It is the ability of the Volvo oxygen sensor to identify trace changes in the oxygen levels of both the exhaust gases your engine produces and the atmosphere surrounding the vehicle that helps improve your mileage and horsepower. On all Volvos being manufactured today, there is an oxygen sensor. It is located in the forward section of the exhaust system just after the exhaust manifold. It is connected to the onboard computer via a small wire. If the oxygen sensor fails or this wire is damaged in any manner, the check engine light will become illuminated on the dashboard of the vehicle. Many owners do not even consider the minute changes in the oxygen levels really matter, but the Volvo engineers know that it does in the calculation for the correct fuel to air mixture. The oxygen sensor is just one of the many data points that are now required for the correct operations of a modern day engine. When the check engine light comes on, there will be a code stored in the onboard computer that has to be retrieved. This is the process since it is not practical to have an individual light on the dash for the many sensors that are collecting data on a Volvo engine. Once a Volvo oxygen sensor is determined to be faulty, then it has to be replaced for your engine to perform as it is expected to. The first step in the replacement of the oxygen sensor is to disconnect the battery. This will prevent any chance of an electrical shock when the wiring harness is touched. It will also clear all of the codes that are stored in the onboard computer so a false reading will not appear once the repair procedure is finished.