Oxygen sensor

Oxygen sensor measures the amount of the oxygen in the exhaust gases. This information is used by the automotive engine computer system to control engine operation. There are few types of oxygen sensors available, but here we will consider most commonly used - voltage-generating type.

Front (upstream) oxygen sensor.
Front or upstream oxygen sensor located in the exhaust manifold or in the downpipe before catalytic converter. It monitors the amount of oxygen in the exhaust gases and provides the "feedback" signal to the engine computer.
If the sensor senses high level of oxygen, the engine is running too lean (not enough fuel). The engine computer adds more fuel.
If the level of oxygen in the exhaust is too low, the computer decides that the engine is running too rich (too much fuel) and subtracts fuel accordingly.
This process is continuous - the engine computer constantly cycles between slightly lean and slightly rich to keep the air/fuel ratio at the optimum level. If you look at the front oxygen sensor voltage signal, it will be cycling somewhere between 0.2 and 0.8 Volts (see lower picture)

Rear (downstream) oxygen sensor
Rear or downstream oxygen sensor located after catalytic converter. It monitors the efficiency of the catalytic converter.

Oxygen sensor problems
Oxygen sensor problems are common. Faulty oxygen sensor may cause poor gas mileage, emission test failure and various driveability problems (hesitation, poor acceleration, surging, etc.).

If you are looking for Do-It-Yourself car repair information, there are couple of websites where for a fee you can get an access to an online repair manual for your car or truck. Follow these links:

• Alldata DIY - online automotive diagnostic and repair information
• eAutoRepair - professional service and repair information to automotive enthusiasts.