Mercedes GLC Diesel Glow Control Unit Problem

Glow control unit issues on diesel engines are pretty common. The persistent preheating lamp and check engine light definitely point in that direction. The smoke when accelerating could be due to engine misfire during cold start, since the glow plugs aren't heating the cylinders properly. Before replacing the unit, check the voltage at the glow plugs themselves. Are you comfortable testing that with a multimeter?

Thank you for your swift response. I don't have a multimeter at hand. Where exactly should I perform the diagnostic test with a multimeter?

Without the right tools, you may not fix it yourself. If you don't have a multimeter, or the know-how to use it, it's probably best to take it to a workshop. A faulty connection can sometimes be fixed easily, but if the glow control unit itself is damaged, it will need replacing. It looks like a faulty cable or plug connection of the component is defective. Replacing the glow control unit would cost 95€.

I appreciate your advice. I took the Mercedes to a workshop, and as you suggested, the glow control unit was indeed defective due to a corroded connector. They replaced the connector, and the preheating lamp now switches off as expected. Thank you once again for the help.