P0420 OBD2 Code on 2001 Nissan Primera: How to Diagnose and Fix Without Replacing Both Catalytic Converters?
I own a 2001 Nissan Primera and have been experiencing a persistent Check Engine Light. A diagnostic scan revealed the OBD2 fault code P0420 - Catalyst System Efficiency Below Threshold (Bank 1). The mechanic initially recommended replacing the oxygen (O2) sensor, but after the replacement, the issue has not resolved. The same code reappeared, and now the technician is suggesting a catalytic converter replacement. However, the Nissan Primera has two catalytic converters, and I'm concerned about the cost of replacing both. My question: Does the P042-20 code specifically point to one of the two catalytic converters being faulty? If so, which one should be replaced? Are there any other components or systems I should investigate before considering a converter replacement? For example, could issues with the Mass Airflow (MAF) sensor affect O2 sensor readings and trigger this code? I'm looking for cost-effective troubleshooting steps to avoid unnecessary repairs. Any guidance on diagnosing this issue would be greatly appreciated, especially since my car is due for an MOT and I don't want this fault to cause failure.
First, consider whether your mechanic is the right person to handle this. If the Check Engine Light was only on without any performance issues, you should have erased the P0420 code using a diagnostic tool — doing so would typically turn off the warning light. Second, inspect your Mass Airflow (MAF) sensor. A faulty or dirty MAF can interfere with oxygen sensor readings and trigger the P0420 code. Key checks for the MAF sensor: - Look for a small diode inside the sensor housing — if present, it may be vulnerable to contamination. - Clean the sensor using CARB cleaner or brake cleaner (avoid harsh chemicals). - Check for leaks in the sealing gasket or molding that holds the sensor in place. Any moisture ingress can damage performance. MAF sensors for a 2001 Nissan Primera typically cost between £40 and £60 at salvage yards. If you find one, cleaning it may resolve the issue without needing to replace any catalytic converters. I’ve shared other troubleshooting tips in my posts about the Nissan Primera SE Plus (2000) — feel free to check them for more insights.