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P0420 Bank 1 Diagnosis and Fix: O2 Sensor Replacement for Nissan Altima GXE

Model: Nissan Altima GXE Fault Code: P0420 Posted: 2008-01-13 09:43

If you're experiencing a P0420 (Bank 1) diagnostic trouble code on your Nissan Altima GXE, start by replacing the oxygen (O2) sensors. These components are relatively inexpensive compared to catalytic converter replacements and play a critical role in monitoring exhaust emissions. The catalytic converter reduces harmful pollutants from engine exhaust using heat and precious metal catalysts that convert toxic gases into safer elements. However, if the engine is not properly tuned or calibrated to OEM specifications, the catalytic converter's efficiency drops significantly—potentially leading to failure. On OBDII-equipped vehicles like the Nissan Altima GXE, oxygen sensors are used by the powertrain control module to assess catalytic converter performance. The system compares the voltage signal from an O2 sensor located upstream (before) the converter with one downstream (after). If the signals match closely, it indicates proper converter function. A common fix is replacing all O2 sensors—some vehicles have two, others up to four. After this step, if the P0420 code persists, the catalytic converter may be faulty. While rare today due to improved emissions technology, a failing converter can still occur and requires replacement—a costly repair. Note: Some users have attempted to bypass the catalytic converter by installing a straight pipe exhaust, but this is illegal and violates emissions regulations. Always ensure any modifications comply with EPA standards. For accurate diagnosis, use a scan tool with live data streaming capability. If the upstream and downstream O2 sensor voltages mirror each other, it suggests poor converter performance. Even if you don't plan to replace parts immediately, consulting a qualified mechanic for an hour of labor is recommended—especially when dealing with complex emissions issues. Regarding replacement costs: Dealerships often charge high prices for catalytic converters. Independent exhaust shops can typically provide certified aftermarket converters at lower prices and still meet EPA performance standards.

Related fault codes
P0420
Comments (2)
Anonymous 2008-09-12 02:03

I recently had the same P0420 issue on my Nissan Altima GXE. I started by replacing all O2 sensors. When that didn't resolve the code, I replaced the catalytic converter. It wasn't expensive—my replacement from RockAuto.com cost $79.00 and included a $30 mail-in rebate. This was a straightforward fix for my vehicle.

Anonymous 2008-09-12 15:36

O2 sensor replacements can be more costly than expected, especially if multiple sensors are involved. In my experience, a bad O2 sensor will typically trigger its own diagnostic code—PO420 is one of the few codes that can generally be trusted as an accurate indicator of catalytic converter inefficiency. With a scan tool capable of live data streaming, diagnosing this issue is straightforward. If the catalytic converter is failing, you'll observe that the upstream and downstream O2 sensor voltages mirror each other. I recommend scheduling an hour with a reputable mechanic to confirm the diagnosis—this ensures accuracy even if you're not planning immediate repairs. As for replacement parts, dealership prices for catalytic converters are often inflated. Certified aftermarket converters from independent exhaust shops offer comparable performance at lower costs and meet EPA emissions standards.