my '99 CA emissions 4runner has 142K on it. back in late 2003 when it had 61K on it, i replaced the bank one (front) A/F sensor in order to resolve trouble code P0135. about a month ago i got trouble code P0136. so i replaced the bank 2 (rear) O2 sensor. all fine for a couple weeks, and then i get P0420 (catalyst efficiency below threshold). could this be because the front sensor is bad again, or am i going to have to replace the cats? and, if the later, will i need to replace the front sensor anyway? i've read prior posts on this topic, and it seems possible that just replacing the front sensor might resolve the "imbalance" between the two sensors. obviously, i'm hoping to avoid replacing the cats. thanks in advance for any guidance!

The 420 is indicating the catalytic converter. It takes good signals from the O2 sensors to be able to test, and flag the converter's efficiency dropping.

Refer to this article and in particular the sine wave appearance of a normal O2 sensor signal..

The way the catalytic converter is tested is that the front A/F sensor on your vehicle will see the characteristic sine wave appearance as the ECU swings the mixture slightly, but the rear sensor should see only a straight line signal with a good converter burning any remaining hydrocarbons and the downstream exhaust having a unifrom reading of whatever O2 is left in the exhaust stream.

As the converter begins to degrade, the rear sensor signal starts looking more like the front sensors swinging signal.. At some point, the ECU is seeing similar sine wave like signals from both sensors, indicating that the converter is no longer catalyzing the remaining gases.. At that point the ECU will give the P0420 error.

You mention "imbalance" in the sensors. That is not an issue.. In your case since you have a California emissions vehicle the front and rear sensors produce different profile signals anyway since the front one is a wide band A/F sensor and the rear is the standard narrow band O2 sensor.. Since the sensors are new, I can't think of any reason you would need to replace them again when replacing the catalytic converter..

One question, was the front sensor that you replaced purchased from Toyota (and know it is the exact right part)? Since this sensors signal is the reference to which the rear one is compared to, it needs to be known that it output is correct, as opposed to unknowns introduced into the system by an aftermarket sensor that may or may not be right for your vehicle.

Some aftermarket sensors that don't work well with Toyotas. One issue is the length of the sensor not matching the length of the "bung" the fitting on the exhaust that the sensor is mounted in.. If the sensor doesn't center properly in the exhaust it's readings can be off.

Thanks very much for the response. This makes sense. Yes, the front A/F sensor was replaced with a Toyota sensor. I just wasn't sure that if, since it was replaced 80K miles ago, that I might need a new one now anyway. Eliminating that as a variable leaves me with just the CAT as the issue, do you agree? Surprisingly, my gas efficiency seems to be about the same as it normally is.

Yep there'll be two new cats in your future. The toyota units will set you back $1800. Aftermarket will be about half that. It'll run a little better too.

I am in pretty much the same boat with a '99 Cali. 4Runner and I did the A/F sensor at 90k--I'm at 110k now. I get a P0420 every few months (I keep a log). Last one was in June. My sensor voltage profiles do look normal (I have an OBDII linkup to my laptop that lets me see.) This has been going on for 2 1/2 years and I passed emissions about nine months ago. In NJ they just pull codes anyway. I believe the trigger algorithm for the P0420 is oversensitive or perhaps can just be triggered by some unusual driving profile. Anyway, my suggestion is reset and see what happens before you shell out $1800 bucks for cats. I would rather reset and ignore 3 or 4 times per year, especially when I can see the performance is actually there.

Well, I took this advice some months ago, and have tried to get by, but i'm at the point where the trouble code is staying on all the time. and now, my mileage is suffering. so, my question is, can anyone suggest the best aftermarket replacement scenario? and should i replace the entire system? manifold back? i'm thinking this is the cleanest approach. although the original muffler is still acceptable, the muffler to tail pipe connection is on it's last legs. open to suggestions on type and where to obtain a new exhaust system.