Mercedes SLC reduced engine performance

Reduced engine performance on an SLC, especially with an error stored, could point to a few things beyond just the main relay. While a failing relay is possible, it's worth considering other control system components too. Has the error code been read? Knowing the specific code will help narrow it down. Also, has anything else seemed off lately, like unusual idling or difficulty starting? A failing relay can sometimes cause intermittent issues before completely failing. Another thing to check is the battery voltage, a weak battery can cause weird ECU problems sometimes.

The error code points to the ECU itself being defective. Battery voltage is good. Other symptoms are limited to the reduced engine power and the error code. What else could cause the ECU to report itself as faulty, other than the ECU?

Okay, an ECU fault code specifically. While it's not common for an ECU to falsely report itself as defective, it's not impossible. Before condemning the ECU and opting for ECU repair, make absolutely sure the wiring harness and all connections to the ECU are in perfect condition. A damaged wire or a corroded connector can definitely cause false readings. If all wiring checks out, and you're certain the battery voltage is stable, then it's probably time to see a qualified workshop about ECU problems.

Wiring seemed fine, so I took it to the shop. Turns out it was indeed the main relay as suspected. After replacement and a system reset, engine performance is back to normal. Total cost: 120€.