The Oxygen Sensor

The Oxygen Sensor
Hello 93636. My name is Warren Parr.
I have been an ASE Master Certified (A1-A8) and Advanced Level (L1) Automotive Technician for more than 25 years. I have also been a California Smog Technician for about 16 years. The editor of the Ranchos Independent and I thought it would be a good idea to write a monthly column regarding the automotive industry and answer your car questions about your personal vehicles. I have decided to begin my column by talking about what I believe is the most important and misunderstood sensor in the modern vehicle. It makes electronic fuel injection possible. If it goes bad, your fuel mileage will decrease but the engine will run fine.

The Oxygen Sensor
I have replaced hundreds of oxygen sensors in my career but rarely for a drivability complaint. Most of the oxygen sensors I replace are because they wear down and become slow. Oxygen sensors are located in the engine’s exhaust and are in charge of “Quality Control” of the fuel management system (FMS). They are tattle tales! Their job is to measure the unused oxygen in the exhaust and feedback that information to the FMS. The FMS uses that information to modify its fuel calculation.

Oxygen sensors also play other important roles such as in the self-evaluation tests vehicles preform on their own emission systems. These tests are called OBDII monitors and the results of those monitors are retrieved from the vehicle’s powertrain control module (PCM or computer) during emissions testing. If an OBDII monitor fails or fails to complete, the vehicle fails its emissions test even if the malfunction indicator lamp (MIL) is off and it passes out the tail pipe.

Let me give you a very simple example of how an oxygen sensor participates in the OBDII monitor testing. Some vehicles have air pumps. One of the jobs of the air pump is to blow air into the exhaust in front of the catalytic converter. The oxygen in the air is used to light off the converter more quickly just like fanning a fire will create a hotter fire. During the air pump monitor, the PCM will momentarily energize the air pump and monitor the oxygen sensor for increased oxygen content in the exhaust. The oxygen sensor is the primary component used in evaluating the air pump’s performance.

Running on Two Fuels The internal combustion engine runs on two fuels: fossil fuel and oxygen. The air drawn into the engine is measured by the mass air flow (MAF) sensor. The approximate oxygen content of air is 21 percent. The PCM searches its fuel look up table to find the correct amount of gasoline needed for the oxygen content entering the engine. The fuel injectors are opened for the exact amount of time to deliver the precise amount of gasoline. The perfect air/fuel ratio is 14.7 parts of oxygen to one part of gasoline. After combustion, the cylinders discharge their contents into the exhaust where the oxygen sensor is waiting. Too much unused oxygen means the correct air fuel ratio was not achieved and not enough gasoline was injected. Too little unused oxygen means too much gas was injected. With today’s modern engines, it’s all about the oxygen. The oxygen entering the engine and exiting the engine is measured. From those two measurements, all of the other engine management calculations are made. If the base calculations and adjustments are correct, all the fuels are combusted resulting in good fuel mileage and low emissions. By the way, unburned or partially burned fuel is called tail pipe emissions.

A Real Life Example Today I was diagnosing a 2001 Toyota 4Runner for a failed smog inspection. Tail pipe emissions were very high, the MIL was on and trouble code P0171 was stored. The standard definition for trouble code P0171 is “Lean Exhaust Cylinder Bank One.” That means the oxygen content is too high in the engine’s bank one exhaust according to the oxygen sensor.

Where did I start? The same place I always start. I tested the sensor to see if it was lying. I connected my lab scope to the sensor and added propane to the running engine. This procedure creates an overfueling condition, thereby lowering the oxygen content in the exhaust. The sensor saw the low oxygen content and responded correctly. Next, I created a large vacuum leak which caused an under-fueling condition or excessive oxygen content in the exhaust. Again, the sensor saw the high oxygen content in the exhaust and responded correctly. OK, the oxygen sensor was functioning correctly. It’s not my problem and I can trust what it’s telling me so I move my diagnostics forward by looking for the cause of the under-fueling condition. The problem was a contaminated mass air flow (MAF) sensor. The customer neglected to change his air filter on a timely basis and dirt got into the MAF sensor causing it to underestimate the air entering the engine. Underestimated air means the FMS made its fuel calculation based on the amount of air it thought was entering the engine instead of the amount of air actually entering the engine. The oxygen sensor did its job. Many times I have seen oxygen sensors replaced for these kinds of codes. Let me state right now, “THE MOST EXPENSIVE WAY TO REPAIR A VEHICLE IS TO GUESS.” Replacing good parts with new parts will only increase the cost of the repair. It will not correct the problem. The proper method for a quality and accurate repair starts with testing.

$20 or $500? Too bad for this customer. He could have avoided his failed smog test and the $500 bill by simply performing his routine maintenance on time including replacing the $20 air filter when needed. Whoever had been servicing his vehicle let him down by not performing complete and proper maintenance inspections. Most likely he thought he was saving money by doing the vehicle maintenance himself. Saving money by servicing your own vehicle is very possible if you use and follow the recommendations and instructions in the vehicle’s owner’s manual. I am here to help you with that and other things if you would like my advice. If you follow it, I believe you will save money, increase the reliability, life and performance of your vehicle.

If you have any questions regarding this article, my industry and, most importantly, your vehicle, please email me at complete_car_care@hotmail.com or send your questions to the Ranchos Independent. I will gather the information and answer your questions in future columns. Thanks again and happy driving.