I am getting code P0420, catalyst system efficiency below threshold.
Hi. There are a few things to check out when receiving this code as it doesn't always mean the catalytic converter is going bad. The car has multiple oxygen sensors that test how efficient the converter is performing. If the sensors are failing, they will give incorrect readings which will convince the computer that the converter is at fault. Exhaust leaks can also throw off the readings the oxygen sensors get, which will confuse the computer as well. Lastly, the catalytic converter itself may be failing which appears to be a somewhat common issue with the Altimas. You should have a certified technician, such as one from YourMechanic, help you inspect the whole exhaust system to find the cause of the problem.
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