Veh: 2001 Honda Accord LX Sedan
Engine: 4-2254cc 2.3L VTEC 4cyl. F23A5
Mi: 61,707
Last MPG was ~25 (400 mi on 15.6 gal)

My "Check Engine" light has been on since January. When I took it to the dealer for an oil change and they told me the "code" indicated there was probably an O2 sensor problem, but to not worry about it until I needed a smog check. The paperwork from the dealer reads "S1 sensor failure." I decided to go to Auto Zone and use their OBD-II device, which gave me 2 codes (that I don't have with me) but basically said there was a non-specific emissions problem.

Today, I returned to the dealer for the "further diagnosis" they recommended, and another oil change. After 4.5 hours they told me they narrowed it down to the Primary O2 sensor that would cost $315 + a $90 diagnostic fee. Since they didn't have the part, they only charged me for the oil change until I return for the repair. Coincidentally, they checked off during their inspection that my engine air filter needed "immediate attention." I thought that was interesting because I cleaned my K&N filter last month. (I checked it when I got home and it was spotless.)

My question is: how can I be certain that this is truly the problem before I fork out over $400? I don't mind paying for a legitimate repair, but if they're saying my clean K&N needs "immediate repair," then I am skeptical as to whether they accurately diagnosed my "check engine" light. I also understand that there is a secondary O2 sensor that costs about $200 less, so I would like to be able to confirm the difference myself. Is there anyway I can check the O2 sensors more specifically?

Thanks.

Here's what I would do. Buy a Denso O.E. oxygen sensor (Denso Part No. 234-4620) from RockAuto.com for $72.79. This is the front oxygen sensor, which goes to the exhaust manifold. Usually, this is the culprit. Most Honda oxygen sensors are made by Denso and come with the Denso name on them. Then, I would follow the directions below and install it yourself:

https://www.askmehelpdesk.com/cars-trucks/faq-how-troubleshoot-repair-maintain-hondas-46563.html#post223376

This is not a difficult task.

You might not agree with this, but I would not use an oil impregnated air filter on any modern-day car. Many car manufacturers explicitly recommend that you don't use them. The reason is that the oil can cause sensor failure; e.g. MAF and oxygen sensor. It's extremely rare to have an oxygen sensor fail on such a new, low mileage Honda. Therefore, I would remove the K&N and use a standard pleated paper air filter. Every study I have seen has demonstrated superior filtering by pleated paper air filters. Less is more.

The secondary oxygen sensor is the "downstream" sensor at the rear of the catalytic converter, Denso Part No. 234-4621 ($38.79).

One of the best problem questions I ever read on this forum, Ad1234. Thank you for giving such complete information and I only hope others will follow.
I completely agree with the above post. Txgreasemonkey is right on the money. Take that flippin' high dollar air filter and chuck it. If you only knew what it was doing to your mass air flow sensor readings!

Veh: 2001 Honda Accord LX Sedan
Engine: 4-2254cc 2.3L VTEC 4cyl. F23A5
Mi: 61,707
Last MPG was ~25 (400 mi on 15.6 gal)

My "Check Engine" light has been on since January. When I took it to the dealer for an oil change and they told me the "code" indicated there was probably an O2 sensor problem, but to not worry about it until I needed a smog check. The paperwork from the dealer reads "S1 sensor failure." I decided to go to Auto Zone and use their OBD-II device, which gave me 2 codes (that I don't have with me) but basically said there was a non-specific emissions problem.

Today, I returned to the dealer for the "further diagnosis" they recommended, and another oil change. After 4.5 hours they told me they narrowed it down to the Primary O2 sensor that would cost $315 + a $90 diagnostic fee. Since they didn't have the part, they only charged me for the oil change until I return for the repair. Coincidentally, they checked off during their inspection that my engine air filter needed "immediate attention." I thought that was interesting because I cleaned my K&N filter last month. (I checked it when I got home and it was spotless.)

My question is: how can I be certain that this is truely the problem before I fork out over $400? I don't mind paying for a legitimate repair, but if they're saying my clean K&N needs "immediate repair," then I am skeptical as to whether or not they accurately diagnosed my "check engine" light. I also understand that there is a secondary O2 sensor that costs about $200 less, so I would like to be able to confirm the difference myself. Is there anyway I can check the O2 sensors more specifically?

Thanks.
You might be able to buy the sensors @ autozone or another auto parts store and see if they are easy enough to change yourself, you'll be able to tell some time when the 02 sensors are going bad, your car might get kind of hard to start, might cut off when idling, but it will start right back up. If you replace find out how to reset your computer so the light will go off, try taking the pos battery cable off for 5min and then put it back on. If you can't buy the parts or just can't replace yourself then you might have to pay for that, but see if the dealer will give you a deal if you replace both sensors.