Solving the P0420 in a 1st Gen, 2nd O2 sensor

[DIYmatrixDad]

New reader, old auto guy, but not too aware of Matrix details...

I am looking over my son's 2003 Matrix, the model is 1ZZ (1st gen) and it is a basic FWD auto model. [I think its a 1ZZ, the the door plate model numebr does not have the digit one, but that makes sense for this year and the ODB codes match to a 1st gen not a 2nd] At 143k miles, we are well past warranty but may need to reprogram the ECU when this is all done to make sure original factory programing will not damage the next one (All 2004~2004 model years are known to dump more fuel then they should have, in part causing this problem)

I understand what a bad cat is (and how to cope with that) but for the life of me I do not see a 2nd Oxygen sensor anywhere along the pipe. There definitively not one just after the single stage cat, where one would normally have it, but I can see not the entire mid pipe run until I rack it. When I look at some of the images of the $$$ replacement part, I do not see a sensor (other then the 1st O2) either.

I would have concluded there is no such sensor (and that the upstream O2 sensors was more likely at fault until tested) but three facts dispute this. 1st: Many people here seem to be have a 2nd one (but the 1zz 2zz thing is confusing to me on that). 2nd: The only 'good' way I am aware of to "monitor" the cat (short of a honest tail pipe sniffer) is a down stream O2 measurement.

And 3rd: My ODB tools in fact show an active 2nd O2 senor measurement (no fault detected on either it or the heater for each, and the dynamic voltage measurement swings on both of the are reasonable and follow what you would expect them to do). [I have reset the codes and am waiting for the interval to clear but at ~15 miles the cat and erg systems are both still thinking about it, the original code was the normal P0420, no other codes so far]

Two questions then. Is the 2nd downstream O2 sensor really there (and not synthetic derived measurement I am seeing in the data stream). And if so, precisley where is it along the mid pipe?

A lesser question is which precise Matrix models DID NOT use a 2nd O2 units? I certainly see a lot of these in web vendors outside CA. But even the CARB board approved vendor image shows one with only one O2 sensor. Wife wants me to get another Matrix for her use, and I want to know what to avoid. [Soapbox: For non CA readers: CARB is our ill conceived regulatory body that has both destroyed the local aftermarket industry and which reguates what can be sold in our state. End soapbox]

My thanks to those that know this model for their help in solving this.

[Matrix_Jim]

The wiring and connector goes thru the floor of the car just below the shifter/ dash area. Crawl underneath and look for this cable.

[DIYmatrixDad]

Okay, soon as he brings it back that is what I will do. Even if MIL stay off you this problem is still coming back in time. I saw the upstream wiring in the passenger leg room along the edge of the b console. But could find any wires in the back areas, and I did not pull the console and look there.

So tell me if you (or others) can, what models of Matrix do in fact have one and only one 02 sensor?

[fishexpo101]

On all earlier first gen Matrix models (both 1ZZ-FE and 2ZZ-GE) they have two O2 sensors - the later 1ZZ-GE models switched from a conventional O2 sensor to an AFR sensor when they switched to the DBW setup. Still two sensors - one before and one after the cat. As far as I understand, as soon as OBD-II was implemented, all manufacturers were compelled to use two O2 sensors instead of one.

Here is a pic that poster Evilthorne posted on Matrixowners that might help:

[DIYmatrixDad]

Well this just keeps getting weirder. That pic is what I expected and see elsewhere, but it does not match my son's pipe. Thanks fishexpo101 for the pic.

But what I seems to have does match what I see the CA CARB cert guys are selling (one O2 only) and looks more like this.



Is is possible someone swapped pipes and has put a cheater on this car? I think is is time to dig into the console. But is folks are selling single O2 units in the rest of the country so they must fit on some 2003 year models of Matrix. Can not make these facts add up just yet.

>> As far as I understand, as soon as OBD-II was implemented, all
>> manufacturers were compelled to use two O2 sensors instead of one.
I think the 2nd sensor occurs soon after, but they were not connected, (ODB-II was a '96 Fed mandate (CA has its own set of rules as well), some OEMs converted early).

>> ..posted on Matrixowners...
The constant problems with the email validation stopped me cold on that board last week, perhaps it is having issues. I can not believe anyone would use European letters for the security part.

Again, thanks for all the help.

[fishexpo101]

Some of the aftermarket pipes I've seen do not include an O2 sensor bung, they are expecting that you weld one on yourself.

As for physically locating the the second O2 sensor or not, the ECM will "expect" two sensors, otherwise it will set a CEL constantly for the missing O2 sensor.

Dropping off the second (post-cat O2 sensor) is pretty common practice for most of the tuning crowd as many run into the P0420 issue.

Some use a modified sparkplug defouler to space the sensor further away from the stream, others completely disconnect the O2 sensor and assembly and install an O2 simulator to mimic signals from the downstream O2 sensor. Problem is, the Toyota ECM is pretty smart/picky, has several layers of redundancy and may still flag a CEL even with the simulator.

Probably the most definitive way to find out exact if the car is supposed to have one or two sensors, is to have the Toyota dealership run your VIN and lookup the appropriate part numbers for you. I would be shocked to find that the car only needs one sensor - all the 1ZZ-FE apps that I've seen all have two sensors. Visually inspecting might shed light if there are two or one sensors, but if someone modded the car, that won't be the definitive answer you need to fix the car.

[Mr.Nutcase]

I think someone removed the post 02 sensor