Skip the auto shop - Our mechanics make house calls

Q: Does the OBD System Measure Gases in the Car's Exhaust?

asked by on

Does the OBD system measure gases in the car's exhaust?

A: The onboard diagnostic (OBD) system utilize...

The onboard diagnostic (OBD) system utilizes sensors mounted in the engine exhaust stream to measure the amount of oxygen that comes out of the engine after the combustion process. When an engine intakes a certain amount of air into the engine, the computer will inject the correct amount of fuel using its internal programming and input from several sensors, including the O2 (oxygen) sensors mounted in the exhaust manifold to test the exhaust oxygen content. The input of the O2 sensor is used to maintain a fuel-to-air ratio to be as close to stoichiometric 14.7 parts air to 1 part fuel.

The O2 sensor can measure the amount of oxygen in the exhaust stream using its internal design and the heat in the exhaust after the engine has completed the combustion process. If the combustion process was incomplete or too much fuel is injected, the O2 sensor will sense an increase in oxygen in the exhaust stream indicating the engine is not burning all the fuel. If the computer lowers the amount of fuel injected into the engine cylinders, then the O2 sensor may indicate a lower oxygen content in the exhaust. The computer will use this exhaust O2 sensor as feedback to control the fuel going into the engine.

The engine may use multiple O2 sensors to monitor the engine fuel control by having sensors on both sides of a V-style engine. This is done so dual exhaust can be utilized and monitoring of both sides of the engine can be done. Another O2 sensor can be used after a catalytic converter to measure the catalyst’s ability to burn off excessive fuel in the exhaust. This is done by comparing the before-catalyst O2 sensor and after-catalyst sensor readings.

If the engine gets a reading from an O2 sensor that indicates a low oxygen content, then the engine computer will start to lower the fuel amount injected until it gets to its programming limits or the O2 sensor starts to indicate a high oxygen content. If the computer never gets a change in the O2 sensor output after reaching its programmed limit on fuel control, then it will set a code in memory for the O2 sensor. This indicates a problem with the O2 sensor or other problems in the fuel system causing excessive fuel in the exhaust.

If the engine gets a reading from an O2 sensor that indicates too much oxygen is in the exhaust, the computer will try and compensate by adding more fuel to its programmed limits. If this limit is reached and the oxygen content is still high, then a code is set. This is due to the fact that the injection system cannot inject enough fuel or there is excessive oxygen getting into the engine unmetered. The O2 sensor may also be malfunctioning and might need to be replaced.

Was this answer helpful?

Need advice from certified mechanic? Get help now!

Over 1000 mechanics are ready to answer your question.
The statements expressed above are only for informational purposes and should be independently verified. Please see our terms of service for more details

Get an instant quote for your car

Our certified mechanics come to you ・Backed by 12-month, 12,000-mile guarantee・Save up to 30%

Get a quote

What others are asking

Q: Check Engine light won't go off

Hi there. The vapor purge solenoid releases the fumes in the gas tank. If the solenoid is stuck closed, the gasses can't escape and try to come out the fill tube. The nozzles at the gas pump are designed to...

Q: Car stalling at first start, black smoke from exhaust

Hello. There are a few possibilities for why your car is stalling when you first start it. Your car’s coolant temperature sensor could be sending the wrong information to the PCM, which will cause the PCM to send the wrong...

Q: My car has the OBD-II codes P0030 and P0135 for the heater control circuit and sensor. Where is this sensor located?

This is a code that pertains to the heater side of your oxygen sensor. The heater is required to raise the temperature of the sensor so that it can operate properly. Simply unplugging relays/fuses will not keep the code from...

Related articles

How Long Does a Distributor O Ring Last?
The distributor is part of the ignition system in your vehicle and its purpose is to route high voltage from the ignition coil to the spark plug. The spark plug then...
How to Renew Your Car Registration in Oklahoma(DELETED)
Having your car registered with the Oklahoma Tax Commission is an important part of driving legally in this state. You will need to do this within 30 days of moving to Oklahoma...
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...