Why mercedes w168 cannot start?

Starter Motor does not operate - does not turn engine. Check Starter Lockout Relay (large green relay with 5 terminals, located behind ECU cover in engine compartment). Check all electrical connections, especially the one located under the screen washer bottle on the chassis. Check wiring from ignition switch. If Auto Transmission, check inhibitor switch on gear lever linkage. If battery is over 2 years old, have it tested - the A Class is susceptible to even slightly low battery voltage.