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1999 Mazda Miata P0420 Code Reappears After O2 Sensor and Aftermarket Cat Replacement

Model: 99 Mazda Miata Fault Code: P0420 Posted: 2008-01-14 07:57

My 1999 Mazda Miata with 70,000 miles has been displaying the Check Engine Light (CEL) with a persistent P0420 code. I replaced both oxygen sensors and an aftermarket catalytic converter (non-OEM), then reset the codes. The CEL returned within 40 miles of driving. After three resets, the light comes back within 50 miles under city conditions—only staying off during highway-speed driving. When driven for 400 miles at highway speeds without the CEL illuminating, it reappeared after just 30 miles of city driving. The vehicle is completely stock except for the aftermarket catalytic converter. I'm seeking advice on how to resolve this issue or identify potential underlying causes.

Related fault codes
P0420
Comments (4)
Anonymous 2008-01-14 09:47

Aftermarket catalytic converters are not always reliable. Please ensure there are no exhaust leaks before the catalytic converter. Monitoring the oxygen sensors could provide additional diagnostic insight.

Anonymous 2008-01-14 11:42

I cannot monitor the O2 sensors myself. The previous owner reported the CEL appearing around 63,000 miles. He took it to a dealer who advised replacing the catalytic converter for $1,000. He then had an aftermarket cat and front O2 sensor installed at a local muffler shop. When the light returned, he simply continued driving without further action. I purchased this car with 68,000 miles on it and have already replaced the rear O2 sensor. The CEL has remained on for over 7,000 miles—unless there's a cost-effective solution (other than a $1,000 catalytic converter), I may need to continue driving it as is.

Anonymous 2009-10-07 12:23

The user explicitly stated the installation was a non-OEM catalytic converter.

Anonymous 2009-10-07 15:35

If the catalyst is failing so rapidly, there may be an underlying fuel control issue or a problem upstream of the catalytic converter. Many aftermarket converters do not meet the required operating temperatures due to lower-quality precious metal content and will fail the catalyst monitor. However, if the brick itself fails that quickly, it won't matter what's installed downstream—the failure will occur soon after.