The automobile has evolved in the last thirty years to include many computerized systems, which has led to the appearance of the check engine light. When the light comes on, it means that the computer has found a problem that requires attention.
Sometimes the onboard computer will attempt to automatically fix the problem. If it is successful, or if you fix it yourself, the light will go off. Other times it may attempt to compensate for the weakened system to keep the car running, but the problem is not fixed, and the light will not go off. Even if you notice no difference in performance, a persistent check engine light means it�s time to visit your mechanic.
Many people ask us to simply �turn off� their check engine light. We really wish it were that simple, but there are about 1700 reasons that could cause that light to turn on. We could reset it, but if the problem is not fixed, the computer will simply turn it on again. Some of the causes are very simple, such as a loose gas cap. Other causes are early warnings of very severe complications. It requires testing, and sometimes a lot of it.
The factories have designed our cars so that the computer monitors and controls all systems, but also monitors them. When the computer recognizes a problem with one of these systems, it also records the reason in the computer. This is called a fault code. For each of these fault codes, there is a standard diagnostic procedure defined by the manufacturer.
There is sometimes more than one possible cause for a particular fault code, and the manufacturers have developed very specific troubleshooting procedures. All of these procedures require expensive equipment and training, standard at any decent automotive repair shop. The time required to run these tests varies, and therefore so does the cost. Estimation of repair costs is difficult without knowing the fault code or the results of required tests.
P0301 means the computer has detected a misfire in cylinder #1. Testing procedures include: spark plugs, ignition cables, ignition coil, and cylinder compression. We must run all tests prescribed, to pinpoint and repair the problem with this engine. The time it takes to run these tests also vary by make, model, and engine type. This fault code is one of several hundred others, and all have different tests to run. That and multiple fault codes can registers simultaneously!