morrisg wrote:P0421 is titled Warm up catalyst system efficiency below threshold in the Mazda repair manual. Under possible causes it lists exhaust gas leakage, A/F sensor and/or HO2S sensor looseness, TWC deterioration or malfunction and PCM malfunction. TWC means the catalyst, PCM means the engine computer. In the repair section it steps through these 4 causes in order: fix any exhaust leaks, check the O2 sensors are tight, then replace the catalyst and PCM in that order to "fix" it. Not much help from the official manual, sorry.
As far as inspection for the state goes, use your Dash Dyno to check that there are no codes and no pending codes before you go in for an inspection and you should pass. I don't know if Illinois does a visual or tailpipe check in addition to plugging into the OBDII port.
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