Check Engine Light turned on with codes P1166 and P1167?

The codes are for the heater circuit, which is common to go out, you can use an OHMs meter to confirm the connection failure inside the sensor heating element. Here is a generic guide to help you step by step with instructions in the diagrams below to show you how on your car:

https://www.2carpros.com/articles/how-to-replace-an-oxygen-sensor

The error codes P1166 and P1167 typically relate to the air/fuel ratio (A/F) sensor located in the exhaust system of the vehicle.

Check out the images (below). Please let us know what happens.

I was able to test for voltage coming to the sensor from the PCM but got no continuity on the heater side of the circuit on the sensor itself, so that seemed to indicate a bad sensor.
Trying to save as much money as possible, I picked up a Bosch o2 sensor for about $50, but the wires are about 3" too short to be able to use it...and that was after driving to 3 different auto parts stores just to find one that fit my '01 Accord 4cyl for Upstream.
Any suggestions for a reliable auto parts place online that I can order from that will get me a proper fitting o2 sensor at a reasonable price?

Sure, here is the part you need for $22.00:

https://amzn.to/3Y3J2Uc

Please let us know if you need anything else to get the problem fixed.

Unfortunately, the upstream o2 sensor replacement did not fix the problem.

Just to give you a little extra info. I reset the check engine light before I replaced the o2 sensor, just to see if it would reset, and when I started the car after resetting, the CE light immediately came back on the moment I started the engine.

When I replaced the o2 sensor late last night and started the car the first time, I only let it run for about 30 seconds, but the CE light did not come on. When I left for work this morning, it did not come on right away, but when I went down my driveway and got out to open the gate, by the time I got back in the car the CE light was on. so that was before even making it out to the street and beginning to drive.

Not sure if all that info helps, but I thought I would mention it just in case.
So now I'm back to square one and need to continue diagnosing. I guess my testing of the old sensor was not accurate, because I was assuming the sensor was bad since I was getting no continuity on the sensor itself on the heater side of the terminals.

Should I move on to the downstream o2 sensor, or would you go a different way with diagnosing from this point?
Thanks!

I don't think it is an Oxygen sensor issue, this will be a vacuum leak, engine misfire or MAP sensor that needs to be replaced. Here are two guides to go over, please clear the codes after the work is done.

https://www.2carpros.com/articles/how-to-tune-up-a-car-engine

and use this guide when the engine is cold only:

https://www.2carpros.com/articles/how-to-use-an-engine-vacuum-gauge

Here is the location of the MAP sensor so you can change it out. Also how is the engine running? Check out the images (below). Please let us know what you find.

Thanks for your suggestions, I will start working on these items. To answer your question, the car is running great. If there was no CE light, I would have had no idea there was any issue.

I am getting P1164 now as well. Still running great. Can’t seem to find any vacuum leak.

What is the reason that you are no longer suspecting the o2 sensor?

As far as the sensing part of an oxygen sensor they don't go out very often. Did you change out the MAP sensor? Also, you could have a partially clogged catalytic converter. Please check out this guide:

https://www.2carpros.com/articles/how-to-test-a-catalytic-converter

Please go over this guide and get back to us.

I have not changed out the MAP sensor yet, but I did some checks on it and all the numbers were coming out as expected, such as the 5v reference voltage, and the voltage at idle, which changed as the throttle was increased, so it seemed like the MAP sensor was good, but if you think that may fix it then I could just swap it out and see what happens.

The sensor may work but the parameters could be off causing the system to lean out. It is worth a try I think, if you get it from Amazon, you can send it back if it does work.

https://amzn.to/3rydDge

Please let us know what happens.

Okay, thanks Ken, I will order one and if that a shot. By the way, what is your rule of thumb/recommendation for ordering auto parts (especially sensors) on Amazon? Are there certain brands that you will try to stick with (or stay away from)? I have always gone to AutoZone or Oriley for many years, and obviously you just go with whatever part or brand that they have, but I see that many of the parts listed on amazon are 1/4 of the price that it would be at AutoZone or Oriley, at least in the case of this MAP sensor.

Buying from Amazon is like paying wholesale prices. I don't have any parts to stay away from at the moment back I do like the name brands.

I put in a new MAP sensor and cleared the codes before starting the car again. Unfortunately, the CE light came on instantly, just like it did after replacing both the upstream and downstream o2 sensors. Does it indicate anything to you...the fact that the CE light comes on instantly the moment the car is started after clearing codes, before even putting it into gear and actually starting to drive it?

I will follow your guide to check the catalytic converter now...but I also wanted to ask you if a cat issue would mean that the engine would be running poorly...which is not the case. It is still running great, not missing, good idle, etc.

Yep, that is a hard code, that fact that you don't have to drive it and the code is generated, it could mean the PCM is out and needs replacement. I would send it out for testing. Check out the images (below). Please let us know what happens.

Finally got this issue solved, so I just wanted to follow-up for anyone else that might find this helpful. It turns out that the issue was only with the original sensor, and since my Accord is a ULEV engine (ultra-low emission), it uses a special and very expensive air/fuel ratio sensor, which explains why the Check Engine light would instantly come back on again when I would replace the sensor with a standard $40 sensor.

Unfortunately, the correct Denso original factory "wide-range" sensor for this specific engine cost me $250, but after driving about 2 hours so far with the new sensor, the CE light has not come back on, and I suspect it will not come on again, since previously it would come on within a few seconds of starting the engine after clearing the codes.

Thanks again for your input on this.

Thanks for letting us know, we are here to help, please use 2CarPros anytime.