1. Start the engine at idle speed.
2. Check the operation of each individual valve with a stethoscope. Clicks indicate the opening and closing of the injector valves.
3. If you don't have a stethoscope, you can perform the same operation using a screwdriver or your finger.
4. If unusual sounds are detected or no sounds are detected at all, then it is necessary to check the voltage, resistance and tightness of the injectors.
Checking voltage and resistance
1. Check the voltage. To do this, disconnect the injector connectors and connect a diode test lamp between both wire contacts. Turn on the starter (assistant). The LED should blink.
2. If the LED does not blink, the fault may be in the wire itself or in the control unit.
(Original source of information: Mercedesman.ru)
3. Turn off the ignition.
4. Connect an ohmmeter between the contacts of each injector and measure the resistance. If the resistance is different on any injector, replace it.