What does a smog test actually check?
A smog test (or smog check) is a comprehensive inspection that ensures your vehicle's emission control system is complete, functional, and effective in reducing air pollution. It is typically broken down into three major parts:
1. Functional Inspection
This is the primary method for testing most vehicles built in the year 2000 and newer.
On-Board Diagnostics (OBD-II) System Check: The technician connects a scanner to your car's OBD port. The scanner checks the vehicle's computer for two things:
Stored Error Codes (DTCs): Any existing trouble codes that indicate a malfunction in the emission control system. If your "Check Engine" light (or MIL) is on, you will automatically fail.
Readiness Monitors: The computer must show that its internal self-tests for all emission systems (like the catalytic converter, oxygen sensors, and EVAP system) have been completed. If you recently disconnected the battery or cleared a code, these monitors may be "incomplete," resulting in a failure.
2. Visual Inspection
The technician visually inspects several physical components to ensure they are present, properly connected, and have not been tampered with or modified.
Emissions Components: They confirm the presence and condition of key parts like the Catalytic Converter, Oxygen Sensors, and the Exhaust Gas Recirculation (EGR) system.
Hoses and Wires: They check for broken, damaged, or disconnected vacuum lines and electrical wiring related to the emissions system.
Visible Smoke: They will observe the exhaust for any excessive black, white, or blue smoke, which indicates a severe engine or fuel issue.
3. Emissions Measurement
This is the traditional method, primarily used for older vehicles (typically pre-2000 models, though exact cutoffs vary by state).
Pollutant Measurement: A probe is inserted into your tailpipe to measure the actual amounts of harmful gases your vehicle is emitting. The levels must be below the state's maximum legal limit for your vehicle's make, model, and year.
Gases Measured: The inspection typically measures three primary air pollutants:
Hydrocarbons (HC): Unburned fuel.
Carbon Monoxide (CO): A product of incomplete combustion.
Nitrogen Oxides (NOx): Pollutants that form smog and acid rain.
The smog test is essentially a multi-point inspection to guarantee that the system designed to keep your car running clean is working as intended.