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In 1996 the EPA began requiring a universal system for diagnosing and testing emission controls on all motor vehicles sold in the US. This required auto manufacturers to utilize the same connection and language for vehicle diagnostics.
With this in place, we can connect to all vehicles and check the condition of the emission control system for inspection requirements.
As you drive your car the fuel system computer tests the components and records the result as a completed test. If all the tests have been run and passed, and the "check engine light" works, your car passes inspection for emissions. If a test fails the computer commands the MIL "malfunction indicator light" or "check engine light" to be on. When it does it also records sensor readings and stores a code the mechanic can use to track down the problem.
If your car's battery cable is disconnected or the computer is cleared by a mechanics scan tool, the test records are cleared. If you take your car for inspection in this condition it is rejected as "not ready". In most cases all you need to do is drive the car for a day or two and the tests will run and your car will be ready for inspection. If the MIL "check engine light" comes on during driving for readiness, the problem will have to be repaired and the readiness process will start all over.
With the newer more sensitive computer systems, getting the check engine lamp repaired as soon as possible can help insure you get the best gas mileage, prevent problems from creating other problems, and keep you from the headache of trying to get the system ready for inspection at the last minute.
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