Kinda Weird
 

Last night while driving about 70ish on the freeway with our 2012 ML-350 Bluetec, the check engine light came on steady. The vehicle ran just fine, no difference in performance or odd noises. Read the codes this morning with 2 different OBDII readers and they both showed no codes stored! I started the car and the light stayed off. The rig had been sitting for about 8 days while we were on vacation, so I'm wondering if this had anything to do with it, or if it was a fluke, or????

Several weeks ago I had the same thing happen to my 2012 ML350. After sitting for about 4 days, I went to get gas. On the trip to the gas station everything was fine. Starting up from the gas station the engine had a slight miss and the check engine light came on. I stopped and re-checked the gas cap. It was loose so I gave it 4 turns. Started engine, ran fine, no light, and has been running fine since.

Well, the light stayed off for a 45 mile trip. Sat for about an hour and a half and on re-start, light came on again. Drove it home, with no issues, ran fine, etc.....put a code reader on it and no codes. Started it again and no light.......LOL

You need a reader that can read all the cars systems.

That's the plan if it keeps coming up

Ok, the code kept coming back each time I cleared it. Pretty much about 40 miles to the mile after clearing the code it would pop the check engine light. I borrowed a friends ICarSoft MB II code reader for Mercedes and it came up with the code.......P20BD, which is for "Redundant heater "B" (DEF) circuit low or no voltage.

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

Anyone??

Well, it did turn out to be the DEF heater was bad. Not a huge deal to repair, just a needless PITA......

At that price, I would have looked into the various ECU def bypass hacks. $400 would infuriated me :(

Cheapest hack I found was $899 for the tune and another $800 for the DPF delete pipe and EGR plates. At those prices $400 Is not too bad. However this is the car's one "freebie" so to speak. Anything else happens with the Bluetec system and I am ripping it out.