Mercedes GLS diesel AdBlue issue

That sounds like a severe AdBlue issue. I had a similar problem with my BMW 3 series. The Selective Catalytic Reduction (SCR) catalyst system relies on Diesel Exhaust Fluid (DEF), which is essentially AdBlue, to reduce nitrogen oxide emissions by injecting the DEF fluid into the exhaust stream. If the system detects a problem, like an empty tank or a malfunctioning injector, it can prevent the engine from starting to comply with emissions regulations. Have you checked the DEF fluid level sensor? Is there any unusual smell around the AdBlue tank or the injection system? A leak could cause the system to register a low fluid level even if the tank capacity is adequate.

I have not checked the DEF fluid level sensor specifically. Is this complicated? I also have not noticed any unusual smells. What is the purpose of the injection system in relation to the SCR catalyst, and could a problem there really stop the car from starting?

The injection system's role is critical. It precisely meters the DEF fluid into the exhaust stream, allowing the SCR catalyst to convert harmful NOx into nitrogen and water. If the system suspects a malfunction, like a blockage or incorrect injection quantity, it will trigger a fault. I am not familiar with the Mercedes system, so I cannot explain the procedure for checking the sensor. I recommend a workshop visit. It sounds like the AdBlue output from tank to dosing unit is defective as you mentioned. If that is the case, you may be looking at a repair cost in the region of 250€.

I took the Mercedes to a workshop, and the diagnosis confirmed a defective AdBlue output from the tank to the dosing unit, as suspected. The emissions control system is now functioning correctly after replacement. Your advice regarding the injection system and its relationship to the SCR catalyst helped me to understand the problem better.