My 4runner is my secondary car and I drive it very rarely (average 2-3k/year). Last year (3k miles ago) I noticed my 4runner sounded like it wasn't running right so after some looking around the forums I decided to clean the MAF, IAC and TB. All went well except soon after I did this I got P0171 and P0446 CEL. I checked all the hoses (I thought) to make sure I hooked everything up correctly but everything looked good. After some looking around everything pointed to the upstream sensor and said if you have a Cali emissions 4runner then you would need to buy an AF sensor rather than a standard O2 sensor. I looked at the sticker under the hood and under the truck at the exhaust to see if I had a Cali truck or not. Stupidly I convinced myself that I did due to the mention of Cali on the sticker and by confusing the cat+muffler as 2 cats. I then replaced my upstream sensor with a Denso 234-9003 AF sensor but continued to have a CEL.

For the last year I have been fighting with this thing off and on and have received various CEL codes (P0171, P0446, P0125, P0130, P0133). I replaced the MAF, intake tube hose, coolant/thermostat and vacuum hoses. When doing the vacuum hoses I noticed that the big hose for the IAC was not connected. I was so angry at myself for missing this but so relieved that I found my emissions problem. After hooking it up the CEL still did not go away. I didn't know what else to do at this point but then thought that maybe the AF sensor was faulty so I looked around some more online to see if this was a common possibility.

This is when I stumbled upon some threads I hadn't read before which made me question my truck being a Cali emissions truck. Now I know that only 99-00 4runners could possibly be Cali emissions, that a muffler isn't a cat (I know I know) and that the sticker under the hood will mention Cali regardless but that the 2nd line at the top left is what is actually relevant. So I put the wrong damn sensor in. I ordered the correct O2 sensor (234-4162), installed it yesterday and within 5 miles my CEL was gone. All of this could have been prevented if I hadn't missed the IAC hose to begin with.



So now I have a Denso 234-9003 that was used for roughly 3k miles. I'm not sure if it would be damaged due to being installed in the wrong truck or not. If it's still worth keeping and somebody wants it, it's your's as long as you cover the shipping fee.

Sorry for the long post but maybe this will help some other noob from making the same mistake as me in the future.

Just curious....an AF sensor will not plug up to an O2 truck and vice versa, the plugs are designed that way. Did you cut off the plug to make it fit the first time or did you buy a universal type sensor and so you expected to have to rewire? Or none of the above that I didn't think of...

I thought about that but forgot to look yesterday when I swapped them. The studs were rusted like crazy so I ended up destroying them and the bracket in order to get the original O2 sensor out. I brought the truck to a shop to have them weld up a new bracket so they were the ones who actually connected the AF sensor. Not sure if they rigged something or not. I'll compare the AF pin connector with the ones from Denso's site.

Ah bummer, you confirmed what I was hoping I was wrong on. I actually WANT to run a A/F ratio sensor in place of my old narrowband but the ECU cannot decipher the voltage signals.

That was a bit of an expensive mistake huh? I think A/F sensors are $100+. If you want, try selling it on Ebay. There's a large market for used parts and you get a few bucks back.

I checked the old AF sensor that I removed and the pin connector looks exactly like the O2 sensor plug. Like this:



Denso's site shows that it should be a little different and look like this:



Also the wire length is only about 5". Denso's site says the AF sensor 234-9003 is 16.5" and the O2 sensor is 10.5". I have no idea if the shop somehow swapped the pin connectors but it doesn't look like the wire's were spliced at all. Maybe they just re-used the old O2 sensor (it's pretty dirty).

We went round and round between the O2 narrow band vs the O2 wide band.

Honestly in a technical sense they all do the same thing. Jared is correct the narrow band and wide band are not interchangeable.

However, people will argue and say we are wrong. But all front O2 sensors are the A/F ones. The issue arises because the old narrow band range is very small range of voltage change and the ECU knows this.

The wide band O2 named changed to A/F sensor when in the front position.


What it comes down to is that all of these wide or narrow are measuring O2 in the exhaust.


The wide band just has more of a voltage band on them so naturally they are more accurate. Where the narrow band of voltage.

California emmisions in the 96 thru 00 have more sensors then the federal of the same years as federal have 2.

01 and 02 all have the California as that went federal in 01 in the 3rd gens.

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Ironically I did this halfway accidentally the other month when trying to diagnose my CEL. Long story short it was fine initially but soon threw codes and drove very funky - very strange and juttery acceleration, especially on hills, but also hardly seemed consistent. Sometimes it's like my car didn't want to shift either, which was weird. All of it stopped once I got the correct sensors put in.

Cali and Federal '96-'02 all have the same number of sensors which is two: one front and one rear.

The incompatibility is more than just the range. The A/Fs run at a fixed voltage and the signal is a variation in current. A Fed ECU has no clue what do with that sensor. The O2 signal varies in voltage as you might expect. The FSM makes this clear, if somewhat complicated.

I understand that.. I just stating that the front O2 senor is always the A/F one.. Nothing more..

The respective parts have to be correct..

However, they must be the correct part for the system and the vehicle. No mix and matching etc. was my point..

Thanks TheDurk for the extension and explain.