Why Need TOPDON AL300, an OBD2 Code Reader?. TOPDON AL300, an excellent OBD2 code reader enables more people to perform pre-inspections for smog tests on their own. By retrieving the I/M Readiness Status and O2 sensor data, this OBD2 scanner checks if emissions-related systems are ready for Inspection and Maintenance testing to avoid the fine and rejection in the inspection.