The meter must have a diode test function if it's a digital in order to test diodes and transistors.

The cheapies aren't able to source enough voltage at constant current to forward bias the PN junction.

The diode appears to be there to keep the PCM POWER relay coil from experiencing back EMF from the ignition coil which might cause it to chatter. It will also keep the PCM from receiving power in the event that battery power is reversed.

Steve