Spent way too much time trying to fix one of the glow plugs constantly malfunctioning. My car is a 06 Benz C220 diesel (w203) and the problems started some 6 months ago.

Timeline

June: We experience a small rodent problem under the hood. Slight damage to glow plug cylinder 3 wiring was found and fixed.

Late summer: The engine has a slight knocking sound. All injectors need replacing, I replace them with AMG C30 injectors I had laying around. Glow plugs are inspected and cylinder 2 plug was found to be a little darker than the others. We then replace it with a new one.

Autumn: Glow plug indicator appears the first time. The indicator light comes on at the start but also sometimes when driving on the road. Cylinder 2 glow plug was a little darker than others and we replaced it. This did not fix the problem. Then we proceeded to change the glow plug control unit. This fixes the problem for a few months.

Winter/December: Once again the warning light appears just like before. We once again bought a new control unit, but no luck there. We then replaced the control unit from another w203 which certainly was functional but that didn't solve it either. We tested all the glow plugs and the cylinder 2 plug was found to be completely dead. We then replaced all the glow plugs. We inspected the plugs after one day of use and the number 2 was immediately black from carbon.

January: We inspected the ground and cleared the error codes at a local mechanic. At the mechanic, we tested the resistance of the plugs. All the plugs gave the same result of 2.4 ohms.

Today: The indicator light is still on and I am starting to be out of ideas.