Hmmm...P0420, eh?

[Jcp123]

Hey guys...new here. Technically I do not have a Yaris, but rather a 2000 Echo. Hey, we all are in the 1NZFE family, right? I thought I might post here since there was some good technical discussion on this code in my yahoo search.

Anyhow, this Echo was inherited from my wife, who in turn inherited it from her parents. Currently it has 240k miles. Naturally she was vague about maintenance history but said it definitely received regular oil changes. Owing to the fact I had to replace the absolutely decrepit original spark plugs at about 195k, I tend to believe it had nothing but those regular oil changes. I had some overheating and driveability problems cured by a MAF sensor cleaning. I also got my first P0420 coded around this time. The cat was duly replaced and for about 2 years and 30k miles, all was good. It still drives strong with good power and fantastic gas mileage in the 39mpg range for my commute. I run nothing but Texaco or Chevron since they carry Techron.

About 6 months ago, P0420 started to reappear periodically. It has gradually started to appear more often, about once per week. I just got done with an attempted cleaning of the IAC...IAC is dead. Cleaned out a lot of gunk while I was at it. But on my first full day of driving, about 35mi total, it popped up no less than four times.

At this point, I am at a bit of a crossroads. It was a super cheap cat which was put on - $130 installed, so I guess I wouldn't be surprised if it had failed. I am also pretty sure (actually almost positive) that the O2 sensors are original as well. O2 sensors are hardly immortal either.

I run an UltraGauge and can monitor my O2s. Voltage seems to stay under 1v on up- and downstream sensors. But fuel trims trouble me a bit more. Long trim seems fixed at 7.81, short term seems to vacillate between -1 and +4. I was taught that -10 to +10 is a normal range, but also that they should generally add to a sum close to zero? I.e. with a long term of 7.8, should I not see short terms generally stay around -7 to -8 but for throttle and/or load changes? Constant throttle seems to yield short terms of about +1 to +2.

Technical maybe...but I would like to know what I am facing. This sounds like it could go either way, O2s or cat. I'd kinda rather put the money back into my other car...it's a classic and is appreciating as opposed to this abused little workhorse. But I hate to think this car is giving up the ghost. I have actually gotten to like the car and triple the gas mileage is...tempting...and honestly the car is pretty damn reliable.