Just above the brake pedal
No.
None. A Diesel Oxidation Catalyst and a catalytic converter (as used on gas engines) are extremely different, as is their construction.
Fouled, damaged injectors or incorrect timing on fuel injector pump
The port for the On Board Diagnostic (OBD) computer is about 6 inches below the parking brake release on the driver's left. It is behind a panel marked OBD-DCS.
Rectangular connector under the driver side of the dash, could be blue or black. It will have 6 or 8 wires in it.
To clear diagnostic codes on a Mazda 6, you can use an OBD-II scanner. Connect the scanner to the vehicle’s OBD-II port, usually located under the dashboard, and turn on the ignition without starting the engine. Follow the scanner's prompts to read and then erase the codes. Alternatively, you can disconnect the car battery for about 10-15 minutes, which may also reset the system and clear the codes, but this may reset other settings as well.
To reset the dash codes on a 2006 Mazda 6, first, turn the ignition key to the "ON" position without starting the engine. Then, press the gas pedal three times within five seconds. After that, turn the ignition off and then back on. This should clear the diagnostic trouble codes from the dashboard. If the codes persist, it may require a more thorough diagnostic check.
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Mazda 6