Changing an air filter on a vehicle is a very basic procedure – open the air box, take out the old filter, pop a new one in and close the air box. Years ago, most North American cars had the big round filter that sat proudly on top of the large engine. Now there are many different types of filters crammed into tight spots. “Air filter sizes are down, but so are engine displacements which means lower air usage at the same RPM; however, in many applications RPMs may be higher than they were 20 years ago and the use of turbochargers may be more prevalent which would increase air flow,” said Susan Storey, product manager – Customer Care & Aftersales for General Motors of Canada Ltd. “Using ‘cheap’ air filters can possibly increase upper engine wear or possible MAS (Mass Airflow Sensor) failure.” “The capacity of the filter to hold contaminants and how well the filter removes contaminants are as important today as ever,” said Bruce Coffey, technical service manager at WIX Filtration Products. “However, air filters in today’s engines are more an integral part of the vehicle’s air induction system. With fuel injection and on-board computers, mass airflow measurements are used to optimize fuel usage and exhaust emission. It certainly can be more critical for some applications than others; but, in general, if the air filter causes the intake air stream to be changed greatly then fuel usage and exhaust emission are compromised. Filters are now more an integral part of the engine air induction system. They often need more moisture resistance since they can be remotely mounted and are more likely to come in contact with water or snow in extreme conditions (or at the car wash).” Coffey added that the visual inspection method still works very well. “Just a monthly check goes a long way toward good maintenance. This works well for both the air induction filter as well as the cabin air filter.” Storey cautions that disturbing the filter and the air box can lead to problems. “If you open the air box to look at the filter then you might as well change it. Anytime you open the air box you disturb the seal and dislodge contaminant trapped in the filter. We should not recommend anything other than following the owner’s manual for mileage and driving conditions,” she said. An important element for making cars run smoothly and efficiently is the oxygen sensor. “Oxygen sensors are extremely important,” said Charles Gonwa, Oxygen Sensor Specialist for Bosch. “They keep the air/fuel ratio as at optimum, keep the engine operating at top efficiency, maintain vehicle performance, and protect the catalytic converter from damage due to excess fuel in the exhaust.” By more closely controlling a vehicle’s air/fuel ratio (AFR), fuel efficiency can be increased, while tailpipe emissions are reduced,” Sukhneet Mavi, technical service coordinator for NGK. “The oxygen sensor functions as the “eyes” and “ears” for the ECU, communicating whether the vehicle is running rich or lean. Theoretically, perfect or optimal combustion occurs when the AFR equals 14.7 parts air to one part fuel also referred to as the Stoichiometric Point. At the Stoichiometric Point, there is little or no pollution present in the exhaust stream. If the AFR is less than 14.7, the fuel mixture is RICH and harmful pollutants such as carbon monoxide (CO) and un-burnt hydrocarbons (HC) are released into the environment. If the AFR is greater than 14.7, the fuel mixture is LEAN and is releasing harmful oxides of nitrogen (NOx) into the environment. Constant exposure to excessive amounts of harmful pollutants (NOx, CO, and HC) will eventually lead to catalyst failure, resulting in failed emission tests, poor fuel economy and costly repairs.” While most oxygen sensors get replaced when there is a warning light on the dashboard, “They should be replaced per the vehicle manufacturer’s recommended replacement interval,” said Gonwa. “This interval could range anywhere from 60,000 miles to 130,000 miles depending on the application. As always, when a DTC code appears and is O2 sensor related, a full and proper diagnosis of the engine management system should be performed to determine if the sensor is indeed defective or worn, and to determine if other system components and conditions caused the DTC illumination.” It should also be kept top-of-mind that problems with an oxygen sensor could also be a sign of a problem somewhere else in the engine. Think of the oxygen sensor as a canary in the coal mine. Oxygen sensors can be damaged by contamination, from excessively rich fuel or oil or additives in the fuel. They may also suffer damage when sealants around the engine have started to fail. Novice technicians should be made aware that the oxygen sensor is sometimes not the cause of a trouble code that is pulled during a routine use of the scan tool. If there is a lean condition, the oxygen sensor will tell the vehicle’s onboard systems to enrich the fuel mixture. A vacuum leak can also appear on diagnostic scan as a faulty oxygen sensor. So it is always a good idea to take a little time to see if there is something else causing the ‘problems’ with the oxygen sensor before laying the blame on the sensor itself.