If your vehicle is showing failed emissions test or smog check, here are the most likely causes and the parts you'll need to fix each one.
A loose, damaged, or missing gas cap allows fuel vapors to escape into the atmosphere. This triggers an EVAP system leak code and an automatic failure during an OBD-II emissions test.
If the battery was recently disconnected or a mechanic cleared the Check Engine Light codes, the engine computer's self-tests (readiness monitors) reset. The vehicle will automatically fail the smog check until it is driven enough to complete these tests.
Degraded oxygen sensors send inaccurate exhaust data to the engine computer. This causes the engine to run too rich (too much fuel) or too lean (too much air), increasing harmful tailpipe emissions and triggering a Check Engine Light.
Old spark plugs or weak ignition coils cause incomplete combustion in the engine cylinders. This dumps unburned fuel directly into the exhaust, causing a massive spike in Hydrocarbon (HC) emissions and potentially damaging the catalytic converter.
The Evaporative Emissions (EVAP) system traps and burns fuel vapors. A stuck purge valve, faulty vent valve, or saturated vapor canister will trigger a Check Engine Light and cause an automatic emissions failure.
A dirty MAF sensor miscalculates the volume of air entering the engine. This leads to improper fuel delivery, causing the engine to run rich or lean and elevating Carbon Monoxide (CO) or Hydrocarbon (HC) emissions.
The catalytic converter is responsible for scrubbing harmful gases (NOx, CO, HC) from the exhaust. Over time, or due to chronic engine misfires, the internal catalyst breaks down and loses its ability to clean the exhaust.
The Exhaust Gas Recirculation (EGR) valve lowers combustion temperatures to reduce Nitrogen Oxide (NOx) emissions. If the valve fails or its passages clog with carbon, NOx emissions will spike, causing a tailpipe test failure.