P0420 code only w/ city driving

[jimmyb.1]

My 02 Sienna has been getting the P0420 code for the past 50K miles but only when I'm doing city driving or going slowly down a gravel road. The van has 200K on it. I just went on a family vacation 1600 miles each way, all interstate and the CE light never came on the entire trip. Typically I'll get the 420 code roughly every other tank for my normal driving habits of half highway half in town. If I get non-ethanol gas it comes on every tank.

June 2009 @ 150K I had the dealership dyno the van and the results were the cats were good enough, all sensors fine and most likely a bad computer.

The question I have is: Are my cats still good enough at 200K? Based on not getting a 420 code in 48 hours of interstate driving is this still a bad computer giving me this code?

Thanks for any help.
Jim

[SiennaDriver]

Messed up sensor. Try some injection cleaner with something called Techron. That products been mentioned before, and I don't know how it does it, but it's cleared up the same code for us. But it eventually comes back.

[wiswind]

From the reading that I have done....there are 3 likely causes of your issue.
Catalytic converter
Sensor
Computer
The dealership is saying computer after a good test of the system.
Sadly, that is the most expensive, so if they are wrong.....it is an expensive try.
However, I would lean toward what they are saying.