Mazda Premacy 2001 1840cc P0420 and P0421: Is Catalytic Converter Replacement Necessary? Diagnosing OBD-II Error Codes for Older Vehicles

It seems you're being given misleading information. The current emission testing standards are becoming stricter, and a lit check engine light is now sufficient to fail the annual test. The Mazda Premacy is not common in North America, and catalytic converter issues are widespread. A proper diagnosis requires using a scan tool to read live data. Given your car's age (10 years) and history of ignoring the check engine light, a new catalytic converter is likely needed. I strongly recommend seeking help from an independent mechanic or your local Mazda dealer — someone with experience in diagnosing these systems.

kev2 wrote: It seems you're being given misleading information... The Mazda Premacy is not common in North America, and catalytic converter issues are widespread. A proper diagnosis requires using a scan tool to read live data. Yes — that's exactly what I feel. Regarding the catalytic converter: it has been working correctly, as confirmed by passing the annual emissions test just weeks ago. The issue is not necessarily with the converter failing, but rather with an incorrect reading or a related system fault. For P0420: Recommended troubleshooting steps include: - Check for exhaust leaks at the manifold, pipes, and catalytic converter. Repair any found. - Use an oscilloscope to examine oxygen sensor operation (Tip: The front O2 sensor before the converter typically shows a fluctuating waveform; the one after should be steady). - Inspect the downstream heated oxygen sensor (HO2S). Replace if faulty. - Consider replacing the catalytic converter as a last resort. For P0421: Measure the voltage of the downstream O2 sensor (Bank 1, post-converter). It's also wise to test all oxygen sensors during inspection. I'm wondering whether both P0420 and P0421 appearing in sequence could provide more insight into the system’s performance?

There is a common root cause between these two codes — the catalytic converter may be failing, even if it passed tests recently. Before replacing the converter, take your vehicle to a qualified technician who can follow established diagnostic procedures. They should use a scan tool in 'Mode 6' to analyze real-time sensor data for accurate diagnosis.

-- Mohawkmtrs wrote: There is a common root cause between these two codes — the catalytic converter may be failing, even if it passed tests recently. Before replacing the converter, take your vehicle to a qualified technician who can follow established diagnostic procedures. They should use a scan tool in 'Mode 6' to analyze real-time sensor data for accurate diagnosis. The system is currently working — it passed the annual test just weeks ago. My main concern now is that after two attempts at repair, I still can't find a competent expert. I want to do as much research as possible before contacting another service provider. I already paid for a software reflash. If I understand correctly (please confirm), this should not be necessary — is that right? Also: What exactly does 'Mode 6' on a scan tool mean?

Trying to explain in simple terms: The vehicle's OBD-II system monitors exhaust gases using oxygen sensors located before and after the catalytic converter — referred to as S1 (pre-cat) and S2 (post-cat). The system compares the voltage readings from these two sensors. If the catalytic converter is working properly, the exhaust gas after the converter should be cleaner, resulting in a different voltage reading for S2 compared to S1. This expected difference is programmed into the vehicle's PCM (Powertrain Control Module). If the PCM detects no significant difference between pre- and post-cat readings, it sets an error code — such as P0420 or P0421. It’s possible for exhaust emissions to be physically clean during testing yet still have a catalytic converter that isn’t performing optimally. A scan tool in 'Mode 6' allows access to real-time raw data from the oxygen sensors, enabling you to compare pre- and post-cat voltage waveforms and assess converter performance directly.

Thanks for this clear explanation — it's exactly what I needed. But: 1. I believe I need more detailed guidance on the actual diagnostic steps. 2. Both P0420 and P0421 suggest specific checks before replacing the catalytic converter. Can a technician simply replace only the oxygen sensors? What should they do? Are there any other steps that must be taken before considering a full converter replacement? I’d appreciate more clarity on this process.

Another helpful resource: https://www.converterwarehouse.com/catalytic-converter-replacement-1840cc-mazda-premacy.html What are your thoughts on this page? Does it support the idea that a catalytic converter replacement is necessary for older vehicles like mine?