Originally Posted by big john

What I did for the last 30 years was diagnosis, engine performance, emissions, electronics, the stuff nobody wants to do. There was a dramatic change in 1996 with the introduction of protocols and laws for vehicles to monitor emission related stuff. This is called OBD 2 and the connector under the dash is called the data link connector. This connector allows access to the up to 30 computers on a communication link.

If checking a used car you can see any trouble codes which are set, even if warning lights are not on. If a car has a bad catalytic converter, for example, the seller can clear the light with a code reader and it won't come back on until the computer has failed it's test. A good code reader will tell you if the tests have been run, or not.

In California a car will fail a smog check if those tests have not run. This is a source of much frustration for technicians trying to get the tests (called monitors) to pass. It can be a giant PITA.[/QUOTE]
Yeah. We were on the verge of despair when Mrs. GeneJockey's Sienna wouldn't pass because ONE TEST kept showing as "Not Ready".