Start the engine and let it idle.
Check each injector with a running engine with a stethoscope to see if it clicks. These clicks occur as the nozzle opens and closes.
If there is no stethoscope, then you can check the injectors with a screwdriver or simply by touching your finger.
If an unusual noise about operation is determined or there is none at all, then you need to check the voltage supply, resistance and density of the nozzles.
Checking the supply voltage and resistance
Check voltage supply. To do this, remove the plug from the nozzle and connect the LED probe (or voltmeter) between two contacts. Have an assistant turn on the starter. Probe LED should blink (voltmeter should show voltage spikes).
If the LED does not flash, then the voltage supply circuit or the control unit itself is faulty.
Switch off the ignition.
Connect an ohmmeter between both terminals on each injector and measure the resistance. If the resistance on one of the injectors is very different from the others, then it must be replaced.
Density check
Remove the injector, leaving the valves and fuel lines connected to the fuel distributor pipe.
Put 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 you need to compare the spray pattern of the jet at different nozzles. The jet of fuel should be cone-shaped and look about the same for all injectors.
Switch 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 - they should not leak more than one drop of fuel per minute.
Install injectors.