I have had P0420 problems, usually in Mexico, with my 2002 Sienna. At times there is a failure, then it cleans up again. Strange.
Recently, it happened again, but this time there may be a possible clue.
It last failed, I think, last summer, and since then has worked perfectly.
I was driving to Tehuacan on a shopping trip and stopped at a convenient Pemex to fill up. Alas, they did not have Premium, so I had them put in 4 or 5 gallons of regular (Magna).
I drove maybe 35 miles to Sam’s Club, shut off, then moved the car to Aurrera (mini-Walmart), and went down town looking for a charger for my wireless phone. I drove the mile or two back to Sam’s Club, to pick up my pictures, then went home. A few minutes later, the Check Engine Light came on. P0420.
I drove back to my village, next day went and filled up with Premium. I turned off the P0420 and it hasn’t come on again, though I took a 200+ mile trip to Cordoba for a special birthday party.
It may be coincidence, but that CEL definitely came on soon after putting regular (Magna) in the tank. And, stayed off when I burned Premium again.
Here is something I learned last year. A cousin had told me that they sometimes have problems with regular, but never Premium. I thought that was weird, but bumped into a URL which told how Mexico gets its gasoline.
They send crude oil to Houston, then buy gasoline back from the Houston refineries. The premium gets its additives put in at Houston, the regular gets additives added in Vera Cruz when the gasoline gets back to Mexico.
My current theory is that somehow they mess up the additives in regular, but not in Premium. I have no idea how this could cause a CEL, probably from a dirty cat. But, it was certainly an amazing coincidence if it wasn’t the gasoline. Any ideas or contrary opinions?