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2 Main Oxygen Sensors Location


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Hi guys i have 90 lex ls400 and i get code 28 and track light is on as well. Track light just keep blinking when i pull the code infinitely fast but CEL shows code 28. I'm assuming track light will go off once i fix code 28 but i don't know? I also pulled track light code and it's 51 (ECU issue). From what i've read that shows because CEL 28 lights up. I'm assuming track light will clear once i fix CEL 28 code with sensors. If anyone had similar issue please confirm.

But main question is where are 2 main oxygen sensors located on 90 lex ls400
I was looking at this tutorial but i think it only shows first one and not sure what RH and LH mean.

Right hand or right header? Then tutorial jumps right away to harness disconnect so i don't see where LH oxygen sensor is located. BTW where should i be looking for those sensors at driver or passenger side and by what major engine part?

Thank you in advance


3a) Unscrew the RH main oxygen sensor using a 22mm or 7/8" wrench. Here you can see that there is just enough room to use a standard 22mm or 7/8" wrench. If your sensors are original it will probably take a little extra elbow grease to get them out.

3b) Unscrew the LH main oxygen sensor. This side is a little harder because the steering rack is in the way, but a standard wrench still fits.



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LH is the drivers side and RH is the passenger side. Both sensors are right before the catalytic converters. Get underneath the car and look toward the back of both front wheels. You might have to remove the engine undercover.

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Thank you. That helps. Oh i forgot about under cover. Is main o2 considered upsteam or downstream and what is the difference between the two.

When i look at o2 listing is states both so not sure which is right one. Should i go with Walker or Bosch for same price? I think Bosch is OEM for lex correct?

I see some are 2 wire and some are 4 wire. Did anyone had this with track light and had track light go away after o2 service?

Does anyone here have Gen1 Lex service manual in pdf that someone can post. That would be greatly appreciated.

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I got Denso, The O2 sensors you would most be concerned with are the up stream ones as in the pictures. They are in the exhaust manifold and just before the catylitic converter on both sides. You will first have to disconnect the electrical connector, then I used I think its a 22 mm box end of the wrench and loosened it. It turned by hand real easy.

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Thank you. Would this sensor work. According to rockauto it would and it fits 90 lex ls400. I'm assuming this buy gets me 2 sensors. One right and one left the way i understand.

One concern i have is that in pic it shows 5 wires. I've seen only 4 on walker product. Would that work. It looks according to description that i won't need to splice wires with this sensor

as plug is oe fit. Am i reading this right? Thoughts guys? Thank you in advance.

So the way i understand upstream means main (not sub) oxygen sensor?

APWI Part # AP465 Heart.png
Upstream Right
Upstream Left


apwiLogo.jpg RAdetailLOGO.gif

More Information for APWI AP465

APWI Oxygen Sensor Features:

Sensor element is made with highly pure and concentrated platinum paste imported from Germany. This will result in increased sensor life.

Supplied with high temperature nickel plated Teflon wire with Dupont material. Wires protected with fiberglass protective sleeve.

Made with Viton Flourine rubber boots. Viton is water resistant and will decrease oil steam and vapor residue.

Sensor Body
• Made with high temperature stainless steel for long sensor life.

Frontal Tube
• Double layered with high quality stainless steel imported from Germany. Increases life expentancy of the sensor.

• OE direct fit connectors. No consolidations.

Hex Nut
• Improved Hex nut design for easier grip and installation.

NOTE: Part number prefix will tell you how many wires each sensor has.
AP1 = 1 Wire
AP2 = 2 Wires
AP3 = 3 Wires
AP4 = 4 Wires
AP5 = 5 Wires

Common O2 Sensor Locations:

APWI Warranty Information
12 months / 12,000 miles

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There is also this. It shows two parts 8946524050, 8946550010} so i'm not clear if this means 2 sensors right and left or just left. ?

DENSO Part # 2344211 {#8946524050, 8946550010} OE Style Heart.png
Upstream; DENSO is the O.E. Manufacturer / Oxygen Sensor

denso.gif RAdetailLOGO.gif
More Information for DENSO 2344211
Point to Zoom
OE Style o2SensCut.jpg
  • Illustration Details
  1. Porous PTFE Filter - Allows atmospheric oxygen to enter the sensor without permitting water or engine contaminants to seep into the casing.
  2. Robotic Laser Weld
  3. Aluminum-Oxide Trap Layer - Keeps silicone and lead outside the ceramic elient through DENSO's exclusive design, preventing damage to the platinum electrode.
  4. Stainless Steel Housing - Resists corrosion and contamination with a rugged, watertight body.
  5. Double Protection Cover - Maintains proper unit temperature for quicker response times and protects the ceramic element against silicone and lead poisoning.
  • Features
  • Corrosion-resistant stainless steel construction
  • PTFE filter blocks efficiency-robbing contaminants
  • Refined zirconia element improves sensor response
  • Benefits
  • Superior design maximizes engine performance and fuel efficiency while reducing fuel consumption and harmful emissions
  • First Time Fit® delivers the right part the first time
  • Ensures smog test compliance

