My 2011 ML350 Bluetec got 20B91B (The output for the heating element of the AdBlue pressure line has an electrical fault or an open circuit) and 20BA91B (this one is secondary/collateral - ) errors. After investigating I realized that this is NOT the error for the DEF tank heater but rather some (several) heaters of the DEF line. Looks like these heaters wrap around the DEF line to defrost it.

I identified 2 line heaters, one of them is A1644700124 (item 20 in the diagram), about 4ft of line which connects to another one that is long A1644700064 or A1644709064 (item 200 in the diagram) and connects to the DEF injector.

I have to identify which one of these heaters is malfunctioning and replace it.

Question(s):
- How many of these inline heaters are ? I measured the A1644700124 (next to DEF tank) at 115Ohms which for a heater is very low.
- What is supposed to be the resistance for A1644700124 (item 20 in the diagram) / A1644700064 or A1644709064 (item 200 in the diagram) ?
- Some debugging experience (what to measure) would help

Thanks,