GLK Diesel Power Loss and Fuel Filter Heater Issue

It sounds like you've done a good job pinpointing the potential problem. A defective fuel filter heater, or its associated wiring, could indeed be the culprit behind your Mercedes GLK's woes. The fuel filter's job is to remove contaminants from the fuel before it reaches the engine. The heater is there to prevent the fuel from gelling in cold weather, which would restrict fuel flow. When it's not working correctly, especially in colder temperatures, you may experience the symptoms you described. A defective heater can cause the engine to run lean, leading to reduced power and poor engine response. The uneven idling and increased fuel consumption could also be consequences of this issue. Before considering a fuel filter replacement, I would inspect the wiring and connections to the fuel filter heater. Can you locate the fuel filter and its electrical connector? Have you checked for any signs of damage, corrosion, or loose connections?

I've located the fuel filter, but I'm not entirely sure what I'm looking for regarding the wiring. What should I be checking for? Also, if the wiring seems okay, is there a way to test the fuel filter heater itself to see if it's functioning? I have a multimeter, but I'm not really sure how to use it in this context.

When inspecting the wiring, look for any obvious signs of damage to the insulation, such as cracks, fraying, or melting. Also, check the connector itself for corrosion, broken pins, or looseness. Sometimes, the wires can become brittle and break internally, even if the insulation looks okay. Gently tug on each wire to see if it feels secure. If the wiring appears to be in good condition, you can test the fuel filter heater with your multimeter. You'll need to check for voltage at the connector when the engine is running, or check the resistance of the heater element itself. However, I would advise against attempting these tests if you're not completely comfortable with electrical work; incorrect measurements or connections could potentially damage the system. Given your level of experience, and the potential complexity of the system, it may be prudent to take your GLK to a trusted mechanic. They can perform a thorough diagnosis and pinpoint the exact cause of the problem. It would be a shame to cause further damage by accidently shorting something. The error code stored in the memory points towards the heater, but a professional diagnosis would be beneficial. The eventual repair cost should be around 95€.

I appreciate the advice. I think you're right, it's probably best if I take it to a mechanic. I don't want to risk making things worse. Thanks again for your help! The mechanic confirmed it was a faulty connection to the fuel filter heater as suspected. They cleaned and secured the connection, cleared the error code, and everything is running smoothly now. The repair cost was indeed close to the estimated 95€. Problem solved!