johnnyutah36

I have just replaced my O2 sensor on a 94 4runner V6. I noticed before the replacement the RMPs would fluxuate quite a bit like up too 300 rpm's, and I was having difficulty geting the idle set correct. I would set it then it would run up and down 300 rpm's or so. Well with the new O2 in there the needle sit perfectly steady at idle. If you have the same condition maybe your O2 sensor as well.

havic

I'm having the same problem with the idling. It fluctuates about 100 rpms when I'm stopped. I did a tune up, changed the air and fuel filter, cleaned the throttle body and it still fluctuates. My last resort was checking for a vacuum leak, until I read this. I'll do this first because it is cheaper, and the last resort is changing the O2 sensor. Do you have two sensors? If so which one did you change and how did you know to change it? I have a 98 4Runner V6 4WD, so it might be a little different.

johnnyutah36

THe check engine light came on like 2-3 times over a 5 month period. I only have one O2 sensor right before the cat, it was a piece of cake to change. I got the sensor from www.sparkplugs.com pretty good price there.

bike4miles

Slightly off topic, I got 2 mpg increase when I replaced my O2 sensor and I just ordered the second one today so my freaking check engine light will go off for good. It's always on.

goldtaco9

My mileage appears to have improved as well since replacing my rear O2.
Mike

havic

Mike, do you know which one is more important, the one before the cat or after the cat? Also, how much did it cost?

goldtaco9

I do not know. I got an error code that told me that the rear was bad. Actually the error indicated a cat problem. The rear sensor wants to see a difference in emissions from the front or it thinks the cat is not working. The sensor was a factory one and it was over a hundred bucks.
Mike

caylorm

As for the better gas mileage, from what I understand, when the "Check engine" light comes on, the computer goes into a "default" mode. It runs the engine on one setting without adjusting for temp, exhaust gas, etc. since it is ignoring the O2 sensor and such, therefore your gas mileage decreases until you fix problem and reset computer. I believe this is true any time the light comes on for any reason.

goldtaco9

I don't know about Toyota but this is absolutely not true with the Subaru OBD I & II computers.
They only go into default mode for specific, potentially engine damaging errors. The ECU's are pretty smart and I would guess that Toyota's are the same.
My 4Runner was getting very consistent MPG. I only drove a few miles once I got the light/code. Since the new O2, the fuel mileage has improved beyond what it was since I bought the truck.
Mike

caylorm

If the ECU know's that the O2 sensor is bad, then how can it use data from that sensor which is obviously giving incorrect data? It has to use some sort of default values to replace the erroneous data in order to make all the calculations for air and fuel, correct? I guess the light could come on when it sees the O2 sensor is going bad/out of specs. I'm sure someone here knows the answer.

goldtaco9

I am not saying that a bad O2 does not trip default mode. I am saying that a "check engine light" does not necessarily cause default mode. In fact, a bad O2 is exactly the type of critical error that should cause default mode.
A speed sensor or neutral switch code, for example, probably would not cause the system to go into default mode but would light up the check engine.
Generally you will not get a light until the computer can no longer adjust for a failing reading. Fuel injectors are a good example of this. They can get quite clogged and someone with a reader can see this but no real errors will pop nor will the light come on, until the ECU can no longer adjust.
Our '95 Jeep just went through this. It was only a couple "points" from popping an error. The injectors were bad but not quite bad enough.
Also, I had no error codes at all and my mileage was steady at around 17 MPG.
After I got the error and replaced the O2 sensor, my mileage popped up to around 19 MPG. Could the old sensor have been failing? Sure, not enough to pop any codes but enough to make the engine's ECU adjust timing or something.
Mike

Mike

CMD93

For my 1993 4runner, I got a free o2 sensor from the dealer, parts and labor. I read about this on here a while back. But if you look closely in the owners manual it says that toyota will cover the cost of the sensor and labor. If you have ~80K on your engine. Search for the tread. I forget the page number of the manual.