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Using your car's on board diagnostic system
 

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P0400 code on a 1998 Mazda Millenia

This model is a great car but mine has a persistent problem with the check engine light (MIL). Having already replaced the knock sensor ($500) and EGR valve (another $500 in addition to the $100 diagnostic charge each time the dealer uses his scan tool), my car was due for inspection and I had to get that light off. By borrowing a friend's code reader, I found that it was the EGR valve again (P0400 - Exhaust Gas Recirculation Flow), and this time I checked out Mazda internet sites & discovered that often just cleaning the valve solves the problem. I went to a local gas station who charged me $10 to clean the EGR valve, had a friend use his code reader to turn off the check engine light, and then passed inspection. The light did eventually come back on, but the car runs great and I saved $500. I plan to keep monitoring the scan codes, researching repair advice, and finally getting this fully under control.

This illustrates the savings that can result from reading the scan codes, researching typical causes, and using this information to make simple and inexpensive repairs. 

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ServiceEngineSoon.org 2005 Leland R. Beaumont