Trouble code P0421 means: Catalyst Efficiency Below Normal (Bank 1)
The 2011 Mazda MX-5-Miata has an inline 4 engine.
The 2012 Mazda MX-5-Miata has an inline 4 engine.
The 2006 Mazda MX-5-Miata has an inline 4 engine.
The 2010 Mazda MX-5-Miata has an inline 4 engine.
The 2002 Mazda MX-5-Miata has an inline 4 engine.
The 2004 Mazda MX-5-Miata has an inline 4 engine.
The 2009 Mazda MX-5-Miata has an inline 4 engine.
The 2008 Mazda MX-5-Miata has an inline 4 engine.
The 2005 Mazda MX-5-Miata has an inline 4 engine.
The 2013 Mazda MX-5-Miata has an inline 4 engine.
The 2007 Mazda MX-5-Miata has an inline 4 engine.