First, the technician does a visual inspection of the vehicle’s relevant emissions components. Once the technician is satisfied that everything meets or exceeds standard, the vehicle’s gas cap is tested to ensure that it is airtight. This is an often a cause for a check engine light!
Finally, a computerized emissions testing machine is connected to the vehicle which runs a full set of On-Board Diagnostic (OBD) tests and tests the exhaust gasses. If everything is functioning as it should, the emissions tester will report no errors and the vehicle has passed the Emissions Test. If anything is malfunctioning, the emissions tester will deliver an error code which is used to identify what repairs are needed. These computerized emissions machines are regulated by the PA Department of Transportation (PENNdot) and are quality and calibrated for accuracy twice a year.