Hello, I have a 2001 Honda Accord, 2.3L, 4 cyl, LX, 260,000 miles. It is running great, and if not for the check engine light I would not have thought there was an issue with it. I don't have a scan tool, just a basic code reader...but I would be open to getting a scan tool in the $100 range that connects to a phone or tablet (no screen) that can do advanced functions and bi-directional controls....like a ThinkDiag or something like that.

The P1166 and P1167 codes point to the o2 sensors, and I have already replaced both the upstream and downstream o2 sensors in hopes that would fix the CE light, but each time I clear the codes, when I start the engine again the CE light comes back on instantly, without any driving and without the engine idling for any amount of time at all. I was told by one person that this means it is a "hard code", since the CE light comes back on before the engine even has time to calculate any values of the running conditions (such as the fuel/air ratio at the o2 sensors, etc)...and the recommendation I got was to send the ECU in for testing since it may be a bad ECU.

I'm really hoping to avoid that, partly because of the cost, but also because I can't be without a car during that time. Again, the car runs great, so I have been continuing to drive it for the past 2 weeks while I'm trying to diagnose it.

I was also told it could be a vacuum leak or a bad MAP sensor throwing things off, but I could not find a vacuum leak, and after putting in a new MAP sensor and clearing the codes, the CE light came back on instantly the moment I started the engine. I also cleaned out the throttle bore about a month or two ago, so that should still be pretty clean (that was another suggestion I got).

I would greatly appreciate any suggestions for other things I could check or repair to possibly solve this issue!

 

Those codes are for the heater in the primary (upstream) af (aka O2) sensor. That circuit is fairly simple. The power is from the number 6 20A LAF fuse in the passenger's under dash fuse box. Check that the fuse isn't blown. Hopefully the old a/f sensor shorted and blew the fuse. If not, then you'll have to do some electrical troubleshooting.

 

That fuse is not blown, so I guess I will need to do more troubleshooting. Do you have any suggestions for what and how to check this electrical circuit?

 

There are a few setups for various engine on the O2 sensor. Also check the no 6 ECU/Cruise control fuse in the driver's side under dash fuse box 15A.

What brand and part number O2 sensor did you use on your car?

The testing is fairly simple. The A/F sensor has 4 wires. 2 wires are the signal to the PCM and you don't need to mess with them. The other 2 wires are + and - for the heater circuit. When working properly, the + wire should provide enough voltage to light up a test light connected to ground. The - wire should light up a test light when the test light is connected to battery +.

What is the engine type in your car? It will be stamped on a metal plate near where the upper radiator hose connects to the engine. It will be F23A#-XXXXXX. I just need the number after the letter A. This will help me figure out how the A/F sensor is powered.

 

Thanks PAhonda, I will check the engine type and get back to you on that, as well as the sensor brand and part number.

Quick question about testing the o2 sensor...do I need to leave the sensor connected when running this test, or am I just testing that voltage is getting to the harness side of the connector? I wasn't sure if I should be back-probing with the sensor connected, and if I should have the engine running when checking this.
Thanks!

 

You can test with the a/f sensor unplugged. I think you can do this with the key in the II position, but you can also try with the engine running. Essentially, you are hooking up a lightbulb in place of the A/F sensor heater circuit to see how it works. You'll get more information if you just test the + and - wire individually, because you'll find out if you have an issue on the + or the - wire. That is why I suggested hooking the test light to ground to test the + wire to the A/F heater.

You can do the initial test with a volt meter just to make sure you are on the correct wires. Let us know the wire colors on the harness side and also what they match to the sensor side.

Here are some videos for reference. South Main Auto's channel has a lot of good videos on how to diagnose on this topic on youtube. Pine Hollow Diagnostics is a good channel too.

 

Thanks for this info! I will test and report back...

 

Here is the info...
Engine: 23VAGJJG (I could not find a label where you described, but found this one on the underside of the hood. If this is not it then I will keep looking)

Replacement bank 1 (upstream) o2 sensor: Walker 350-34631. I have already taken it back out and put the "original" back in, since the code still came back the instant that I started the car after installing the new sensor and clearing the codes.

I also bought a Bosch o2 sensor first, but the wires were too short and it would not reach the harness to connect it.

The sensor that has been in the car for many years (8+ years) is a DENSO 192400-1020 (10B09). Also has "DU2" stamped on the sensor.

With my test light connected to either battery positive or battery ground (respectively), it is lighting on the ground terminal(#2) and the voltage terminal(#4).

Voltage: between terminal 2 and terminal 4 with key on (not running) I'm getting 10 volts

Resistance: between terminal 3(heater) and terminal 4(+voltage) I'm getting .009 on "auto", or infinite resistance on the "continuity" setting on my DVM.

The resistance would lead me to believe that this sensor is bad (and it may be), but then why would my CE light instantly come back on right after replacing the o2 sensor, clearing the codes, and then re-starting the car for the first time? Again, the CE light is coming back on either instantly or within a few seconds of starting the engine after clearing...so the engine does not really have the normal time to assess the driving conditions yet before setting the code.

I just re-read one of your earlier posts and realized that I forgot to check the wire color on the harness side and the corresponding wire colors on the o2 sensor. I will be able to check that later.

 

Before doing any more testing, the wire colors are important to ensure you are testing the correct wires.

The number you found isn't helping.

Here is a pic of where the engine information is located.

 

I still can't find the metal plate on the engine, only "VA" stamped directly into the block, so I will have to keep looking for that, but I can at least tell you for now that it is an ULEV engine.

Here is the o2 sensor and vehicle harness information:

Looking at the vehicle harness side, it is two white wires, one red, and one blue. If you are looking at the harness with the connector release on top, then it is as follows:
top left: blue
top right: red
bottom left: white
bottom right: white

The Denso o2 sensor wires are: 2 black, one white and one green
Corresponding to the vehicle harness they are as follows:
top left: green
top right: white
bottom left: black
bottom right: black

Just to be clear, this is the existing/original sensor (Denso) that has been on the car for many years (not the Walker or Bosch one that I tried to replace it with).
Thanks!