Mechanics

ENGINE LIGHT

2005 Mazda 3

Engine Mechanical problem
2005 Mazda 3 4 cyl Two Wheel Drive Automatic 110000 miles

My engine light came on after having my alternator replaced. I was told that might be the cause but it failed inspection because of the light and the code that can up was PO421 Low catalytic threshold. What does this mean and what do I have to do to fix it in the most inexpensive way? Mazda wants $95.00 just tp run a diagnostic even though I can give them the code.
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Lucy Verhave
April 23, 2010.



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