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1999 Mazda Protege P0421 OBD2 Code: Causes and Solutions for Catalytic Converter Efficiency Issue

Model: 99 Mazda Protege Fault Code: P0421 Posted: 2014-04-25 18:15

My 1999 Mazda Protege ES with 127,000 miles is currently displaying only the P0421 diagnostic trouble code. I recently replaced the spark plug set and lower radiator hose, and have used a torque-based OBD2 scanner to read data from the oxygen sensors. Despite these efforts, the issue persists. Could anyone provide insight into what might be causing this? Should I consider using an engine combustion cleaner as a potential fix?

Related fault codes
P0421
Comments (1)
Anonymous 2014-04-26 07:08

For accurate diagnosis of P0421, it's important to analyze the pre- and post-catalyst oxygen sensor readings. Graphing these signals helps identify whether the catalytic converter is functioning properly or if there are issues with exhaust gas composition. A detailed analysis of this data can guide appropriate troubleshooting steps. Reference: http://www.obd-codes.com/p0421