My 4Runner has 135K miles on it and a few months ago the CEL comes on code P0420 dealer says the CAT's are bad (they did rattle etc...) wants $2,500 for new CAT's I can't go that high so I have them replaced at another shop for $800 and the CEL stays off for 3 months then comes back same code, I replaced both O2 sensors and the CEL stays off for a couple of months then comes back same code. I reset the code and it comes back every 30-60 miles. I finally got the O2 simulator from URD and put it on 2 weeks ago, 700+ miles and no CEL, but my fuel milage has gone to crap since I put the simulator on. I have put in a new air filter, PCV valve and the plugs only have 23K miles on them, I looked at a couple and they look ok to me.

During the time that the CEL kept coming on my milage was the same about 300 miles out of a tank mixed highway and city, now I am at 270 or so. Any thought would be appreciated I am really confused.

 

Have you checked/replaced the O2 and Air/Fuel ratio sensors?

Are the cats aftermarket? If so...that may be your problem. Aftermarket cats, despite what they advertise, do NOT work as well as factory cats.

 

it does have CA emmissions and the A/F sensor was replaced as well as the O2 sensor. They are aftermarket CAT's, what I don't understand is that they were put on in December and the gas mileage has been fine until 2 weeks ago the A/F & O2 sensors were replaced in February. I also cleaned the MAF Monday night but that did not look really dirty

 

I believe aftermarket cats are not certified for CA trucks. How did they get away with installing them in your truck?!

 

I live in VA and there is no emmissions testing here.

 

cats have nothing to do with performance unless they are plugged or flow too freely

what is making your engine run good or run like crap is the A/F sensor and stuff toward the motor. after the a/f sensor, nothing (bad cat if not plugged, bad o2 sensor) will affect performance, but can throw a MIL.

 

if you got dealer parts for the A/F sensor the the thing should be running great.

however most codes related to emissions can also be one of 7 different subsystems, including the integrity of the actual wiring harnesses...so perhaps it isn't related to the exhaust pipe parts

 

The A/F sensor is a stock one from the dealer I pulled it out and looked at it a couple of weeks back and while it looked like it was in the exhaust stream and had some black soot on it I guess it was ok, it only has 5-6K on it. It does not make sense to me that the milage changed as soon as the simulator was installed.

Has anyone else experienced a drop in fuel mileage with the sim installed? should the A/F sensor look a little sooty?

 

One other thing I did remember I ran some BG44 through it about 2 mo ago then the CEL started to pop P0420 again could this have fouled the A/F sensor? If it did would the fuel milage not have dropped at that point?

 

I'd say it's the aftermarket cats.

 

I am trying to understand how it could be the aftermarket CAT's when they were replaced in early December and the fuel mileage has been the same unitl 2 weeks ago. Maybe I am missing something, but to me if the CAT's were the issue the FM would have been affected before now.

 

Why are you throwing all this money away? Just take it to someone with the right tools, and you will come out ahead cost wise. First thing that needs to be done is someone with an ODBII computer has to look at what the 4Runner's computer sees as far as data coming off all sensors. You can tell if either of the O2 sensors or the A/F meter is good without even popping the hood.