2002 Toyota Camry PO420 Code: Should You Replace O2 Sensor with Aftermarket?

A PO420 code in a 2002 Toyota Camry is not typically caused by the O2 sensor itself. Regarding aftermarket O2 sensors, I strongly advise against using them. Many aftermarket parts don't match the correct part numbers for this application, which can result in installing an oxygen sensor where an air-fuel ratio (A/F) sensor should go — or vice versa. This mismatch leads to serious drivability and emissions issues. This topic has been discussed extensively across forums, including at least three times specifically on Toyota Camry models with PO420 codes. While replacing the sensor might seem like a fix, I'm skeptical it will resolve the underlying problem.

Should I replace the catalytic converter first instead of the O2 sensor? And do air-fuel ratio sensors go bad — could that be causing the PO420 code?

Here's a clear breakdown: Air-fuel ratio (A/F) sensors are similar to oxygen sensors but operate differently. They're located in the exhaust system and serve a related function, yet they are not interchangeable. Using an O2 sensor in place of an A/F sensor — or vice versa — can cause significant issues. Most imported vehicles originally used A/F sensors, so it's critical to use parts that are specifically designed for Toyota applications. Avoid generic Bosch cross-references that work well in other models but fail when installed in a 2002 Toyota Camry. Importantly, replacing just the O2 sensor will not fix a PO420 code. A faulty sensor may have contributed to catalytic converter failure, but many other factors — such as engine misfires or poor fuel quality — can also cause it. Therefore, you'll likely need a new, high-quality catalytic converter. Check with your Toyota dealer for any remaining warranty coverage on the vehicle. If no warranty exists, avoid the cheapest option — it may not resolve the issue. Always compare pricing between OEM Toyota parts and reputable aftermarket converters.

Thanks Jeff! I replaced the catalytic converter on Bank 1, cleared the engine light, and completed a drive cycle. Afterward, I checked the catalyst efficiency monitor — the test was successfully completed. The system now shows no PO420 code.

I've noticed my fuel mileage dropped by 10–20% at some point after the PO420 code appeared. Additionally, the engine idles low at stoplights and sometimes stalls — which I believe indicates a lack of oxygen in the intake stream. Before the PO420 code showed up, I replaced the air filter (which had been overdue), and that improved fuel efficiency slightly — maybe 5%. Tire pressure is also correct, so those two factors are ruled out. The stalling and poor mileage coincided with the appearance of the PO420 code. Does this symptom pattern seem consistent with a failing catalytic converter? Thanks for your insight. ~Shadn

The stalling could be due to a restricted exhaust — possibly from a clogged or failed catalytic converter. However, it might also stem from other issues such as an outdated tune-up. How recently was your last maintenance performed? In any case, I recommend starting with replacing the catalytic converter using a high-quality unit.