Emission System Overview

The combustion process in an engine produces several harmful byproducts. Carbon monoxide — a poisonous gas that kills by displacing oxygen in your blood. Hydrocarbons — unburned fuel that contributes to smog. Oxides of nitrogen — NOx — gases formed under extreme combustion heat that contribute to smog and acid rain. The emission control systems on the vehicle are designed to reduce these harmful outputs to levels mandated by federal and state law. Every system works together — disabling or neglecting one affects the others.

Catalytic converter — the big one. Chemically converts harmful exhaust gases into less harmful ones using precious metal catalysts. EGR — exhaust gas recirculation — feeds a controlled amount of inert exhaust gas back into the intake to lower combustion temperature and reduce NOx formation. EVAP — evaporative emission system — captures fuel vapors from the tank so they do not escape into the atmosphere. PCV — positive crankcase ventilation — routes blow-by gases from the crankcase back through the intake so the engine can burn them instead of venting them to the air. Each system targets a specific pollutant or emission pathway.

Converts carbon monoxide to carbon dioxide. Converts hydrocarbons to water vapor and carbon dioxide. Reduces oxides of nitrogen to plain nitrogen and oxygen. The converter only works above approximately 500 degrees Fahrenheit — called light-off temperature. This is why emission output is highest during cold startup before the converter reaches operating temperature. Modern vehicles use close-coupled converters mounted near the exhaust manifold so they heat up faster.

The PCM continuously monitors every emission-related system through a series of self-tests called monitors. The catalyst monitor compares upstream and downstream O2 sensor signals. The EVAP monitor pressure-tests the fuel vapor system for leaks. The EGR monitor checks for proper exhaust gas flow. The misfire monitor watches for combustion events that fail. When a monitor detects a problem on two consecutive drive cycles, it sets a diagnostic trouble code and turns on the check engine light. Understanding monitors is critical because many state emission inspections require that all monitors have run and passed.

In many states, vehicles must pass an emission inspection to be registered. Failed emission tests are one of the most common reasons customers bring vehicles to the shop. A check engine light is an automatic failure in most states regardless of the code. Understanding how each emission system works and how to diagnose it is a daily bread-and-butter skill. It is also federal law — tampering with or removing emission control devices carries significant fines. You will be asked to do it. The answer is always no.