Does a Bad Oxygen Sensor Decrease Gas Mileage?

The average engine is a beast with 10,000 inward-turned eyes, a nearly omniscient organism fully obsessed with its own internal condition. The oxygen sensor is one of your engine's primary tools of self-diagnosis; without it, the computer goes from a state of zen-like efficiency to a state of child-like confusion. Count on it: temper tantrums shall ensue.

O2 Sensor Basics

An oxygen sensor is essentially a temperature probe -- a thermometer. Engines work by combining air and fuel in a very specific (stoichiometric) fuel ratio, typically about 14 parts air to one part fuel. Engines need this specific ratio because it provides enough air to burn the fuel, but leaves a little fuel leftover to carry heat away from the cylinder. In a gasoline engine, a "lean" condition -- where there's too much air -- will increase exhaust gas temperatures, while a "rich" mixture -- too much fuel -- will drop the temperatures.

Computer Function

An O2 sensor reads the exhaust gas temperatures, and the computer extrapolates from that data to deduce the engine's current air/fuel ratio. The computer's basic job is to perform a delicate balancing act, introducing just enough air and fuel to keep the ratio where it should be. The computer usually operates in "closed loop" setup, where it gets data about air and fuel going into the engine, and the ratio of air and fuel coming out. If the oxygen sensor malfunctions, then the computer will go into "open loop" mode, flying blind and using its best guess to keep the engine running.

Front O2 Sensor Failure

The front O2 sensor -- the one on the engine-side of the catalytic converter -- is the one that the computer uses to adjust air and fuel ratio. An O2 sensor is the only sensor on your car that produces its own voltage, and it does so by (basically) converting heat to electricity. The voltage variance from one extreme to the other is tiny, and any malfunction in the front sensor will ultimately throw the computer into open loop mode. Since lean mixtures are hotter and more dangerous to the engine, the computer will adopt a much more conservative, fuel-rich mixture to keep temperatures down. The end result is a noticeable drop in power and fuel economy.

Rear O2 Sensor Failure

The O2 sensor mounted just past the catalytic converter doesn't monitor the engine itself, but a failure here will create the same end result. Part of a converter's job is to burn off whatever unburned fuel comes out of the engine, which creates a massive amount of heat. Thus, the gases coming out of a cat are far leaner and hotter than those going in. If the rear O2 sensor fails, your engine will assume that something's gone wrong with the cat and will go into a "limp home" mode till you get it fixed. Limp home mode is kind of a nasty trick on the part of vehicle manufacturers, a state wherein the engine will run just well enough for you to get home to fix the converter. Limp home mode will typically create a massive drop in power and fuel economy; enough to convince you to fix the converter ASAP.

Repair

Most computers will self-diagnose and spit out a trouble code to tell you that there's been a malfunction in the O2 sensor. But don't believe it; an "O2 sensor malfunction" code only tells you that the sensor's readings are way out of range, and that they're reading lean or rich. That malfunction code could easily indicate another problem -- one causing a run-lean or rich condition in the engine. Before you go replacing the O2 sensor, make sure that you've properly diagnosed the engine to make sure that it isn't a sensor failure, vacuum leak, air channel clog or fuel system malfunction.

References

• "Sensors Applications: Sensors for Automotive Technology"; Jiri Marek, Hans-Peter Trah, Yatsutoshi Suzuki and Iwao Yokomori; 2005
• "Bosch Automotive Handbook, 4th Edition; Horst Bauer; 1996
• "A technicians Guide to Advanced Automotive Emissions Systems"; Richard Escalambre; 1997