What gases does OBD detect in the exhaust?
Your engine runs on combustion—fire—which creates exhaust gases. A wide range of gases are produced during normal operation and must be controlled as many become pollutants when released into the atmosphere. It's actually a common misconception that your vehicle's on-board diagnostic (OBD) system detects gases, but that's not the case. Detects faults in exhaust equipment (catalytic converter, oxygen sensors, fuel tank purge valve, etc.).
Oxygen sensors
Part of the confusion here has to do with the catalytic converter and the vehicle's oxygen sensor(s). Your vehicle may have one or two catalytic converters and one or more oxygen sensors (some have multiple oxygen sensors located at different points in the exhaust system).
The catalytic converter is located approximately in the middle of the exhaust pipe on most vehicles (although this may vary). Its job is to heat and burn the exhaust gases present in all cars. However, the OBD system does not measure these gases, with the exception of oxygen.
Oxygen sensors (or O2 sensors) are responsible for measuring the amount of unburned oxygen in your car's exhaust and then relaying that information to the car's computer. Based on information from the O2 sensors, the computer can adjust the air-fuel mixture so that it does not run lean or rich (too little oxygen or too much oxygen, respectively).
Other components controlled by the OBD system
The OBD system monitors a number of different components related to the fuel/evaporation system, emissions system, and other systems, including:
- EGR valve
- Thermostat
- catalytic heater
- Forced crankcase ventilation system
- Some components of the AC system
However, the OBD system does not monitor gases - it monitors voltage and resistance, which can indicate a problem with these components (and therefore the overall emissions of the vehicle itself).
