Thank you for reading this and any reply you could give me.
I worked on my sister's truck this weekend (2000 Toyota 4runner sr5 auto 135k miles California emissions 3.4L V6). MAF was trowing codes P0171 and there was also code P0420. Here is some of the data from the freeze data:
Frame 1 Mod#1
P0171
rpm: 2140
Load=41.5%
temp=194F
intake temp=51F
speed=43mph
fuel system 1=CLSD
fuel system 2=n/a
short term fuel trim1=0.7%
long term fuel trim1=37.5%
short term fuel trim2=-100.6%
long term fuel trim2=-100.6%

I cleanup the MAF, truck passed NJ inspection,but light came back after leaving inspection station. I failed to clean the two wires completely inside the MAF. The two wires were still covered by dirt and now are clean as silver (I used CRC MAF cleaner).
PCV was old and stuck open, intake was covered by crude. Air filter was clogged.
I cleaned up the intake and truck run much better. But after 40 miles, the light came back again, with code P0420. The Ox sensor and A/F sensors were replaced around 2 years ago. The dealer wants to replace the catalytic converters. Thank you for any reply, sorry for too long post.

Well seeing at your fuel trim was at 100% I'd say you may have melted down the cat with an insane amount of HCs(unburnt fuel)(=heat!), but without seeing the truck, it's all speculation.
Problem is, it's hard to tell if youreally need a cat just by looking at it. You'd have to get a scan tool on it and watch the A/F ratio and O2 sensors. A/F should do it's normal zig-zag while the O2 should be flatlined. If the O2 is showing a decent amount of movement, your cat is dead.

Here's a link to the FSM for a 2002 4Runner, a lot of the info is the same for the 2000 model. http://www.ncttora.com/fsm/1996/SIL/fsm/repair.html

You can find the testing details for the sensors and test them to see if they are still good or not. You'll also find the complete diagnostics for both codes. Hope this helps.

Thank you for your reply. I will check again the cat. The truck seen to run much better after cleaning the MAF, just the P0420 that keeps coming back.

Cleaning the filters are one of the most things I hated. LoL. By the way, could you help me locate A/F sensor? I have to replace the one I'm using. If you have any idea please put a response to this thread.

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Toyota Parts

Hello, this truck has a 3.4 V6 California emissions, the A/F sensor is located downstream on the exhaust pipe, just follow the pipe as it comes out from the heads. You will find pics at this site: 4x4Wire - Tech: Time to Replace the O2 Sensor on your 3rd Gen Toyota? .
I recommend that if you will replace it use a Denso unit which was the original that came with this truck,
Do you mind if I ask you why are you replacing it ? ( if you have the MIL on and if you know what powertrain - P code - it has store in memory ).

yeah, that may be the problem. Thanks for the help my friend...By the way I have no idea about it. The mechanic just told me it should be replaced. I'm not that techie friend....

Hello. You may just take it to a local Autozone and have it scan for you for free. Just post the P code here.
Good luck.

After exhaustive research into the insanely stupid reg's in cali, I found a place in Upland, Ca. called Speedway Exhaust. The guy there will install both catalytics with the correct california numbers on it for $400. Dealer wanted $1,200.