Oxygen Sensors, otherwise referred to as O2 Sensors, and their related check engine light codes, can be troublesome at the very best! Today we discuss the design and function of these sensors, and some of the pitfalls encountered during the diagnosis of their related fault codes.




It seems as though the check engine light and O2 Sensor codes have always gone hand in hand. I can’t even begin to tell you how many perfectly good sensors I’ve seen replaced for a rich/lean code, or as a cure to a service engine light that comes and goes. Fact is, when oxygen sensors were first being used, they did tend to fail. But manufacturers have been redesigning and tweaking these parts for over twenty years, and nowadays they are fairly bullet proof! Oxygen sensor codes no longer mean "replace the oxygen sensor", and this mindset can be costly! So what goes wrong during the diagnosis of these systems? Well, let me share my experience and you can take it for what it’s worth!

Nearly all engine control systems are comprised of multiple sub-systems. These sub-systems each depend on correct input from other areas of the electronic engine control system in order to perform their individual tasks properly. Confused yet? Keep reading, it’ll make sense soon!

There are a large number of codes relating to oxygen sensor faults. There are also a lot of faults that are often incorrectly thought to be caused by a bad oxygen sensor (also referred to as an O2 Sensor). The O2 codes relating to circuit faults can be tested fairly easily. For help on these codes, check out our Generic OBDII list and definitely read our article on diagnosing the wiring related portion of a trouble code. You will also run into codes for the O2 heater circuits. Newer O2 sensors have heaters as part of their design. This allows them to get up to temperature quicker and begin functioning sooner, resulting in lower overall vehicle emissions. If you ever need specific in depth code help, use the “Get Help” link and we can run down the basics with you as needed. The codes that may give you more of a run for your money are the PO171-P0175 generic (or similar manufacturer specific) codes. These codes tell you that despite the computers attempts to correct fuel trim (the amount of time the injectors stay on, delivering more or less fuel as needed); it can not obtain the correct air/fuel ratio.

These are known as rich and lean codes, and almost as often as I’ve seen new oxygen sensors installed for these codes, I’ve seen the same vehicles need re-diagnosis!



First we need to understand how an oxygen sensor works. The oxygen sensor screws into the exhaust and the sensor end protrudes into the pipe so that exhaust gases pass across the sensors internal element. There is a steel shielding with slots that direct exhaust flow across the actual element. As a note, oxygen sensors used to determine engine fuel ratios are always located in front of the catalytic converter. The sensors behind the cats are called monitors and we will discuss these later. Sensors are normally described with the prefix HO2S meaning Heated Oxygen Sensor and followed with bank and position number. For example HO2S11 would be the oxygen sensor on bank 1 and first in line (pre catalytic converter) and HO2S12 would be on bank 1, but second in line (the monitor) An oxygen sensor creates a voltage between 0 and 1 volt by means of a chemical reaction between the sensor element and the oxygen in the exhaust passing across that element. Outside air also passes through the sensor and it is a comparison between the oxygen content in the exhaust and fresh air that actually determines voltage output (information for trivial purposes only and for curious techs). Believe it or not, the fresh air on many sensors actually travels through the wiring insulation! Anyway, a voltage closer to one volt indicates a rich condition and closer to zero indicates lean. The computer uses this valve to constantly adjust fuel trim to maintain a 14.7:1 air fuel ratio. We will be putting up an article on fuel trim soon but this is a huge topic all on its own. Okay so this is where it gets somewhat complicated. Just kidding! It’s all nuts and bolts; and wires, and chemicals and never mind!



Okay so the more oxygen in the exhaust (lean condition) the lower the voltage from the O2 sensor and the more fuel the computer calls for. Then the oxygen content in the exhaust drops (because it is being used up in the combustion process), and the voltage increases (rich condition) and the process repeats, for as long as the car is running, hundreds of times a minute. A gasoline internal combustion engine needs oxygen to burn the fuel. If the mixture is ideal (or 14.7:1) then all of the oxygen is consumed as the fuel is burned. The exact amount of fuel needed to produce a 14.7:1 air/fuel ratio varies with barometric pressure (altitude), relative humidity and fuel quality and condition; thus the need for oxygen sensors.

So knowing all of this what should we check when we have those pesky rich or lean codes? The most common issues for lean codes are:

1. Vacuum leaks - check for failed or loose vacuum lines, leaking intake gaskets, intake air tubes loose or any other source of un-metered air leaks (leaks after the Mass Air Flow Sensor)
2. Restricted fuel filter or bent/pinched fuel system lines
3. Incorrect input from other sensors, such as the Mass Air Flow Sensor, which may not always drop a separate code
4. Engine misfire – Yes I know this one may seem weird. You might think that if there is a misfire then you will have all that unburned fuel and it should read rich; right? Well the O2 sensors read only oxygen content in the exhaust, so if you have all that unburned fuel from incomplete combustion then, you guessed it, you also have all that unburned oxygen. High O2 content in exhaust equals a lean reading!



There are also some other possibilities such as an internally leaking EGR system, (but this will typically set a separate code). A leak in the exhaust system before the O2 sensor will also cause incorrect readings. And always check for after- market modifications. These can throw a wrench into the works! The only other possibilities (however unlikely), are wiring issues, computer concerns or a bad O2 sensor! There now that I’ve said it, on to rich codes.
The possible causes of rich codes are:

1. A leaking or faulty fuel injector
2. Fuel injector driver in computer shorted, or wiring short for injectors (likely a ground short)
3. Leaking or faulty fuel pressure regulator or restricted return line
4. Faulty evaporative emissions system - bleeding fuel vapors into engine (not commanded by computer)
5. On newer models a faulty fuel pump or fuel pump driver module
6. Faulty readings from other sensors such as a Mass Air Flow Sensor. You may actually be getting more air than the MAF tells the computer
7. Exhaust leaks before the sensor will cause erratic readings
8. After market components or performance chips
9. And yes, if I dare say it, possibly a computer, wiring issue or even a faulty O2 sensor!



The other codes we should address are those related to the sensors located after the catalytic converter. Though these may appear identical to the oxygen sensors pre-converter, they perform an entirely different task and are known as Monitors. The only job of these sensors is to “monitor” the efficiency of the catalytic converters. The readings from these sensors should be much more stable and not fluctuating like the front O2 sensors. The computer compares the readings from the oxygen sensors (pre cats) and the monitors (post cat) to determine if the catalytic converters are doing their job and “cleaning” the exhaust. You never want to replace a monitor for a rich/lean concern as they have no bearing on these codes. As the converters begin to fail, you will see the monitors voltage readings follow the oxygen sensor readings. Technically these are all “oxygen sensors” but it is important to distinguish the difference between pre-converter & post converter sensors, so I find it easiest to stick to calling the back ones monitors.



And that pretty much wraps up my ideas on this topic. If you’ve run into other causes of these codes for gasoline engines then use the contact form and educate me! The quickest way to learn is through somebody else’s experience! Hope we helped to shed some light.



The engine-light-help.com Team