Oxygen Sensor

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A Check Engine light most often means one of the oxygen sensors in the exhaust system is sending the OBDII system an error code. Many times the solution is to replace the sensor itself, which can be both difficult and expensive. Less often, the sensor is actually doing it’s job, signaling a problem upstream in the fuel injection system.

In modern vehicles fuel injection has become very complex, with multiple sensors and valves working together to optimize the fuel-air ratio for low emissions and good performance. A fault in any one of those could lead to a oxygen sensor turning on the Check Engine light. However, with an understanding of how these components work together it’s possible to carry out some quick checks. This starts with learning how sensors in the exhaust can monitor engine combustion.

Oxygen sensors and combustion

Every vehicle manufacturerd since the mid 1990’s has two oxygen sensors in each exhaust system. (So that’s four on a V8-powered vehcle like the Chevy Suburban or Silverado.) These are mounted in the exhaust, one each side of the catalyst. (On some vehicles the upstream sensor is actually in the exhaust manifold.) Their job is, as the name suggests, to sense the amount of oxygen in the exhaust gas.

This oxygen comes from the air drawn in through the induction system and into the cylinders. At some point on its journey, either in the throttle body, the inlet ports, or directly in the cylinder, the injection system sprays gasoline into the air, in a ratio of 14.7 parts air to 1 part gas.

Maintaining this air-fuel ratio is important because it’s the mixture strength that burns most efficiently. All the gasoline and almost all the air will be consumed and there will be no unburnt hydrocarbons going down the exhaust. Should the mixture be richer, say 14.0:1, (meaning less air and more gas,) some of the gasoline won’t be consumed during combustion. Less power will be produced, emissions of noxious gases will be higher, and it’s possible the catalyst will be damaged. Conversely, too much oxygen means the engine is running lean, (the O2 isn’t all being consumed during combustion.) This will reduce power output and can potentially damage the engine.

Sensor construction

At the heart of most oxygen sensors is an element made from Zirconia, (some use Titania, but it works in much the same way.) Zirconia is able to conduct oxygen ions when it gets hot and this effect is used to sense the ratio of oxygen in the exhaust gas to that in the air. The way this works is that the outside of the sensor is in contact with the exhaust and the inside contains reference air. A thin layer of platinum coats both sides of the Zirconia, and acts as a pair of conductive electrodes.

ECU signals

Oxygen ions entering the Zirconia cause the voltage between the two layers of platinum to vary. Low oxygen levels result in high resistance, and so high voltage. When more oxygen is present the voltage is lower. The Engine Control Unit (ECU) monitors this signal, adjusting the air-fuel ratio as necessary by changing the length of time for which the injectors are open.

Inside each injector is a solenoid whose job is to open and close a valve, allowing gasoline to spray through a a small hole or holes. (Some of the newest injectors use a piezo actuator to open and close the valve.) On a signal from the ECU the injector opens, allowing gas to spray until a second signal tells it to close.

Fuel injection pressure

The injection system works by holding gasoline under pressure behind the injectors. In port and direct injection engines this happens in a steel pipe called the fuel rail. The fuel pump supplies the rail with more gas than the engine can use, returning the excess to the tank. At the output end of the rail there’s a spring-loaded valve called the fuel pressure regulator. This holds fuel in the rail, only opening when the pressure exceeds the spring setting.

Oxygen Sensor

The twist in this is that air pressure at the inlet port varies depending on how hard the engine is working. Technically, this is referred to as vacuum because it’s less than atmospheric pressure. To maintain the pressure differential the spring load in the regulator must be adjusted to account for changes in this vacuum, and this is done with a vacuum hose connecting it to the inlet manifold.

Vacuum and the Check Engine light

If there’s no vacuum acting on the regulator it needs higher pressure in the rail to make it open. Higher pressure means more fuel spraying when the injector is opened, and that leads to an over-rich air-fuel mixture.

So, here’s something to look for when the Check Engine light comes on: find the fuel pressure regulator and verify that it’s hooked up at both ends. If it is, the problem is either a bad sensor or something else is going on, but at least you’ve eliminated one possible cause.

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