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All You Ever Wanted To Know About Oxygen Sensors

Oxygen sensors (also called lambda sensors or O2 sensors) measure the amount of oxygen in gas mixtures in the vehicle. Without these sensors, electronic fuel injection and emissions control would not be possible. Oxygen sensors help determine the fuel/air mixture in the engine and control it in order to improve emissions and performance. Most modern vehicles utilizing fuel injection will have at least one oxygen sensor in the exhaust stream.

The determination of oxygen rich or lean variations tells the engine's computer to increase or decrease the amount of fuel being injected into the chambers for combustion. High amounts of oxygen in the exhaust will cause higher emissions at the tailpipe and usually means the engine is not running efficiency, lowering fuel economy. Conversely, low amounts of oxygen means that fuel is likely not being completely burnt, resulting in loss of efficiency.

O2 Sensors

How an Oxygen Sensor Works

O2 sensors work by reacting to O2 molecules in the exhaust compared with the amount of O2 in the ambient air. A rich (over-oxygenated) mixture will result in an ionic buildup inside the sensor as the internal richness creates more friction than the outside oxygen concentration. This creates a voltage that is read by the engine computer. The opposite happens when a lean mixture is in the exhaust gases being measured, and the computer reads a draw (drain) from the sensor.

Oxygen sensors are relatively simple constructs with no moving parts and are reliant on the transference of ions through a Zirconia membrane. Most often, when they fail, it is the connections to this membrane that have broken or corroded with time and use.

Bad O2 Sensors

When an oxygen sensor fails, it will be noted by the driver in two ways:

  • First, the vehicles' computer will trip an engine code, lighting the dashboard's engine light.
  • Second, the vehicle's fuel economy will suffer and the vehicle will perform poorly on emissions testing.

At least one oxygen sensor will be located on the engine's exhaust manifold. On V-type engines (V6, V8, etc), there will be a primary and secondary, one on each side's exhaust manifold. The primary will be on the cylinder-1 side of the engine while the secondary will be on the opposing side. Another sensor may be located just before or just after the catalytic converter in the exhaust stream, depending on vehicle makeup.

On most Toyota vehicles, they are easy to locate and replace, requiring only an appropriately-sized wrench and a plug to remove and replace.