What Gases Does the OBD Detect in the Exhaust?

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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).


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Arturo

25 years of experience
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Arturo
25 years of experience
Volkswagen Tiguan - Brake Pads Replacement (Rear) - La Jolla, California
Arturo was efficient and extremely professional. I'll refer him to friends and family and call him for any issues related to my car. I am very happy with the service!
Kia Forte - Brake Pads Replacement (Front) - San Diego, California
ARTURO is great! very professional , fast, replaced my front brake pads and oil an filter change. everything now works and works well. Brakes have much improved stopping power and engine is smooth and quiet.Arturo is also kind and thoughtful. "Your mechanic" is very fortunate to have Arturo Lopez represent your company. I enthusiastically recommend him and "Your mechanic> as a fine operation Best regards to the Team Peter Marto

Travis

11 years of experience
312 reviews
Travis
11 years of experience
Ford Explorer Sport Trac - Brake Pads Replacement (Rear) - Minneapolis, Minnesota
Travis is great! This is the second time he has come and serviced our truck. I recommend Travis for any service ned you may have!
Subaru Forester - Brake Pads Replacement (Front, Rear) - Rosemount, Minnesota
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Scott

32 years of experience
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Scott
32 years of experience
Hyundai Sonata - Brake Pads Replacement (Front, Rear) - Denver, Colorado
Arrived on time and worked quickly. Gave me detailed information on his completed work and the details of my car. Answered all of my questions thoroughly.
Lexus IS250 - Brake Pads Replacement (Rear) - Denver, Colorado
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9 years of experience
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Nicholas
9 years of experience
Lexus GX470 - Brake Pads Replacement (Rear) - Clayton, North Carolina
Nicholas texted me 30 minutes prior to arrival. Showed up on time as scheduled. He confirmed the work that I requested be performed. He completed the work, performed the road tested, and delivered my keys along with recommendations of other work that might be in my best interest. Was very pleasant, professional, and cleaned up after he was done. I was so impressed with the service he performed and the speed at which the services were completed. I have already scheduled a future service with Nicholas and will schedule any and all future repairs with him if he is available.

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