That is my view, too, Joe, and have been taken to task for it. My 2002 Sienna has had intermittent P0420. in October, my Son-in-law replaced the other two sensors. (A year or two ago, the heater in one went out, so it was new already.)
It has been several months now and no P0420 yet.
Irlandes:
That’s been my experience with tired sensors.
is it generally true that by replacing the O2 sensors I might possibly see a slight uptick in my MPG?
The upstream sensor is the one that has an effect on fuel mixture (and hence MPG).
The downstream sensor, after the cat, is the one that triggers the P0420 code. That sensor has no effect on fuel mixture or MPG. It’s only role in life is to check upstream operation. (If the car is on it’s last legs, using a simple spark plug non-fouler begins to look attractive).
as a mechanic who performs smog inspections, including the “real” dyno run smogs . . .
I’ve seen many cats which have gone bad
I guarantee you 100% it wasn’t a bad sensor causing the problem
How do I know this . . . I replaced the cat and did NOT replace the sensor
If the numbers are a LOT lower with the new cat and the old sensors, it sure does seem to imply the sensors were not the problem
I’m talking hc numbers in the HUNDREDS, versus less than 20 with the new cat
I’m only talking about bad cats, not what may or may not have caused the condition
I’m not saying a bad sensor couldn’t cause a p0420
I’m saying that I’ve probably had more experience with cats, sensors and smogs than many of the non wrenches on this website
By the way, I also deal with non-OBD2 vehicles. Which do not even monitor the catalyst, and therefore don’t have rear sensors. You can’t rely on some code to tell you the cat’s condition. It’s a different world.
These non-OBD2 vehicles I’m referring to are not pre-1996. They are non-OBD2 because they are class 4 and larger. Such vehicles are OBD2 exempt.
My point is this . . . I don’t always “rely” on some code and sensor(s) to determine if a cat is bad
If anybody wishes to condemn me now . . . go for it 