spider445

Hi,

New to the board and had a question regarding my 01 Tacoma 3400 V6 4wd. I had the CEL come on and had a code reading of 420B, a friend used his scan tool and the front 02 sensor showed a reading of 3.29 to 3.45 at 2000 rpm, this reading did not drop down when rpm went to idle. The rear 02 sensor showed readings of 0.5 to 0.45 at 2000 rpm then dropped to 0 at idle.

He thinks the front 02 sensor is bad, the rear cat has also been replace with an aftermarket unit prior to me purchasing the truck, and both 02 sensors look like they have been replaced also.

Thanks for your help in advance.

fish0281

The '01 and '02 (maybe '00 too) have a pretty tight bandwith that the cat must operate within... you can replace the AF sensor (upstream of the CC) and the O2 sensor for pretty cheap, but it's likely that the aftermarket cat is just not quite as good as the factory ones. The truck''s emission sensors are set up for tight emissions controls (say, 95% removal) regardless of your local requirements (maybe less than 95%) and it's entirely possible that the cat you have is at 94% or something - it'd probably pass emissions, but the sensor is looking for something better than that. It's a real crapshoot and I've been dealing with it for a while now.

I'd suggest replacing the sensors as a first try - look up the part numbers on URD's site, then buy them at Amazon. Make sure you don't have any leaks too, that'll cause a code to pop up. If the light keeps coming back on with all of that, go to URD's site and buy an "O2 simulator" or ... spend about $2k on Toyota cats.

Also, the AF sensor performance can't be judged by a standard scan tool or looking at just the voltage output. It's an oscillating signal and it's either the frequency or amplitude that determines if it's good/bad.


Tl;dr: It's probably your aftermarket cats. Buy new AF and O2 sensors, check for leaks, and if all else fails, buy the URD O2 Simulator.

fish0281

There is a lot of info out there on this site and similar 4Runner forums, but the general consesus is that aftermarket cats just don't have the same performance as Toyota ones - at least in the long-term - and you'll keep going in circles trying to resolve it w/o buying either a Toyota CC or the sim.

Do you have the old cat still?

Rob510

P0420 is a cataylst inefficiency code. Its is possible that you Air/Fuel ratio sensor is the problem but I would say its your aftermarket cat, like fish said Toyota systems are very strict as to the allowable tolerance. I would save the money on the new sensors and go buy a sparkplug non-fouler from any parts store. You have to drill the middle part out to allow the O2 sensor to fit inside but you can get a pack of 2 for like $5. Since you have a converter its not going to take much to keep the light from coming back on.

fish0281

Eeek, I would not recommend doing that - it will force the engine to incorrectly adjust the fuel-air ratio and if it goes lean (as I assume it would), this will lead to bigger problems! A lean engine is more dangerous than a rich CC - the ever-so-slightly rich condition engines cycle though allow liquid fuel to exist on the cylinder walls in order to help heat transfer out of the cylinder and into the cooling system.

Rob510

Installing it on the rear O2 isnt going to cause the engine to change air fuel mixture. You are just limiting the sampling pool that the rear sensor sees. I have done this on many cars to keep that code away with no problems.

fish0281

Ah, I thought you were saying to put it on the AF sensor (which wouldn't work anyway since it's a flange-style sensor). That'd probably be fine on the O2 (rear) sensor, I'd be interested to know if it works on the '01+ system.

Rob510

Ok, I see. Even if you could, putting on the front sensor would be a bad idea. Basically you are tricking the ECU to think the catalyst is working really well.

spider445

Thanks for the information, if I replace the cat with one from Toyota do I also have to replace the unit coming from the manifold to the cat just prior to the AF sensor or just the one between the sensors?