P0420 THREE WAY CATALYST FUNCTION
P0325 KNOCK SENSOR
The code you need to fix is P0420. It indicates the catalyst is not able to store enough oxygen.
Possible causes are: (in order)
1. Warm-up three way catalyst
2. Exhaust tube
3. Intake air leaks
5. Injector leak
6. Spark plug
7. Improper ignition timing
The quickest way to check the catalyst is taking the van to your Nissan dealer. They can use Nissan’s scanner to monitor the O2 sensors readings. If you want to do some Do-It-Yourself troubleshooting, then do the following:
1. Check exhaust tubes and muffler for dents or cracks. If OK, search for leaks in exhaust piping before the catalyst. Replace exhaust tube(s) if found any damaged.
2. If all pipes are ok, proceed to search for air leaks after the air filter. Repair leaks if find any.
3. If no air leaks are detected, check the engine timing. Adjust if necessary. If timing is OK, check injectors. (For this, I recommend take the van to a Nissan dealer)
4. If injectors are OK, replace spark plugs, ignition wires, distributor cap and rotor. (If the van is high mileage (over 100K+) and you have not replaced these parts or replaced with wrong parts, replace them with correct parts.
5. If everything above is in good working condition, you need to replace the warm-up three way catalyst.
There is a very small chance the front O2 sensor might be going bad too. However, the computer is not reporting code P0133.
If all above is OK (good working condition) replace the three way catalyst. It is the located in the center of the van.
I have given some tips based on the few information you have provided.
By the way, How many miles are in your van?