2006 Mazda MX5 1.8L P0421 Fault After Using Terraclean - Cat Clog Suspected
I own a 2006 Mazda MX5 with a 1.8L MXR engine and 5-speed transmission (known as Miata in the US). After performing a Terraclean service, I began receiving a P042-1 diagnostic error, which indicates a catalytic converter efficiency issue post-catalyst. During the Terraclean process, engine RPMs were kept below 2000, but my vehicle experienced sudden drops in revs and required constant accelerator input to maintain speed. Since then, the P0421 code has persisted. I have already replaced the pre-cat O2 sensor, but I suspect the Terraclean treatment may have clogged or damaged the catalytic converter. Is this a known issue among other 2006 Mazda MX5 owners? Could the Terraclean procedure be causing catalytic converter blockage?
This sounds like a self-inflicted problem. Why was Terraclean chosen over standard maintenance? Consider using an O2 sensor graphing scanner to compare pre- and post-catalyst sensor readings—this can help identify if the sensors are functioning correctly. A back pressure test or vacuum gauge check may reveal exhaust restriction. Options are limited: either continue driving and hope the catalytic converter clears up over time (requiring long, sustained drives to reach optimal operating temperature), or consider replacing the catalytic converter. I'm optimistic—take a long drive and report your results. *Note: A faulty O2 sensor could also be misreporting data, so it's worth checking both pre- and post-cat sensors.