Trying to keep this short. I'm the CEL is on guy in the shop. Below is what I look for when it's P0420. Unless you have an exhaust leak upstream of the downstream O2 sensor, P0420 is a dead cat. It is rarely due to a bad downstream O2. You need a scanner that does live data, and updates the screen at least twice a second. Here's what to watch. Start the car, both upstream and downstream will read around .45V. As the car warms up, in about 30 sec, the upstream starts to "swing" from .1V to .9V. Soon after the downstream starts to do the same. This is where you watch for O2 health. They should be changing, the highest most of the time is around .75V to .15V. Actually, the lower the downward swings are the healthier the sensors are. The car should be in closed loop with a minute. That means the upstream O2 is alive and the computer is using it to control the fuel. My car is in closed loop in 25 sec from a 40 deg start. Here's where the computer starts to watch the cat. From a cold start, a minute to warm up, then drive a mile or so, the downstream O2 (B1S2 on the scanner) should stop the wide swinging and vary from .4 to .7 and with another minute be pretty steady around .7V. This is a healthy cat. These engines shut the injectors off when you coast and the tach is over 1300 RPM. Now they call it PZEV.....when you are coasting, the front O2 instantly drops to 0V, and about a second later the downstream. As soon as you get back on the gas, or drop below 1300, the downstream should climb back to .7V. If the downstream is always swinging, the cat is dead. If it's only flat at idle, the cat is dead. Sometimes it's only flat at a steady speed, it's dead. This is what the computer looks at to if it should throw the P0420. Again, unless you have an exhaust leak, P0420 means dead cat. 99%.
Does stuff like Guaranteed To Pass Emmissions, Lucas or Techron clean a cat? Yes, if it's borderline. What kills a cat? Cheap gas, to much short trip driving, oil burning, never changing the oil so now the rings are gummed up (Penzoil high mileage works wonders over a few thousand miles) , head gasket leak, running to rich "cuz the the thermostat is running too cold, running premium gas and doing short trips, and driving with skip for more than a couple of days. The upstream O2 is tired and running rich. Stick with gas that claims to clean the engine. Shell, Mobil, Irving, Conoco, BP. Use a can of Techron at every oil change. Techron is safe for the Nissan injectors. Use Shell VPower when you are going to be on the highway for the weekend, it's got Techron. Hope this helps, S