I have a 03 ssei. Started having issues with p0420 shortly after I moved to the twin cities area. My Cat was rattling, so as part of the fix to another drivability issue, I replaced it with an advance aftermarket. The code keeps coming back. I only have a couple of clamps on the exhaust and not welded - it could be leaking. However, I started noticing when I fill the tank while traveling, the light is out almost as long as the new gas lasts, then comes back on with a new fill up when home. Tested that theory with a tank of pure gas - 91 octane no ethanol. Turned light off manually and light came on in a couple days. Went to a Holiday, and put their 91 in - likely with ethanol - same one I first noticed this anomaly. And I left the code, and within a few days, the light is out. Drove it 200 miles this week and light is still out.

So what'* everyone'* thoughts that gas formulation can trip the light? I always use 91 or better octane if available. Will running lower octane cause this and is that possibly the issue with my local gas station? I've read that gas is constantly breaking down, and if you find some that'* been around awhile, chances are, it won't have the advertised octane rating anymore. My thought is that the 91 octane pure gas I found hardly ever gets used, and my local gas station is not being accurate in their advertised octane rating if these can be related.

This is third time the light has gone out on its own following a fill-up.

you have to be sure you arent leaking, the system is sensative. same for the aftermaket cat they dont allways read good. you can get o2 sims or have it tuned out of your computer. or get a oem cat which is probably the most expensive option.

as far as the gas no doubt there are differences but normally the car has enough adjustment to compensate and not set off any codes

Many aftermarket cats will set codes on our cars. There are threads in the forum that discuss which ones work, and which don't. IIRC Dan seems to know something on this, so perhaps he could chime in with some information. You don't need to buy an OE cat, but you do need to buy the correct after market one.

Leaking at the cat will also set codes, but find out if it is a good cat or not before you weld it.