We often panic when the check engine light appears on the dashboard. Relax—the light comes on as a warning that something might not be quite right with your vehicle. Let a skilled technician take a look, assess the situation, and confirm—or dismiss—any concerns. Familiarize yourself with the most common sources of a check engine light, and you’ll be better prepared to discuss the repair with your shop's service advisor and understand what your car needs.

The Five Most Frequent Check Engine Codes

  1. Misfire - A misfire in your engine can be traced back to any number of reasons, ranging from a problem with the spark plugs or plug wires to the distributor cap coils of the fuel injectors.
  2. Emissions - Your check engine light can also come on to indicate a problem with your evaporative emissions system, which prohibits harmful vapors from escaping from your vehicle and into the atmosphere. The failure can arise from a variety of reasons, including a loose or defective gas cap, a broken gas cap seal or a leak in a plastic tube that runs between the engine compartment and the fuel tank.
  3. Failed oxygen sensor - Your car’s oxygen sensor (O2 sensor) measures and regulates the amount of unburned oxygen in your vehicle’s exhaust system. Faulty sensors can damage your car’s spark plugs and catalytic converter, as well as cause your car to burn more fuel than necessary.
  4. Catalytic converter - Your catalytic converter changes harmful carbon monoxide into carbon dioxide. Faulty oxygen sensors or damaged spark plugs or wires can damage a catalytic converter.
  5. Mass airflow system - Your mass airflow system (MAS) measures the amount of air that enters the engine of your car and determines how much fuel your engine requires to run efficiently. A damaged mass airflow system can cause reduce your vehicle’s performance and result in low fuel economy, as well as damage or ruin your spark plugs

Understanding System Components

To control exhaust emissions, which are responsible for two-thirds of the total engine pollutants, two types of systems are used: the air-injection system and the exhaust gas recirculation (EGR) system. In EGR a certain portion of exhaust gases is directed back to the cylinder head, where they are combined with the fuel-air mixture and enter the combustion chamber. The recirculated exhaust gases serve to lower the temperature of combustion, a condition that favours lower production of nitrogen oxides as combustion products (though at some loss of engine efficiency). In a typical air-injection system, an engine-driven pump injects air into the exhaust manifold, where the air combines with unburned hydrocarbons and carbon monoxide at a high temperature and, in effect, continues the combustion process. In this way, a large percentage of the pollutants that were formerly discharged through the exhaust system is burned (though with no additional generation of power).

Another area for additional combustion is the catalytic converter, consisting of an insulated chamber containing ceramic pellets or a ceramic honeycomb structure coated with a thin layer of metals such as platinum and palladium. As the exhaust gases are passed through the packed beads or the honeycomb, the metals act as catalysts to induce the hydrocarbons, carbon monoxide, and nitrogen oxides in the exhaust to convert to water vapour, carbon dioxide, and nitrogen. These systems are not completely effective: during warm-up, the temperatures are so low that emissions cannot be catalyzed. Preheating the catalytic converter is a possible solution to this problem; the high-voltage batteries in hybrid cars, for example, can provide enough power to heat up the converter very quickly.


Finding the cause of a Check Engine Light (CEL) error code can be as simple as getting a much needed tune-up, fixing worn out exhaust components or replacing a worn gas cap. Most times a CEL is pointing to a simple problem or regular maintenance, that when repaired, will improve fuel efficiency, provide better performance, and keep you on the road.

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