The Oxygen Sensor: A Brief Overview

There are dozens of sensors in the modern automobile, measuring temperature, pressure, speed, chemistry, which electronic control modules (ECM) use to deliver a comfortable and efficient ride. One of these is the oxygen sensor (O2S) which, as the name suggests, measures oxygen, though they are also called heated O2S (HO2S), lambda sensor or air-fuel ratio (AFR) sensor.

What does this sensor do exactly, and how does it do it? As a DIYer, is there anything you need to know about testing or replacing O₂ sensors?

Oxygen Sensor Functions

First, a quick chemistry lesson: gasoline + oxygen → carbon dioxide + water + energy or (2 C₈H₁₈ + 25 O₂ → 16 CO₂ + 18 H₂O + E). In an ideal combustion chamber, the exact amount of oxygen and gasoline would be ignited to produce perfect combustion, but real-world chemistry is more complex.

Air is composed of 78% N₂ (nitrogen) 21% O₂, and 1% other gases, but up to 5% H₂O (water vapor) may displace other gases in the mixture. Other variables make perfect combustion even more complex, such as barometric pressure, air temperature, engine condition, speed, temperature and load.

The O2S measures oxygen differential — that is, the difference in oxygen content between the exhaust and atmosphere. The sensing portion of the oxygen sensor is a hollow bulb of zirconia (zirconium dioxide – ZrO₂) or titania (titanium oxide – TiO₂), which have unique properties in relation to oxygen differential. Above 350 °C (660 °F), when exposed to a high oxygen differential, zirconia generates an electrical charge (titania changes electrical resistance), which the ECM uses to indirectly measure air-fuel ratio.

ECM and Oxygen Sensor Readings

The ECM is constantly working toward stoichiometry, a chemically perfect air-fuel ratio of 14.7:1 (14.7 parts air, one part fuel). When AFR is rich, there is little oxygen left in the exhaust and the O2S generates a higher voltage. When AFR is lean, there is more oxygen in the exhaust and a lower voltage is generated.

When the ECM sees near 1 V or 5 V (depending on type), it reduces fuel input until O2S voltage switches low, close to 0.1 V or 1 V. Then ECM increases fuel input when O2S voltage is high until O2S voltage goes low. This cycle typically occurs in less than a second at idle, faster at higher speeds. The ECM also uses a second set of O2S to monitor three-way catalytic converter (TWC) function.

Oxygen Sensor Diagnostic Tips

• The best way to test oxygen sensors is via scan tool. You can measure zirconia sensor voltage directly, but not titania sensors because their voltage is calculated from resistance and current values.
• Oxygen sensor replacement will not “fix” AFR or TWC efficiency codes. The ECM verifies O₂ sensor function before running AFR and TWC monitors.
• Check for exhaust leaks when diagnosing AFR problems. An exhaust leak can skew O₂ sensor readings because it allows atmospheric oxygen to enter the exhaust.
• Most vehicles use heated oxygen sensors to get them up to operating temperature more quickly. When diagnosing O₂ sensor problems, check heater circuit fuses and relays.

Check out all the sensors available on NAPA Online or trust one of our 17,000 NAPA AutoCare locations for routine maintenance and repairs. We hope that this primer has been enlightening, but if you need more information on oxygen sensors, chat with a knowledgeable expert at your local NAPA AUTO PARTS store.

Photo courtesy of Wikimedia Commons.