Start the engine and let it idle.
Check each injector with a stethoscope while the engine is running to see if it clicks. These clicks occur when the injector opens and closes.
If you don't have a stethoscope, you can check the injectors with a screwdriver or just by touching them with your finger.
If unusual noise is detected during operation or there is no noise at all, then it is necessary to check the voltage supply, resistance and density of the injectors.
Checking the supply voltage and resistance
Check the voltage supply. To do this, remove the plug from the injector and connect the LED tester (or a voltmeter) between the two contacts. The assistant should turn on the starter. The LED of the tester should blink (the voltmeter should show voltage surges).
If the LED does not blink, then the voltage supply circuit or the control unit itself is faulty.
Turn off the ignition.
Connect an ohmmeter between both contacts on each of the injectors and measure the resistance. If the resistance on one of the injectors is very different from the others, then it needs to be replaced.
Density check
Remove the injector, leaving the valves and fuel lines connected to the fuel rail.
Place the plug on the nozzle.
Place the nozzle in a suitable measuring container.
The assistant should turn on the starter for a few seconds and compare the spray pattern of the different injectors. The fuel jet should be cone-shaped and look approximately the same for all injectors.
Turn off the ignition.
Pull the plugs off the injectors.
Turn on the ignition for about 5 seconds without turning on the starter. Then check the tightness of the injectors - no more than one drop of fuel per minute should flow out of them.
Install the injectors.