If you have a check engine light (CEL) or MIL (malfunction indicator light) on and you have poor fuel economy, you might already have the answer to your problem. Any time the computer detects a fault with the engine management system, it sets a code. Many of these engine management problems will affect fuel economy and emissions. This is one of the main reasons they started using this system in the first place. It’s there to alert the driver that there is an issue that can affect the engine’s operation and emissions. So if you have a check engine light, pull the code to see what you’re up against.
I’ll touch on some of the engine management systems that can effect fuel economy later in this article. For now, if you have a CEL, pull the code and do what you can to repair it. In the meantime you might consider reading the Solving Automotive Performance Problems article for more information on these issues.