Here at Val & Mike’s Auto Service at least five calls a day are in relation to the check engine light. Unfortunately, without seeing the vehicle and performing a diagnostic test, an accurate diagnosis cannot be made over the telephone. Though the latest computer code readers from the local auto parts store can display the trouble codes, code readers cannot determine the cause or the solution to repairing the check engine light. Take for example the catalytic converter we recently replaced in a 2000 Toyota 4-Runner. When the diagnostic scanner (not code reader) pulled the computer codes it displayed the vehicle’s Catalytic Converter system efficiency was below threshold (not working), but also displayed the 02 sensors not working collectively with the system. It was apparent the catalytic convertor had failed and replacing it was the first step to address the check engine light. After the installation of the catalytic convertor, retesting the computer system and accessing the 02 sensors to test their functioning was imperative to determine if they were working. In the case of the Toyota 4-Runner, the 02 sensors began to function normally and the new catalytic convertor solved our customer’s problem. In some cases though it is found that failed 02 sensors can be the cause of the cat efficiency code and the catalytic convertor is actually functioning normally. It takes up-to-date equipment and, most importantly, a skilled technician that can identify, understand and diagnose the thousands of codes in relation to the check engine light as we did for our customer Valerie A.