The documents are for 2003 Tacomas, but the inspection procedures should be similar. I think that you might have a HOS in the front (rather than an A/F sensor), so the inspection procedure of the front (bank 1 sensor 1) sensor will be different for you. I do not know if it will be exactly the same as the link I gave you above, but it might just be.
If you trust your dealership, you could have them check the sensors and cats, and you could go buy the parts cheap online. http://www.urdusa.com sells O2 sensors, and http://www.1sttoyotaparts.com sells all OEM products. The problem is... the dealership mechanics make too many mistakes, and you might end up replacing something, and not fixing the problem. I don't trust my dealership.
Anyhow, P0420 only means that the cats aren't working properly, and a lot of times, this doesn't cause further damage. If your local DOT enforces smog checks, then it might be an issue that you even have a CEL, as that's a critical criteria.
Hey I haven't even had a chance to even look under my truck. I'm to swamped at work and at home. Let me know If that fixes your problem might be mine to I only comes up ever once in a while. And If anybody knows could my K&N kit be cause a problem to much air not enough fuel to make sensors read wrong let me know. I have 85,000 on my truck just out of warrenty.
if you have the po-420 code I dont think that code would be associated with your intake, I think you would be looking at a 170 or 172 code if it was a result of your intake, but if you throw a code because of your intake I would recommend cleaning your injectors and replacing your plugs a little more frequently, this should balance out the air-fuel ratio a bit