> Your Online Garage

Emissions: Inspections, Failures, and Repairs


Emissions systems first appeared on automobiles in the 60s!  Today, however, with new sticker emissions standards, the emission system has become more complex and important than ever.  Many components of the On-Board Computer System are also used in controlling and diagnosing emission control system failures.  Properly functioning emission control systems not only ensure a cleaner environment, but peak vehicle performance.

Emission Control System

  • Controls the formation of: hydrocarbons, carbon monoxide, oxides of nitrogen
  • Prevents the release of: crankcase fumes, gasoline vapors
  • Alerts drivers if vehicle emits excessive emissions (96 and newer vehicles)

Benefits of a Properly Maintained Emission Control System

  • Low emissions
  • Performance
  • Fuel economy
  • Longer engine life
  • Prevent premature oil contamination
  • Compliance with local emission laws

Air Injection Pump, Check-Valves, Control Valves

  • Pump turned by accessory belt, or electrically driven
  • Pumps outside air into exhaust system to assist in reducing hydrocarbons and carbon monoxide emissions

Symptoms of a malfunctioning air injection system include noise (whining or growling noise), failing state emission test, and warning indicator light (check engine).

Catalytic Converter

  • Installed in exhaust system (as many as four may be present)
  • Contains precious metals (platinum, palladium, rhodium)
  • Reduces hydrocarbons, carbon monoxide, oxides of nitrogen emissions

Symptoms of a malfunctioning catalytic converter include failing state emissions test, warning indicator light (check engine), reduced power (restricted converter), and no start (clogged converter).

Evaporative Emission Control

  • Charcoal canister (stores fuel vapors)
  • Fuel tank pressure sensor (some models) detects pressure losses in evaporative system
  • Gas cap
  • Prevents fuel vapors from escaping into atmosphere
  • Eliminates stored vapors when necessary

Symptoms of a malfunctioning evaporative system include failing state emissions test, warning indicator light (check engine), and fuel odor.

Exhaust Gas Re-circulation (EGR)

  • Located on or near intake manifold
  • Vacuum or electrically activated valve
  • Controls the flow of exhaust gases into intake manifold
  • Reduces combustion chamber temperatures to control Oxides of Nitrogen formation (NOx)

Symptoms of a malfunctioning EGR valve include failing state emissions test, warning indicator light (check engine), hesitation, stalling/rough idle, pinging noise on acceleration.

Oxygen Sensor (O2S or HO2S)

  • Located in the exhaust system upstream and/or downstream of the catalytic converter
  • Detects the amount of oxygen present in the exhaust gases
  • Provides information to the On-Board Computer System to help maintain proper air/fuel mixture
  • Diagnostic sensor to determine catalytic converter efficiency

Symptoms of a malfunctioning oxygen sensor include failing state emissions test, poor fuel economy, warning indicator light (check engine), hesitation, and stalling/rough or unstable idle.

Positive Crankcase Ventilation Valve (PCV)

  • Located in valve cover, intake manifold, or crankcase area
  • Vacuum operated valve
  • Contains spring and movable tapered valve
  • Regulates flow of air into crankcase to remove crankcase fumes

Symptoms of a malfunctioning PCV valve include oil leaks, oil in air filter housing, and high/low or unstable idle.

Sign up for exclusive offers!