2000 Mitsubishi Lancer engine check light

What led you to an oxygen sensor? High octane gas is much harder to ignite and will make some running problems, particularly misfires, worse. You want gas that's easier to ignite. That's low octane.

The Engine Computer detected a problem, set a diagnostic fault code, and turned the Check Engine light on to tell you. There are well over a thousand potential fault codes. They will never say to replace a part or that a part is defective. They only indicate the circuit or system that needs further diagnosis, or the unacceptable operating condition.

The place to start is by having the fault code read and recorded. Many auto parts stores will do that for you for free.

I need to know the exact fault code number to know where to start the diagnosis. There are over a thousand potential codes, and without that, there is no way to know what to look for. Even with oxygen sensors, there are almost a dozen codes for each one, and they all mean vastly different things. Some suggest the sensor is not working properly. Some indicate the sensor is working just fine but it's reporting an unacceptable condition. Even the generic code descriptions from some aftermarket scanners can be confusing. That's why we need the exact code number.

As a general rule, you can judge the severity of an intermittent problem by what it takes to make the light go off. The least serious is when the light turns off while you're driving. At that time the problem has stopped acting up for the moment. If it's more serious, even if the problem stops acting up, the light will stay on until you turn the ignition switch off and restart the engine. The light will stay off until the problem occurs again.

When it's still more serious, the light will be on any time the engine is running, even when the problem hasn't occurred. With the really serious codes, the light will be flashing. That means stop the engine right away. Too much raw fuel is going into the exhaust system and is going to overheat and damage the catalytic converter.

About half of the fault codes do not cause the Check Engine light to turn on at all. Only those problems that could adversely affect emissions must turn the light on.