An oxygen sensor is a device used to regulate the fuel mixture through the use of signals to the computer. There is one O2 sensor inserted into the exhaust pipe in front of and one behind the catalytic converter. An oxygen sensor heats up in the exhaust stream until it is glowing red and senses the amount of oxygen in the exhaust, which is a direct representation of the fuel mixture. The computer keeps the optimum fuel mixture of 14.5 parts air to 1 part fuel based on the signals from the O2 sensor.
If the sensor signal count goes up, there is too much oxygen in the stream, which means that the mixture is too lean. Too much air and not enough fuel causes high cylinder head temperatures, detonation and lack of power. As a result, the computer increases the fuel flow. If the count is low from the O2 sensor there is not enough oxygen--too much fuel and not enough air--so the mixture is rich, leading to fouling of the plugs, increased gas consumption and loss of power. In response, the computer decreases the fuel flow into the engine.
The oxygen sensors have one more function. The computer looks at the difference in the count between the front sensor and the one behind the converter. The front sensor should be very active in count number variation whereas the one behind the converter should be more constant and lower than the front. In other words, the computer wants to see what is going in the converter and what is coming out. If the numbers on the front sensor are close to the ones on the back sensor then the catalytic converter is no good. It would also send a code saying that the catalytic converter is beyond the threshold.
After the introduction of the fuel injection systems and oxygen sensors, it had been noticed that the oxygen sensor needed to be hot in order to function, which took a few minutes. In those few minutes the engine was expelling harmful emissions. This was corrected by adding an electrical heater element to the sensor to bring it to operating temperature faster.
If an oxygen sensor is left inoperative in the system, the computer, in lieu of a signal, will take a best guess at the mixture. There will be a lack of power and a drop in fuel economy plus a lot of associated problems. In a fuel injected system, everything must work or the system fails.
When the check engine light comes on indicating a problem with the sensor, remember this information--there are normally only two codes for the oxygen sensor. One code would say oxygen sensor number 1, which would be in front of the converter, preheater failure. This is the only one that can be taken to the bank that the sensor is bad. However, it does not need to be replaced right away because it will work once it heats up. It is just for the start-up that it does not work. The other code for the sensor says oxygen sensor and the number out of range. This just means that the sensor signal cannot be counteracted by the computer. Oxygen sensors last a long time as long as no abnormalities exist in the engine. When they fail it is because they became coated with unburned gas and oil. Most new mechanics are ready to jump on this code and replace the O2 sensor just to find out that the new one does the same thing or only works for a few miles. The computer has no way of stating that the O2 sensor is definitely bad, just that the signal is out of range. This can be caused by anything from a bad spark plug, wire, vacuum leak, PCV valve, dirty air cleaner and the list goes on. Before the O2 sensor is unnecessarily replaced, tell the mechanic that you have it on expert authority to take a few minutes and check the engine for reasons for the sensor to be out of range. If it must be replaced something in the past or present damaged it.
