I've been getting a P0420 code about once every 25+ mile trip. Lately I'm sometimes getting two per trip. I've been checking the data when the code pops up on my scan gauge, and EVERY time the truck is running in open loop, while going through a flat, lower speed zone after several miles of 60 MPH highway. I've noticed that my truck is kicking into open loop every once in a while under these conditions, but it doesn't always throw the code.

I'm trying to figure out where to start here. The open loop/low speed/low exhaust volume thing is perplexing. It seems like the insufficient catalyst code would pop up under high load - like it did when I burned up the original cat 6 years ago with a trailer in the Sierras. This is the first time I've had a code since replacing the cat way back then, and it's had an easy life these last couple years.

Any notes from similar experiences or wild-ass guesses would be appreciated.

 

It might help to know just what your working on ??

One or two cats ??

It seems to me that every time the catalyst test completes if the problem has not been fixed the light will trip . Without looking I am not sure if this is a one or two test trip

I know this is how my Tacoma was till I fixed the issue

I think your seeing the same thing that is why it looks so random .

Remember it is the complete circuit for some reason the ECM is not seeing enough variation between the upstream and down stream sensors

 

I had the code turn on for the 1st time on my 2nd tank of gas while on my way to New York (from Chandler,AZ) and it seemed to coincide with me flooring it to get onto the freeway with my 01 Taco(180,000 miles). The code re-appeared several times. I was able to clear it with my scanguage2 each time. I noted it seemed to recur shortly after flooring it for getting onto the freeway or similar. I have not seen it happen in a week so was it bad gas?

Th open loop should be a big clue.

hmmmm.....I think mine hit open loop while I floored it several times and that may have aggravated a P0420 to trigger. Read paragraph #1
http://urdusa.com/store/Denso-Produc..._13/index.html



see this
http://www.easterncatalytic.com/educ...he-po42o-code/

 

You know it could have been the different formulas for the gas between AZ and New York

The poor ECM was in a constant state of change and flooring it did not help

 

open loop.....did somebody remove the thermostat causing it t run to cool?

 

It's my 95 T-100. 3.4, single cat. I posted here because hardly anyone looks at the T-100 page and I thought I would get a lot more responses on the 3.4 emissions system here.

Thanks for the links, Zuk! Interesting reading. I was also reminded of a quirky problem prior to the P0420. After pushing the engine hard to pass and returning to idle, my truck would die, and not hold idle after restarting for about 10 minutes. This hasn't happened since the code started tripping. I guess I'll try the front O2 sensor to see if my A/F ratio is off.

 

Oh, and t-stat is OEM. I run about 193-200 degrees.