There are many potential issues that could cause your Check Engine Light to turn on. For example, say you find an oxygen sensor code (any code from P0130 to P0167). The oxygen sensor is a key sensor because it is part of the fuel feedback control system. The engine computer (called the “Powertrain Control Module” or “PCM”) looks at the oxygen sensor signal to see if the engine is running rich or lean. If the engine is running rich (too much fuel), it shortens the control signal to the fuel injectors to reduce the volume of fuel delivered. This rebalances the air/fuel mixture to the right ratio for optimum fuel economy, performance, and emissions. If the oxygen sensor is not sending a good signal to the PCM, it usually makes the engine run rich. This wastes fuel and increases emissions. Consequently, the Onboard Diagnostic System detects the fault, sets a code and turns on the Check Engine Light. A diagnostic scantool enables you to look at some important data. You can look at the output voltage of the oxygen sensor to see if it is behaving normally. A good oxygen sensor should produce a low voltage signal (0.2 volts or less) when the engine (exhaust, actually) is lean, and a higher voltage signal (0.8 volts or more) when the engine (exhaust) is rich. The sensor's output voltage should be bouncing up and down as the PCM constantly adjusts the fuel mixture. The best way to look at this particular signal is to graph it. Graphing the oxygen sensor’s output signal makes it easier to see the up and down changes in the voltage (much easier than a simple numeric readout). The key here is using a scantool that allows you to display and graph this kind of information in your desired format. Figure 2 shows a real time graph of front and rear sensors. More importantly, you can see that the Bank 1 sensor is working, while Bank 2 is not. Once you know your oxygen sensor is not working, you can now do any additional tests that might be needed. Often times, a fault in a sensor circuit is a bad connector and not a bad sensor. Simply cleaning the connector may be all that's necessary to bring the sensor back to life. Other times, you may have to use a simple Ohmmeter to check a resistance value, or a voltmeter to check a circuit voltage or ground connection to isolate a fault. The trick is to NOT jump to conclusions. Make sure you’ve identified and isolated the fault BEFORE you replace any parts. You’ll save yourself a lot of time, money, and aggravation if you remember and practice this simple rule. A perfect example is when the Check Engine Light is on and you find a code P0445. This is an Evaporative Emissions Control System (EVAP) code indicating a large vapor leak. The fuel system on today’s vehicles is sealed so fuel vapors cannot escape into the atmosphere. The EVAP system captures and stores fuel vapors to “recycle” them back through the engine. If the Onboard Diagnostic System detects a leak in the EVAP system, it will set a code and turn on the Check Engine Light. In this case, the code P0445 may be due to nothing more than a loose, missing, or leaky gas gap. Check that the gas cap is tight. If it’s not loose, check the seal between the cap and the filler tube for corrosion. If you’ve corrected the problem, the EVAP code will go away, and the Check Engine Light will turn off, without having to clear the code. This will happen the next time the onboard diagnostic system runs a self-check on the EVAP system. This usually occurs after the vehicle sits overnight and the fuel tank is between 1/4 and 3/4 full.