Skip the auto shop - Our mechanics make house calls

What Gases Does the OBD Detect in the Exhaust?

obd exhaust

Your engine runs on combustion – fire – which creates exhaust gases. A wide range of gases are produced during normal operation, and they must be controlled because many of them become pollutants when released into the atmosphere. It’s actually a common misconception that the on-board diagnostic (OBD) system on your car detects gases, but it doesn’t. It detects malfunctions with emissions equipment (the catalytic converter, oxygen sensors, fuel tank purge valve, etc.).

Oxygen sensors

Part of the confusion here surrounds the catalytic converter and the car’s oxygen sensor(s). Your car may have one or two catalytic converters, and one or more oxygen sensors (some have multiple O2 sensors located at different points in the exhaust system.)

The catalytic converter is located around the midway point in the exhaust on most vehicles (this can vary, though). Its job is to heat up and burn away exhaust gases present in all automobiles. However, the OBD system does not measure these gases, with the sole exception of oxygen.

Oxygen sensors (or O2 sensors), are responsible for measuring the amount of unburned oxygen in your car’s exhaust, and then transmitting that information to the car’s computer. Based on the information from the O2 sensors, the computer can adjust the air/fuel mixture to ensure that it doesn’t run lean or rich (too little oxygen or too much oxygen, respectively).

Other components monitored by the OBD system

The OBD system monitors a number of different components tied to the fuel/evap system, emissions system, and other systems, including:

  • EGR valve
  • Thermostat
  • Catalyst heater
  • Positive crankcase ventilation system
  • Some components of the AC system

However, the OBD system does not monitor gases – it monitors voltage and resistance that could indicate a problem with those components (and therefire with the overall emissions of the vehicle itself).

The statements expressed above are only for informational purposes and should be independently verified. Please see our terms of service for more details
Icon-warranty_badge-02

Skip the repair shop, our top-rated mechanics come to you.

At your home or office

Choose from 600+ repair, maintenance & diagnostic services. Our top-rated mechanics bring all parts & tools to your location.

Fair & transparent pricing

See labor & parts costs upfront, so you can book with confidence.

12-month, 12,000-mile warranty

Our services are backed by a 12-month, 12,000-mile warranty for your peace of mind.

Get A Quote

Need Help With Your Car?

Our certified mobile mechanics make house calls in over 2,000 U.S. cities. Fast, free online quotes for your car repair.

GET A QUOTE

More related articles

Insurance Requirements for Car Registration in Kentucky
The Kentucky Transportation Cabinet requires that all drivers in Kentucky carry liability automotive insurance, or “financial responsibility” in order to operate a vehicle legally and maintain vehicle...
Rules of the Road For Iowa Drivers
Driving on the roads requires knowledge of the rules, many of which are based on common sense and courtesy. However, even though you know the rules in...
Veteran and Military Driver Laws and Benefits in Idaho
The state of Idaho offers a number of benefits and perks for those Americans who have either served in an Armed Forces branch in the...


Related questions

Q: Engine code after repair

If the sensor is defective then the sensor can be replaced without replacing the entire housing again. The sensor will need to be checked with a scan tool to see if its giving the correct readings by comparing the data...

Q: Need a diagnostic.

Most shops charge an hour of labor for a diagnostic. The price for this hour of labor can range anywhere from $90.00 up to $120.00. If you have your diagnostic performed by a technician from YourMechanic you can get this...

Q: Evap solenoid system

I applaud your diligence in reading the code and working the specific problem! There is another angle to this issue that I'm sure was next on your list - cleaning (or replacing) the evaporation vent purge valve, which accumulates dust...