Understanding The Operation Of Oxygen Sensors

Understanding The Operation Of Oxygen Sensors

They play a vital role in achieving the stoichiometric ideal - but there's more to it than just that.

An oxygen sensor (O2) is a vital part of the fuel and emission-control system. It measures the difference in oxygen in the air on the outside of and inside the exhaust. It reports its finding to the ECU to calculate the exact amount of fuel the injector should pulse into the cylinder to create combustion. The main function is to maintain efficient combustion without being too lean or too rich.

In a closed-loop system, the O2 sensor monitors the air/fuel ratio 100 times per second to make minor fuel-trim corrections. Fuel can be added or reduced to ensure this ratio is the stoichiometric ideal, helping the engine burn fuel more efficiently.

The core of most narrowband O2 sensors is made of zirconia, which helps produce a voltage according to the amount of oxygen in the exhaust. The sensor element is a ceramic cylinder that is plated inside and outside with porous platinum electrodes. It measures the difference in oxygen between the exhaust gas and the external air and then produces a voltage or changes its resistance depending on the difference between the two measurements.

Oxygen sensors work best when they’re heated to approximately 600 F. Most newer O2 sensors come with a heating element encased in ceramic that quickly brings the tip up to temperature. Older sensors, without heating elements, will eventually be heated by the exhaust. Still, there’s a lag time between when the engine is started and when the sensor gets to optimum temperature. The time it takes for the exhaust gases to bring the probe to temperature depends on the ambient air temperature and the layout of the exhaust system. It can take several minutes without a heater circuit.about:blank

Today, most O2 sensors have four wires: two for the lambda output and two for the heater circuit. However, some use the metal case to ground the sensor element signal, so they only use three wires. Earlier non-electrically heated sensors had one or two wires.

The lifecycle of a typical heated O2 sensor is about 100,000 miles in normal conditions. Single-wire non-heated sensors tend to fail due to the buildup of soot on the ceramic element, which lengthens its response time and may altogether quit reading oxygen. With heated sensors, deposits are burned off during operation, and failure usually results from a bad heater circuit or clogged converter. This will cause a lean mixture, where the ECU enriches the fuel in the mix. Too much fuel loads the exhaust with carbon monoxide and hydrocarbons, and fuel economy decreases.

Another common cause of the premature failure of O2 sensors is when they become contaminated with grease or silicates, which are commonly used as corrosion inhibitors in some antifreeze types. The deposits on the sensor may appear shiny white and grainy light gray. Oil leaks also can contaminate the probe tip with an oily black residue and lead to slow or no response.about:blank

An overly rich mixture causes a buildup of black powdered deposits on the probe. This may be caused by the probe failing or by a problem elsewhere in the fuel system. You can test an O2 sensor with a probe by attaching one lead to the signal wire and the other to the ground to check for voltage. These sensors read an increase in oxygen as a lean condition and should produce close to 200 millivolts (0.20 volts). The sensor is bad if it doesn’t respond or is slow to respond. The issue could be a vacuum leak or some other component. You also can check the heater circuit for resistance with an ohmmeter. If there’s no resistance, the circuit is bad, and the sensor must be replaced. 

This article is courtesy of Counterman.

You May Also Like

Fuel Pumps and Cranking

Diagnosing the problem comes down to understanding what causes a loss of fuel pressure.

Here is a diagnostic puzzle: An owner comes to your shop complaining that it takes longer than expected to start his car’s engine in the morning. There are no codes for the crankshaft and camshaft sensors. And, the oil pressure is within specifications. If you are lucky, you might have a code for low fuel pressure on the high side of the direct injection system. What could be the source of the problem?

BMW 4-Series Service

The 4-series does not stray far from the typical BMW engineering and operation.

Crankshaft Counterweights And Balancing

Imbalance in the engine exhibits vibration and damage. Balancing becomes critical to any engine build.

Transmission Valve Body Replacement

Learning how to perform drivetrain diagnostics and “in-the-car” repairs is important.

Understanding Underhood Parts for Hybrids

Anything that can happen to an ICE in a standard vehicle can happen to an ICE in a hybrid.

Other Posts

VW Four Motion

A DSG transmission is lighter than a comparable automatic with the same number of speeds.

Cabin Air Filter Replacement

There are three major reasons to replace a cabin air filter in the winter.

Batteries For Stop/Start Systems

On all stop/start vehicles, the life and performance of the battery are measured by the vehicle in several ways.

High Pressure Direct Injection Fuel Systems

The main destroyer of high-pressure fuel pumps is a lack of oil changes.