PO420 code

P0420 is a catalyst code. It basically compares readings from upstream and downstream O2 sensors. If the difference between readings is not enough, then the code is set, indicating that the catalytic converter is notdoing its job well enough.

As for the lacquer trick, I have heard of it. But I have never done it. My personal favorite is to buy a bottle of Seafoam. Raise the hood on the car and take the vacuum hose off of the brake booster. Start the car. Have someone give it some gas (maybe idle it around 1500 rpm's) and stick the disconnected hose into the top of the Seafoam bottle. Then turn the bottle over and let it empty into the intake. Shut the car off when the bottle is empty. Put the hose back on, and let it sit a few hours. Then fire it up and quickly get going down the road (because the car will be smoking quite a bit and just might make the neighbors mad). Open up the throttle a few times. Drive the car at least fifteen miles. Go home and clear the codes. You should be good to go if the catalytic converter was not too far gone.

Note: as I mentioned above, this will make the car smoke. A lot. So do it when no one's around. And have the car set up so you can just pull it out of the driveway and get going down the road. Get out into the country side if you can and keep it moving.

I have run this trick several times and can vouch for it. It does work. It is a temporary fix (makes the code go away for about six months in my experience). You will also notice that the engine runs much better for a while and even a slight increase in gas mileage for a week or two (if you pay attention to it).