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POOR MPG and O2 SENSORS


WHAT'S AN O2 SENSOR?

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Cars have had oxygen sensors on them since the early 80’s, but few people even know what or where they are. Oxygen sensors, more commonly known as O2 sensors, send information to the computer telling it whether the engine is running too rich or too lean. The sensor is a zirconia ceramic probe that usually has a thin coating of platinum. The sensor screws into the exhaust manifold or directly in front of the catalytic convertor. Many new cars have a sensor in of front and behind the converter; the rear one tells if the converter is doing its job. Early O2 sensors had one wire, second generation ones had two and the later ones have four wires.


HOW DO O2 SENSORS WORK?

 

The O2 sensor in front of the converter generates its own voltage, through the interaction of the exhaust heat and the metal coated probe, and the voltage is sent to the computer. The higher the exhaust heat, the higher voltage. The computer processes this information along with input from other sensors, one of which measures outside air temperature. The difference between the outside air temperature and the exhaust is calculated, using voltage and resistance values, by the computer and the fuel injection rate is adjusted. If the O2 sensor is getting sluggish or it fails, the computer gets faulty information, the air/fuel ratio fluctuates, the computer ends up guessing and goes to the default settings which causes fuel economy and driveability to become erratic.


HOW DO YOU KNOW IF IT'S OK?

 

The sensor can be checked with special equipment or with a propane torch and a voltmeter, if you know what you’re doing. If the person testing the sensor uses an ohmmeter they will ruin the sensor. Ohmmeters send an electrical charge through the item being tested and any electrical input, including shorting it out, will ruin the sensor. If you don’t know what you’re doing, don’t do it. Touching the probe and using sealers that are silicone based, can also ruin an O2 sensor.


WHAT ARE THE SYMPTOMS?

 

Some of the symptoms of a bad O2 sensor are fluctuating fuel economy. One you get 30 MPG and then it drops drastically, later it’s not so bad and then goes down again. There can be other problems, but a roller coaster ride involving fuel economy can indicate a faulty O2 sensor. Poor drivability, jerky acceleration or loss of power can all be O2 sensor related.


NOT A CURE ALL.

 

Nothing will cure defective sensors, engines that burn oil or trouble codes that haven’t been remedied. If you have computer and sensor problems, they need to be remedied before spending money on items to increase fuel economy. Some work, many don’t and some are pure “pie in the sky.”


JUST BECAUSE IT'S NEW, DOESN'T MEAN IT WON'T BREAK DOWN.

 

Before we left on our summer adventure, I replaced a faulty 02 sensor on our Aveo. After about 1500 miles, the mileage began fluctuating the same as before. As soon as I could get the necessary equipment, which a friend in Oregon had, I found the new 02 sensor was bad. I replaced it, the mileage increased again and has remained steady since that time. Our son had had the same experience, only his new sensor was bad from the beginning.