I have a 1995 LS400 federal emissions spec not CA spec as verified by stickers under hood and I believe also by ECU (my ECU is 89661-50224)

Chat GPT (I know, use with care and a lot of caution) insists that a 95 LS400 federal spec has no downstream O2 sensors. However, I see the below on both sides. Help me out here because I’m still somewhat new to this, what am I looking at?

I have an OBD2 style port, however consistent with my research it seems to pull limited data and the car doesn’t seem to be full OBD2. The EVAP monitor is “N/A” and the CAT monitor likes to stay at “INC”.

At idle today after driving for a bit, reading live data my short term fuel trim based on B1S1 and B2S1 were each 0.8%. The reader showed short term fuel trims for B1S2 and B2S2 at N/A and 99.2 percent respectively, which chat GPT insists is garbage data / hallucination.

There are no CEL codes and the car drives perfectly (recently did complete overhaul and tune up)

Any thoughts are appreciated regarding what I am seeing below.

 

I haven't read your whole post yet, but ChatGPT is wrong and you are correct

Your car has 4 O2 sensors - 2 upstream and 2 downstream

The one in your photo is the bank 1 downstream O2 Sensor

Diagram:
https://www.amayama.com/en/catalogs/...y-computer-337

 

Ok thanks so I’m not crazy then. I prompted Chat GPT a little differently and it came up with the below, which contradicts in part its earlier output but seems closer to the reality I am seeing.

Are there any cars that don’t have catalyst efficiency monitoring but still have downstream sensors?

Great question — yes, there are some oddball cases where you can seedownstream O₂ sensors present but no true catalyst efficiency monitoring. A few key situations:1. OBD-I “Transitional” Cars (1994–1995)

• Some vehicles in the mid-1990s added extra O₂ sensors in preparation for OBD-II requirements, but the ECU logic wasn’t yet programmed to run thecatalyst efficiency monitor.
• Example: Certain 1994–1995 Toyota/Lexus (including LS400 CA-spec), and some Nissan/Infiniti models, may have rear sensors physically installed but used only for basic closed-loop checks or emissions data collection, not for triggering a P0420-type fault.
• That’s why your ’95 LS400 confusion makes sense: the car can have the downstream sensor hardware without fully running the cat monitor in the ECU.

2. Non-U.S. / Market Variants

• In some European and Japanese Domestic Market (JDM) cars, downstream sensors were used for emissions feedback or meeting local standards butnot tied into OBD-II catalyst monitoring logic, since those regs were phased in differently than in the U.S.

3. Early California-Spec OBD-I Cars

• California pushed for stricter emissions earlier than federal standards. Some late OBD-I California cars included downstream sensors as part of a more sophisticated emissions package, but again — ECU programming didn’t always include catalyst efficiency checks.

4. Non-Efficiency Uses

• On some engines, a downstream sensor may be used for:
  • Air–fuel ratio diagnostics (verifying mixture over time).
  • Secondary air injection checks.
  • EGR diagnostics.
    These aren’t “catalyst efficiency” per se, but they rely on a post-cat sensor to see how exhaust chemistry changes under certain conditions.

✅ Bottom line:
Yes, there are cars (especially mid-’90s transitional OBD-I/early OBD-II models and some market variants) that had downstream O₂ sensors without using them for full catalyst efficiency monitoring. This explains why you can physically see the sensors on certain ’95 cars but never see a cat-efficiency code pop, because the ECU isn’t running that monitor.

 

In my opinion, that ChatGPT explanation is misleading and does not apply at all to the LS 400. It does however apply to some other cars from the period

Since 1989, all LS 400 models had upstream and downstream O2 sensors on bank 1 and bank 2. This includes CA and Federal emissions compliant versions

I read some more of your post now. In 1995, the car had basic OBD-II, which was expanded in 1996 onward. This would be why your data may not be as detailed
https://www.clublexus.com/forums/ls-1st-and-2nd-gen-1990-2000/505727-95-ls400-obd-i-or-obd-ii.html#post5542431

I would consult the service manual. There are procedures for checking O2 sensor health specified in there. I will see if I have a chance to look that up soon..

 

Ok thanks. Well clearly this is helping me further understand the limits of large language models.

At the end of the day I guess what I’m trying to anticipate is an eventual dreaded CEL for catalyst efficiency, as cat replacement in NY is a pain (has to be CA/CARB or OEM which in this case is discontinued)

At this point I know my car has at least physically installed downstream O2 sensors. But the catalyst efficiency monitor doesn’t seem to be running. I cleared codes after doing timing but that was a couple weeks and about 150 miles ago. So I think if it were going to run the catalyst efficiency test would have ran…I think…

 

Well, if I recall, your car is fairly low mileage. The OEM catalytic converters are very high quality and shouldn't need replacement unless abused with super rich fuel mixtures for a long period of time

My 2001 Camry took a while to get all of the readiness monitors completed. The catalyst efficiency one took a while, and I had to vary my driving between local and highway trips

I'll see if there's something in the manual later

 

Ok got it. It may be that I do have catalyst efficiency monitoring and just need to drive it more. Well, at least if this takes a while to complete, I know what I can do come inspection time if necessary. My understanding is in NY given my vehicle’s age I can have up to two monitors incomplete and still pass as long as no CEL.

I had a cat fail on a prior Ls400 that I owned at 127k miles and it was a pain in the -ss to deal with in NY so just obsessing about this because I’ve cleared out a lot of preventative maintenance but this feels like something hanging out there. Having said that, the owner didn’t exactly take great care of other one.

This car has about 119k miles at the moment. The other one spent its life in NY, and this one is a GA car.

 

Just adding to my previous post. I looked in the service manual and I could not find information regarding testing the O2 sensor other than the heather circuit from the connector, as well as info regarding the readiness monitors

Will have to do some more thinking about it. I've posted info regarding testing the O2 sensors for first gens on this forum before. The procedure involves connecting a multimeter or a scope to 2 pins of the Toyota dlc connector and revving the engine to 2,000 rpm. It may or may not apply to gen 2