Mercedes M Klasse hybrid starting issue

The symptoms you describe certainly suggest a potential issue with the power supply to the engine control unit. Given that your vehicle is a hybrid, the battery connections are paramount for proper engine operation. A degradation in these connections can indeed manifest as an engine control malfunction, especially if the ECU is not receiving sufficient power. It would be prudent to inspect the connections thoroughly. Are you equipped to measure the voltage at the ECU itself? Have you checked the ground connections as well?

Voltage at the ECU, eh I hadn't thought about that specifically. How would I go about checking that, and where exactly is the ECU located on this model Also, the ground connections – are there any specific ones I should be targeting, or just giving them all a once-over

Inspecting the ECU voltage requires a multimeter. Consult your vehicle's service manual, or search online resources, to pinpoint the ECU's location and the specific pins to test for voltage. Regarding grounding, give particular attention to the main ground strap connecting the engine block to the chassis. A loose or corroded connection there can disrupt the entire electrical system. However, considering the potential for ECU failure, and the complexity of hybrid systems, a comprehensive diagnostic assessment at a qualified workshop is advisable.

Thanks a bunch for the advice Turns out you were spot on about the battery cable connections. Cleaned them up real good, and she fired right up. The bill came out to a reasonable 155€ to make sure everything was shipshape. Appreciate the help