Common O2 Sensor Locations:

O2sensorLocations.jpg Alternate/OEM Part Number(s): 8946524050, 8946550010
DENSO Warranty Information
12 month. IMPORTANT A/C COMPRESSOR WARRANTY NOTE: In order to ensure that proper warranty credit is issued if needed, the compressor installation instructions must be adhered to. All compressors returned for defect must show proof of purchase of an Orifice Tube/Expansion Valve, Drier, and A/C System Flush or a Work Order showing the purchase of these items and that the system was flushed.
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Yes, "upstream" would equal "main" O2 sensors. You're in the right place if you're looking above the catalytic converter on both right and left side. Denso is the OE part, IIRC. I can't remember if the code clears once you replace the sensors, but worst case, clear the codes after you've done the job by pulling the fuel injection system fuse for 20-30 seconds (or disconnecting the battery for a similar time).


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Thanks Paul. Can you tell if APWI comes in pair, 2 sensors. It says Right and left if i'm reading this right?

The way i read Denso it's only one correct?

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Thanks brother. I will call them tomorrow and ask. If AWPI comes in pair i will get that as that's nice price for both. If not then i will probably go with denso as its OEM or Walker part as tutorial suggests. Still thinking which one to go with tho.

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Just heads up for anyone ever want to use APWI AP465. It doesn't fit as thread is different. It would not screw it. I will be returning it for Denso which i think is OEM manufacturer of lex o2. Also 7/8 wrench strips o2 thread so if you have a chance go with 22mm instead of 7/8. on 7/8 there is play where on 22 there is none or very little that is noticeable. I got right one off and i stripped screw thread but left one wound't budge and 7/8 was sliding on o2. I will have to get 22mm and smash the other end with sledge hammer to fit pipe onto it for leverage. Unless someone has a clue otherwise. I also had a tool but tool was too short to it was useless too.

I'm thinking of getting cat get hot and try it before it cools off to spread the metal a bit. I sprayed it with liquid wrench but in past i had no luck with pb bluster so i won't count on that.

7/8 in = 22.225 mm which explains stripping of thread on o2. Correct tool is 22mm only not 7/8

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And that .225 mm is about .009 fractional. If it was a 6 point wrench or socket it probably would work, but still best to use 22 mm. And I had mentioned that in #4 post above.

Did you strip the thread in the manifold? Or are you talking about stripping off the HEX of the sensor? If there is no hex left, then yes heat the area the sensor screws into and use some good vise grips on the old sensor. Or if you can get a small pipe wrench in there that would even work better.

Hopefully its the passenger side your having problems with, the driver side is horrible for access to the o2 sensor and the plug.

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Thanks Bob. I bought special o2 wrench for $15 so not bad price. I don't want to money around anymore with 3 point wrench. I stripped little bet of thread of the o2 sensor that i unscrewed so no big deal as it will be replaced anyway. Other thread is ok. I have returned apwi as thread was different. Ordered Denso and currently waiting for both tool and sensors. Hopefully that will get rid of my CEL. Thanks for all your help guys. I will update the thread with my findings and if it fixed CEL.

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Guys so i changed right side o2 and CEL and Track light went away after i reset ECU. The left side i could not get it to unscrew. It may need to come down to torching it red hot and then trying it.

For that i would have to go to my mechanic and pay him. Question though.

Is old left o2 delivering correct into to ecu since it's 25 yrs old and right one is new. The way i understand it right size delivers correct into and left doesn't.

Not sure how o2 works. Is old left o2 good still meaning its working right by delivering correct info if no codes show or is it just getting by and should be changed anyway.

I'm asking cause i'm trying to figure out if it's worth extra money to dish out if left side ECU reports no trouble but then again i don't know how o2 works. It may be just getting by for now?

Thank you in advance for your advise.

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If I recall correctly, each O2 sensor has its own engine code, so if the CEL stays out, drive on. In my own experience, the other upstream sensor failed not long after the other. I'd hang on to the other new sensor as you'll likely need it eventually.


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Thanks Paul. Actually CEL and track just came back on after a day of driving so i guess i should change it.

I think after i cleared CEL from ECU (inj fuse) it takes little bit for ECU to learn that there is still an issue so i think that code never really cleared as i only temporary reset it.

It's my best guess.

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