Emission Control Systems
6 LessonsUnderstand catalytic converters, EGR, PCV, EVAP, and how emissions testing really works.
Overview
Emission systems reduce harmful exhaust pollutants to meet federal and state standards. This module covers catalytic converters, EGR systems, PCV valves, EVAP systems, secondary air injection, and the OBD-II monitors that verify everything is working. Understanding emissions is understanding how engines are designed to run clean.
Key Components
- Catalytic converter (TWC)
- EGR valve and passages
- PCV system
- EVAP system (canister, purge, vent)
- Oxygen sensors (upstream and downstream)
How It Works
The catalytic converter chemically converts harmful gases (HC, CO, NOx) into harmless ones (H2O, CO2, N2). The PCM monitors converter efficiency using upstream and downstream O2 sensors. EGR reduces combustion temperatures to limit NOx. The EVAP system captures fuel vapors and routes them to the engine for burning.
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Join the Nation — FreeCommon Problems
- Catalyst efficiency below threshold (P0420)
- EGR passages clogged with carbon
- EVAP leaks from cracked hoses or bad gas cap
- PCV valve stuck causing oil consumption
- O2 sensor degradation causing poor fuel economy
Diagnostic Tips
- Compare upstream and downstream O2 sensor waveforms
- EGR flow can be tested with scan tool bi-directional control
- Smoke machine is essential for EVAP leak diagnosis
- Check PCV system before chasing oil consumption codes
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