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O2 Sensor - Code 28


Ross W.
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Hi all

I haven't written in awhile! Been busy.

Two weeks ago, the check engine light came on, and since the car ( '94 LS400) was due for it's timing belt service, to the mechanic it went. My mechanic called back and the timing belt service went without a hitch, and he let me know that a Code 28 was what activated the c.e.l., basically the right side upstream O2 sensor was faulty, so I replaced them both for good measure. The c. e. l. came back on immediately, and the same code showed up. My mechanic played with it all day today and he's stumped. He's had the car running with the scanner watching E.C.U. and sensor functions in real time, and the computer and both O2 sensors are talking to each other as they should, yet diagnostics insists one of them is bad, hence the code showing up! He's checked the wiring harness and couldn't find anything. I'm taking it back tomorrow for more scan time and detective work. So, has anyone with a '93 or '94 had this happen? One thing that's really ironic with mine is, right after I bought it five years ago, the throttle position switch went bad, open circuit in fact, and it didn't set a fault code or activate the c.e.l.!

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Code 28 - Right main bank O2 sensor signal

Were the new O2 sensors OEM or universal?

Reason being is I ordered a Universal and I checked the resistance of it before installation, and it was above what the www.lexls.com website said was appropriate resistance value for that O2 sensor. I ordered an OEM sensor and the resistance was dead-on where it should be.

I would first try to reset all codes again, maybe even leave the negative terminal off the battery for a couple hours to be sure.

Next I would pull the Right side O2 sensor and measure the resistance of it.

If the resistance is outside of wher it should be, I would remedy that. If the resistance is fine, I would make sure and triple check over and make sure that the new O2 sensors were wired correctly, and that the connections were put together properly as well.

If everything is good up to here, I am unsure what to do next.

Good luck!

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Found the problem!

The right-side O2 sensor's heater is controlled by a latching relay, using a transistor as the trigger, that pulls the heater circuit to ground after engine startup, and it's in the ECU. I should be able to replace the transistor and have it up and running, and along the way, I might replace all the electrolytic capacitors while I've got the lid off. At least I know what it is now! :(

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I took the ECM out and the components are surface-mount devices, for which I have no experience with, never mind the specialized equipment to work with them.... solder, irons, fixtures, etc., so now I need to look at a rebuilt or good used ECM. If anybody has any good leads, I'd really appreciate the assistance!

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My mechanic ordered a remanufactured ECM through a rebuilder they deal with in Florida. $600.00 + freight. I'm keeping my fingers crossed. My hope is the "new" ECM doesn't throw new fault codes for marginal devices that the original ECM adapted to over the years. Now where's that twin turbo kit I keep hearing about? :lol:

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  • 3 weeks later...

*BUMP* :lol:

The Lexus reman. ECM was installed and what a difference! I think there have been more problems with the original ECM than just the Code 28 when the ECM was not pulling the heaters to ground; the car has excellent throttle response which was never there from the beginning, better around-town gas mileage, better and more consistent acceleration, and overall even smoother overall. I'm really pleased!

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