For awhile I noticed my gas mileage going down.

Finally I got a Code 21 and 28

Which is main front and rear. Any idea which side it is?

I plan on buying the "universal" Denso ones because I can get those really cheap. Are they all the same on the SC400's? It appears to be this way, but I have never owned a car that was, so I just want to double check that there are 4 sensors and all are indeed 4 wire screw in types.

Thank you
Matt

 

any ideas?

 

the back 2 don't need to be replaced, they just make sure the fronts are working, which is why the CEL came on. Just change the front to 1st. oh Btw what kind of millage are getting? I am only get 17mph and I havenm't changed mine, NO CEL

 

Um i used to get 20ish and now its like 15-16


So the rears make sure the fronts are working? so the ones in the manifold need to be replaced?

Im just going to buy 2 then, might as well do both!

 

yeah the rear are just monitoring the fronts, they don't really affect much from what I heard. let me know whats happens and if your millage jumps up. I am getting 16-17 (280-300 tank) I used to get like 19-20 (320-350) a tank

 

there's a thread showing the whole replacement procedure and parts descriptions in the LS400 section (pretty mush same as sc400)

 

Code 21 is the front sensor, 28 the rear. And thats for the sc300. If you're going to change them you might as well change both of them to be on the safe side if you don't want to run into anything down the line.

 

so for a code 28 it would be the o2 sensor behind the the cat?

 

For SC400 OBDI:

21 is the Main O2 Left bank sensor signal code.
28 is the Main O2 Right bank sensor signal code.

Good Luck,

Tim

 

yea its the 2 on the exhaust manifold....i just replaced the 2. they're only one wire so if you can splice save the money and get the non plug n play one.

 

Does any one know were I can find a good price on a 1 wire flange 02 sensor ? Am I the only one with this type of 02 sensor?

 

No you're not the only one; they are referencing a V8 motor here; you've got the I6. I think any regular autoparts place can get it for you; and I'm pretty sure NAPA and Carquest can easily get it.

 

Matt...

The front oxygen sensors, located within the manifolds themselves, are monitoring the a/f mixture that the motor is running. If they are -or have been- slightly out of tolerance, the ECU will spit a code after 'X' consecutive start ups.

I am not sure how OBD-I works, but on OBD-II, I believe it is 40 consecutive start ups slightly out of tolerance, 3 consecutive start ups rather out of tolerance, or one wildly out of tolerance event to register the light.

Back to the sensors: the sensors in the manifold register a/f mixture and thus control how much fuel is being delivered to the motor for a given amount of metered air. If they are not reading just right, the computer will -if anything- error on the side of rich so as not to blow the motor from a freak lean condition (thus the gas mileage woes).

The sensors after the cats measure the difference in oxygen from before the cats to after the cats. These sensors are primarily to measure catalytic converter efficiency. If your cat is going bad, the rear sensors will spit a code signifying that the catalyst efficiency is low. However, this will not alter fuel economy as the car will not go into "limp mode" because the front sensors are still measuring the right mixture within the combustion chamber.

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Moral of the story: your situation sounds exactly like the front two sensors are going bad. The fuel economy taking a dive and the codes being given are excellent indicators.

You have your hands full replacing them. You will need to bring your rust penetrator and your patience since the bolts will not give you much clearance, and the exhaust is rather corroded around the manifolds. That being said, you can definitely get the job done.

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For perspective, I am spitting a code on my Tercel for inefficicent catalyst (rear sensor is giving a bad reading). I have a suspicion that my cat is clogged rather than the sensor is busted (although I have not taken a multimeter to the situation to test it out). In Indiana, we have no emissions testing, so I am not worried about failing an inspection. Rather, I just recently got 291 miles on 7.53 gallons of gas from mixed city/hwy driving (over 38mpg). There is no doubt that I am NOT running rich in limp mode since the main oxygen sensor in the manifold is still doing its job at maintaining proper mixture.

Swap your front oxygen sensors, and the car should run like the day it was purchased.