How to Diagnose and Fix P0420 OBD2 Error on 1997 Toyota Camry V6: Catalytic Converter or Exhaust System Issue?

Check for exhaust leaks in the system, especially before and after the catalytic converter. A leak can cause false P0420 readings by disrupting oxygen sensor signals.

Ensure the vehicle is fully warmed up before testing — catalytic converters only function efficiently at operating temperature. Also verify engine coolant temperature (ECT) sensor accuracy. A cold engine may result in open-loop operation, which masks true converter performance.

I replaced the entire exhaust system, including the catalytic converter, with an aftermarket unit from AutoZone. The part was Maremont-branded (only option available). I'm not sure if this is causing the P0420 error. Could there be a way to recalibrate the ECU after installation so the system recognizes it as functional? I plan to retest once I have access to my friend's scanner.

I re-ran the O2 sensor test today. Does this new data suggest a faulty sensor or simply an incompatibility with the aftermarket catalytic converter?

Here are updated screenshots from the latest scan.

Final image of the O2 sensor waveform data set.

Are you monitoring the downstream oxygen sensors (behind the catalytic converter)? This is critical for diagnosing P0420 — if they behave like upstream sensors, it indicates a failing converter.

I am monitoring both Bank 1 and Bank 2, with Sensors 1 and 2. The sensor labels are clearly shown in the top-left corner of each screenshot (below Fuel System Status). For example: B1S2 O2 Sensor Output Voltage refers to Bank 1, Sensor 2.

If the downstream oxygen sensors (like B1S2) are behaving like upstream sensors — showing choppy, rapid waveforms instead of stable, smooth patterns — then your aftermarket catalytic converter is not functioning properly. The screenshots clearly show this pattern.

I'm still unclear on how you determine that the downstream O2 sensors are behaving abnormally. Are you saying the B1S2 sensor (after the converter) should have a flat, stable reading around 0.45V? I see some sensors show no response — it may be due to installation timing after exhaust replacement. Thanks for clarifying.

Exactly — Jeff is right. Your aftermarket converters aren't performing as expected. Look at the output voltage waveforms: Sensor 2s (downstream) should show long, wavy patterns when the converter works properly. Instead, they mirror the upstream sensors' choppy signals. This indicates poor catalytic conversion efficiency.

I now understand — if I compare B1S1 and B1S2 O2 sensor output waveforms, they appear nearly identical. You're correct that downstream sensors (like B1S2) should show a stable, flat signal when the catalytic converter is working properly. Since this code is for Bank 1 (as per http://www.obd-codes.com/trouble_codes/...bank-1.php), I only need to evaluate Bank 1. Sensor 1 (S1) is upstream of the converter, and Sensor 2 (S2) is downstream. The ideal downstream voltage should stabilize near 0.45V. My current data shows no such stabilization — it looks like the post-converter sensors are not responding properly. I’ll need to return to AutoZone to verify the quality or compatibility of the aftermarket converter installed.

It seems that even advanced scan tools don’t provide clear guidance on how to interpret O2 sensor waveforms for catalytic converter diagnosis. They don't indicate which component is failing — just report a code. This makes troubleshooting difficult without expert knowledge.

While scan tools may not explain system logic, they should at least provide sample reference waveforms for normal operation — especially for downstream sensors. The current tool shows the same pre-catalytic waveform across all sensors, which is misleading. A proper diagnostic tool should display a clear example of what a healthy post-converter O2 sensor signal looks like. This would help users distinguish between faulty sensors and converter failure.