Hello everyone! This is my first post and this is also my first Honda! I bought a 2002 Honda Accord EX with the ULEV F23A4 motor in it and the reason the owner sold it was because someone took a hacksaw and stole the catalytic converter off of the vehicle in broad daylight. When they did that they also snipped the rear O2 sensor off, but thankfully they left the harness plugged in and the wires hanging so I still have something to work with. Since my state doesn't do any emissions testing I expected I could scoop this car up, weld a straight pipe on it, and find a way to delete the rear O2 or, if I have to, just live with the CEL since I THOUGHT rear O2 sensors never affected the ECU and don't change how the car runs. Then I was doing some reading about this motor and I saw this on Honda News about the F23A4:

Now that doesn't explicitly say that the rear sensor actually affects the air fuel mixture the ECU is using, but I know this sensor is different from the F23A1 sensor and I don't see why they would bother using a different part unless they are using the information from that sensor differently.

Ultimately I already welded a straight pipe where the Cat was because the car sounded like a single engine Cessna and made no power. Now it seems to be running fine and very quiet and there is no stuttering or hesitation, but I have a check engine light on and for all I know I am losing power even if it still runs this way. I've seen the standard suggested fix for this but I don't really want to have to cut a hole in the exhaust, and weld on a bung, and buy an O2 sensor, AND buy non-foulers all just to get rid of that code if that's all it will do. So please if anyone can confirm how that sensor is used on this particular motor and also please confirm if the O2 sensor actually needs some contact with the exhaust to function or if its possible that I could just buy an O2, plug it into the harness, then zip tie it somewhere safe under the car and go merrily on my way. Also I have seen people wiring up capacitors as simulators to resolve this issue but I can't imagine that would save a ton of money compared to just buying an O2 if I can.

Thank you very much for your time and any information you can provide is much appreciated.

 

A ULEV does use that O2 and it is luckily the same O2 as a the non-ULEV. The ECU is also different from standard, along with the intake and exhaust. It checks more thoroughly so that it could meet Cali standards.

You won't like the answer, but those ULEV engines are a bit more sensitive and trying to hack around them is probably not going to work. You can try, but I think in the end my suggestion is you bit the bullet, put on an cat, and buy the right kinds of O2 for the back of the car. I believe only the front O2 is the expensive one (because it's widband or something) but the secondary is just a normal one. I think it's around $40 for the sensor. I think a cat is about $65 off Ebay. Straighforward and no more errors.

As you said, O2 simluators will cost $30-$40 anyway.

 

I see. Thanks for the reply. I think what I'll do first then is buy a new downstream O2 since ultimately it sounds like I will need one. Then once I have it I will try just installing it outside of the exhaust like I had described and see if it does anything for me. The code I get right now is specifically about open or shorted wiring since again the whole sensor has been snipped right now. But I'll follow up once I get it an install it and maybe I can get lucky but more realistically I will have more work to do. I got the car really cheap and it has bad body damage on it but is otherwise mechanically sound so I don't know that I will be making the full investment of a cat at this point. The way they cut it was such that they got both of the gaskets and fittings on each side so ultimately we would be talking about an entirely new exhaust in order to accommodate a cat again.

Thanks again for your comments though and I'll be sure to post again once I get the new O2.

 

I will be curious to see how it comes out. Like you said, the price for the O2 sensor is not bad, and the mechanical O2 bypass is close to the price of slapping on the cat, but it can work.

If it was non-ULEV I'd be more like "sure, just use the mechanical bypass" but I suspect the computer is more sensitive and if it doesn't move the way it thinks it should, it will still throw the error. However, overall even though it's an experiment, it's interesting. And, the cars fine, so at least you are not trying to fix something mysterious.

Good luck!

 

Unfortunately I also have the ULEV California car. They are more sensitive, and the A/F sensor (the front one) isn't cheap. I was told by Honda the readings from both sensors are used to determine the fuel to air mix. Be VERY gentle with them. I'm hearing even a tap can damage them, mostly the wideband front one. I'm very interested in hearing your results.

 

Is it a 5 wire? (wideband)

 

4 wire B1S1 A/F sensor

 

Which part of the intake is different? I have an 01 accord LX and also another 01 accord LX ULEV and I have found both intake systems to be the same.

 

I thought the upper half of the intake was different, but if you are looking at it directly, then I was mistaken, only the exhaust manifold is different.

 

The ULEV engine uses the same intake manifold contrary to some Accord Forums that will suggest otherwise. The difference in the engine is the certification of low emissions, and it uses a short runner 4 - 1 exhaust manifold w/AFR sensor, and an ECU that makes use of different tuning and the signals form the manifold mounted air fuel ratio sensor (AFR).

 

Confirmed