Q: the car wont pass emmissions the code po420 comes up

Code P0420 is for the efficiency of the catalytic converter (CC). You have two CC’s on your car. There is one built into the exhaust manifold, and there is one in the mid-pipe, under the car.

The one in the manifold is the one monitored by the computer for efficiency. The O2 sensors are used to monitor it. The CC is an important emissions device. The exhaust gases pass through it. Chemical reactions occur to convert harmful gases into less harmful gases.

There are two ways that they stop functioning. The material the gases pass through falls apart, or it gets coated with oil if your engine is using oil. Taking into account the amount of miles you have, and how quickly the code returns, I would guess that the material has come apart. This can create another issue.

When it comes apart, it can clog the second CC. The fact remains that if the CC in the manifold has come apart inside, the code will return consistently. The proper repair is to replace the catalyst converter.

There are aftermarket parts available that are less pricey than Nissan parts. It is against the law for a business to sell a used CC. If you would like this repaired, or just a quote, consider YourMechanic. A certified technician will be able to assist you with a further diagnoses of your Check Engine Light and help you make the necessary repairs. Good luck.