The invention provides for a diagnostic probe assembly for a tester which is used to diagnose printhead integrated circuits. The probe assembly includes a support assembly and a controller board mounted on the support assembly and having a processor configured to generate test signals for testing a printhead integrated circuit. A routing board is in operative signal communication with the controller board and is configured to multiplex the generated test signals for respective dies of the printhead integrated circuits. The probe assembly also includes a probe interface in signal communication with the routing board and configured for relaying the multiplexed test signals to and from the respective dies.