AdBlue tank error on Mercedes M Klasse diesel

That's a frustrating one. I had a similar issue on my Mercedes, though I can’t remember the year. The engine wouldn't start either. The problem lies within the AdBlue system, specifically the delivery of DEF fluid to the diesel exhaust. Have you checked the pump that pushes the AdBlue to the injection system? These pumps sometimes fail, or the lines get blocked. How many kilometers are on the clock? The tank capacity is large, but the system is sensitive to low levels, so the error message is displayed quickly. Did you manage to read the error code from the ECU, or do you only see the message in the speedometer?

Good point about the pump. I haven't checked it directly yet, but I was wondering if that could be the culprit. Is there a way to test the pump's output without specialized tools? Also, the car has about 207573 km on it. I did read the error code, and it specifically pointed to a low AdBlue level, even though the tank is full. Is it likely a faulty sensor in that case?

Based on your description, the sensor might be faulty, but I wouldn’t rule out the pump entirely. Without proper diagnostic tools, testing the pump output is tricky, and you could easily damage something. I would advise taking it to a workshop equipped with Mercedes diagnostic tools. They can properly diagnose the issue, whether it's the pump, the sensor, or a blockage in the lines. A broken pump could also damage the injection system. For me, the garage determined a faulty pump was the culprit and they replaced it along with some clogged lines. I remember it was around 250€, all in. I’m pretty sure they will quickly sort it out for you.

Thanks for the advice! I took it to a workshop, and you were spot on – it was the AdBlue pump. They replaced it, cleared the error codes, and now it's running perfectly. Cost me about 250€ including labor. Appreciate the help!