P0420 only once I hit half a tank of gas

[Qmb9015]

Here recently i have gotten the standard P0420 converter efficiency below threshold bank 1 code but only once i have just crossed the half tank mark on the gauge. Both times i was 55+mph and either just finished a 2 hour trip or was in the middle of it. Both trips the first half of the tank was used to drive the 2 hours it takes to get to my destination so i would think that if it was a leak somewhere then the code would have popped up during the first 2 hours of the trip correct? Im hoping its just bad O2 sensors and i plan on replacing both bank 1 sensors(probably both original) but just in case that doesn't fix the issue what do yall think it could be?


2001 4runner SR5 4wd 233k miles.

 

[Dobich39]

After much experience with these wonderful codes and changing cats and 02 sensors I would go with the cheapest option first. Especially because you said it starts with half of fuel tank, Ive seen vehicles do this and this trick always seems to work, try techron fuel treatment. If that doesn't work after a week I would go for the 02 sensors next.

 

[Qmb9015]

After much experience with these wonderful codes and changing cats and 02 sensors I would go with the cheapest option first. Especially because you said it starts with half of fuel tank, Ive seen vehicles do this and this trick always seems to work, try techron fuel treatment. If that doesn't work after a week I would go for the 02 sensors next.

Thanks Ill get some tomorrow as i just filled up today and plan on doing another 4 hour round trip this weekend.

 

[Qmb9015]

After much experience with these wonderful codes and changing cats and 02 sensors I would go with the cheapest option first. Especially because you said it starts with half of fuel tank, Ive seen vehicles do this and this trick always seems to work, try techron fuel treatment. If that doesn't work after a week I would go for the 02 sensors next.

Well still got the code so it looks like im going to replace the O2 sensors.

 

[JayRolla]

Don't worry about the half tank thing. Most likely coincidence. Start with rear o2 since it's throwing the code.