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17th Annual Technical Forum: Oxygen Sensors

Counterman magazine presents 15 technical and sales topics in an easy-to-read question-and-answer format for the magazine’s annual Technical Forum. This article appeared in the August 2009 issue.


Q. My Check Engine light is on and the code is P0171 (System Lean, Bank 1). Does that mean I need a new oxygen sensor?
A. Probably not. If your oxygen sensor were bad, it usually causes the fuel system to run rich. This would likely set a P0172 rich code, not a lean code.

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Assuming there are no other codes present (no oxygen sensor or other sensor codes), the fuel system may be running lean for a variety of reasons. One common cause is air or vacuum leaks between the throttle body and the cylinder heads. Leaks at vacuum hose connections, intake manifold gaskets, or the PCV or EGR valves can all allow air into the manifold causing a lean fuel condition. Dirty fuel injectors or low fuel pressure can also cause a lean fuel mixture, as can a dirty mass airflow (MAF) sensor (cleaning it with aerosol electronics cleaner can often resolve that problem).


The oxygen sensor’s job is to monitor unburned oxygen in the exhaust. This is an important job because it allows the engine computer maintain a balanced fuel mixture that is essential for good fuel economy, low emissions and peak engine performance.

Air or vacuum leaks in the engine, as well as compression leaks or engine misfires, can fool the O2 sensor into generating a false lean signal when unburned air passes through the engine into the exhaust.

When the O2 sensor detects a higher than normal amount of oxygen, its voltage output drops down to a couple of tenths of a volt. This sends a lean signal to the powertrain control module (PCM), which tries to compensate by adding more fuel to the fuel mixture. Consequently, the engine then runs too rich, wastes fuel and pollutes.
The sensitivity and reaction time of oxygen sensors slows with age, and the zirconium ceramic sensing
element inside the tip can become contaminated if the  engine is burning oil or develops a coolant leak in the head gasket or one of its cylinders.


O2 sensors must react quickly to changing oxygen levels in the exhaust, otherwise the computer response will be too slow to keep things in balance. A good O2 sensor should switch from a rich-to-lean indication in 20 to 40 milliseconds (that’s 20 to 40 thousandths of a second!), and a lean-to-rich transition in 60 to 80 milliseconds.
Most late model O2 sensors should last up to 100,000 miles under normal driving conditions. But if the sensor becomes contaminated, it can fail at any mileage. Replacing high mileage O2 sensors is a good way to restore like-new operation, engine performance and fuel economy. Old sluggish O2 sensors are one of the leading reasons why many vehicles get poor fuel economy or fail an emissions test.


Q. My Toyota has something called an Air/Fuel sensor instead of an oxygen sensor. What’s the difference?
A. An Air/Fuel sensor is just a smarter oxygen sensor. An ordinary O2 sensor reads unburned oxygen in the exhaust, and generates either a high or low voltage signal. The signal is high (0.7 to 0.9 volts) when the air/fuel mixture is rich (little O2 in the exhaust), or low (0.1 to 0.3 volts) when the air/fuel mixture is lean (lots of O2 in the exhaust). As such, it works more like a Rich/Lean indicator switch.

An Air/Fuel sensor, by comparison, does not generate a high/low voltage signal. Rather, it generates a current signal that changes in direct proportion to the air/fuel ratio. As such, it can tell the PCM the exact air/fuel mixture, and not just give a rich or lean indication as conventional oxygen sensors do. An air/fuel sensor can also read very lean air/fuel mixtures with a high degree of accuracy, which helps the PCM minimize exhaust emissions.

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