An O2 sensor is a sensor placed in the exhaust stream that measures the oxygen content of the exhaust. This measurement has a direct relationship with how your engine is burning its air/fuel mixture. After the O2 sensor heats up to operating temperature, the computer begins to use the O2 sensor reading as its primary source to calculate the fuel mixture for your engine. It’s sort of a self-monitoring system to maximize power and efficiency. Modern vehicles don’t use O2 sensors; they use what’s called an AF, or air fuel sensor. These sensors are WAY more accurate than their ancestors. So if your vehicle is equipped with an AF sensor, you can ignore much of this section.
If an AF sensor has a problem, it will likely turn on the check engine light. Trust me, on modern systems, AF sensors are very closely monitored for any faults. The slightest hiccup with an AF sensor calls in the check engine light police. Older vehicles with single-wire O2 sensors are not as good about this. In fact, many of these older systems don’t have any form of O2 sensor monitoring. For this reason, it’s a good idea to check the operation of the O2 sensor if you have poor MPG on one of these older vehicles. It’s simple enough. Just grab a DVOM (digital volt ohm meter) and set it to a low voltage scale (less than 20v if you can). The O2 sensor’s operating range is in the 0 to 1v range. After you’ve warmed up the vehicle, check the voltage output of the O2 sensor. It should be switching rapidly across the .5v mark if it’s healthy. If it’s not healthy, you’ll notice it hanging out at a particular voltage, or it won’t switch very fast at all. This indicates a “lazy” O2 sensor. If you find one of these, replace it and then recheck your MPG. I’d wager you have marked improvements. In fact, we used to recommend O2 sensor replacement as part of a service on some of these older vehicles, usually at the 60K mark. After replacing one of these old O2 sensors, you might be surprised at how well your engine runs. Newer O2 sensors aren’t like this and they’re more expensive. For that reason, I don’t recommend replacing them as part of a service.