The ACV is designed to control air injected from the air pump and direct it
to either the main cat or bypass to a noise muffler in the front passenger
corner of the front bumper. Within the ACV is the anti-burn valve that
is supposed to suppress backfires in the exhaust system. A malfuncitoning
ACV can cause emission problems, and a replacement can only be bought from a Mazda
dealer. Without a properly functioning ACV, it's almost impossible to pass
almost any emissions testing.
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