troubleshooting tips or even better, a schematic of the heater circuit that I could use to narrow it down?

Many thanks

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2012 Mercedes ML350 Bluetec 102K (hers)
2005 Corvette 55K (fun car)
2002 VW Jetta TDI 238K (mine)
1998 Volvo S70 T5 Turbo 211K (kids)
1994 Ford F150 4WD 246K (firewood hauler)
1983 Mercedes 300D 384K (diesel commuter)