If the Car Talk guys were still doing their thing (Rest in peace, Tommy), I'd call them about this one. But I figure the EmpegBBS is the next best thing, right?

Car: Honda Accord SE, 2000 model year.

Check engine light comes on. Home scan tool app+bluetooth-plug says Code: P1166 Meaning: Primary HO2S (no. 1). Some internet research tells me this means that my Primary (upstream) Heated Oxygen Sensor needs to be replaced. The internet also tells me that this is underneath the car and is connected to the exhaust system somewhere beneath the center of the car.

I notice that there is a thing that looks like it might be an oxygen sensor in a much easier-to-reach place in the front of the engine compartment. So easy I could practically do it without getting my hands dirty. But the internet tells me that that's not the O2 sensor, that's the airflow sensor. The O2 sensor is a harder one that involves being under the car and undoing a particularly annoying wiring connector (the ones where I always scrape my knuckles trying to get the fscking connector undone).

So I take the car to a local car repair shop (a Firestone which I've had great service from in the past, these are the only guys who have been able to do my front brakes without warping the discs from overtorqing). Tell them my check engine light is on and that the internet tells me that the thing that needs to be replaced is under the car.

When I get the car back, I see the following when I look at it:
- Under the hood, the "easy" thing has been replaced. The thing that my research told me wasn't the part that needed to be replaced.
- (I haven't looked under the car to see if they ALSO replaced the hard thing.)
- The check engine light is still on.

Take it back to the repair shop, and they tell me:
- The tech said he replaced the correct necessary parts, I'm good, don't worry.
- All I need to do is drive the car a bit, through some city driving and highway driving. The engine computer will eventually figure it out, and the check engine light, and the fault code, will clear themselves.

It's been a couple days, including at least one commute at highway speed, and the check engine light is still on.

Opinions? Should I wait longer and do more driving?

My own mechanic agrees with the "drive it a bit to allow the sensor/computer to re-learn".

But he (and others) are also in agreement that the local Firestone places are borderline fraudulent with their customers.

Cheers

Thanks for confirming their "drive it a while" instructions.

Good point about chains like Firestone. I'm sure that auto shop quality is highly dependent upon the individual staff at that particular shop. The one I used has been extremely competent in the past, never failing to get the work done correctly and quickly. This is the first time I've had even the slightest cause to doubt them.

Have you re-read the OBD codes since the repair?

There may be some other problem occurring.

The heated sensor is powered through a fuse, so if the fuse has failed then it might trigger the same trouble code as a bad sensor.

It makes some sense that the 'upstream' oxygen sensor would be on the exhaust system very close to the engine and may well be the part visible when looking at the engine from the open hood.

Good point about the possibility of multiple simultaneous failures.

When researching on the internet, I'm getting contradicting information as to where the gizmo is located. Some of them agree that the one they replaced is the correct one, others say, no, it's under the car. I guess it's hard to research this thing.

I'll give it another day of driving before I make any assumptions. If the CEL goes out, then all is well. If not, I'll look at the code again (it wasn't my scan tool, I'd have to borrow it again to check it) and see if it's still P1166 or something else.

Thanks, all!

Usually, with those scan tools, you have to read multiple times to get all the codes. If it gives you the same code again, you only have one.

My car had this same failure and I cleared the CEL myself, but it came right back on. I gave it a week and it was still on, it seems it'll stay on until it thinks it's seen all driving modes. So, I took the car out away from everything and let it experience a few wide open throttle to 100mph stints. It's cleared its light (maybe out of fear) and has been fine ever since.

Ah yeah, I hadn't thought about the need for it to see multiple throttle positions. Here in Seattle I rarely have any situations where I can ever accelerate like that. I'll have to invent one somehow. ROAD TRIP!

Might be that your car might have different positions for the catalytic converter (I know the Peugeot 206 does, depending on year and engine. Either one immediately after the exhaust manifold , aligned vertically, or below the car (where the exhaust turns and starts to run horizontally). The lambda sensor[s) (older cars might just have one) move with it.

What was the part they actually replaced?

The part that was actually replaced was the one labeled "Air/Fuel Ratio Sensor" in this photo. This was the "easy" one I talked about.
http://honda-tech.com/attachments/honda-...-bad-engine.jpg

Other sources on the internet said, no, the one you have to replace is the difficult one under the car. I never even looked under there to see if there was even anything there at all. So maybe that was the correct one.

My CEL still isn't off, though. I should check the fuse for the thing. Anyone know where that's located?

Here's an interesting thread which says that you need to use the exactly correct oxygen sensor, or the code will keep coming back:

http://honda-tech.com/honda-accord-1990-...e-help-2315752/

Right so that's not a airflow sensor. That's just another O2 sensor pre catalytic converter. Often there are ones before and after the cat so it can tell how the cat is operating.

If the message means the post cat sensor, then yeah they've replaced the wrong one.

They might be the same part though so if you have the "faulty" one they pulled out you might be able to put it back in.

"P1166 Meaning: Primary HO2S" to me would mean the pre-cat one to me though so I'd say they've replaced the right one based on the code actually.

Awesome, so I know they replaced the right part. Thanks!

According to the internet, then, the check engine light is still on for one of the following reasons:

- Fuse is blown (I need to find the fuse).

- Replacement part is not exactly right (this model of Honda is picky about it).

- Computer needs more drive time at highway speeds (becoming increasingly unlikely as the week wears on).

I'm not a Honda expert. I would just say primary is the first one in the pipe. Might be worth reading up a bit more on that code.

Tried the old disconnect the battery trick? Bit of a long shot, but a car that age should reset a lot of things if you do. Make sure you know the radio code if you have a factory radio.

Something else to try could be to do a live read of the values the sensor(s) are delivering, to make sure it/they are giving sane values... Would be good to know if the shop checked the values after swapping out the sensor.

A code for a sensor could also mean the fault lies in the associated wiring, and would remain despite changing the sensor itself.

I've seen codes for the O2 when it was actually the MAF as well. A dying fuel pump can also kick up that code.

As can a leak in the fuel line, or a bad seal on the fuel filler cap, or a problem with the charcoal canister, etc..

Which translates to having the car serviced by a competent automotive technician who really knows his way around the Honda systems.

Diagnostics, especially on older cars, often involves much more than just reading the trouble codes and replacing the obvious component. There may be multiple factors contributing to the currently reported issue and often there are other issues that should be attended to at the same time.

One of the downsides to maintaining an older car is the increasing number of subsystems and components that may be approaching end of life or no longer operating well within specification. This leads to more diagnostic time spent checking stuff just to verify those things are not also causing trouble, or about to cause trouble.

My bet is - Vacuum hose.

Best fix - Black electrical tape over the light.

Understood, but the particular code I'm getting is that the *heating element* in the oxygen sensor is what's not working. My expectation is that's a pretty specific input line that the computer is monitoring. So though those are still possibilities, I expect it's still actually something related to that part.

I'll also try the battery disconnect thing too, if I can't find a blown fuse.

Oh, sorry, that heating code should clear instantly when the O2 sensor starts drawing current. The shop should have known this.

Current status:

I did:
- Tried looking for a fuse, couldn't find one.
- Can't figure out which relay is the one that controls the O2 sensor.
- Tried pulling the fuse to the computer for a couple of hours, didn't fix it.

Took it back to the repair shop. They did:
- Traced and measured voltage and resistance on the wiring to the O2 sensor, all good.
- Checked voltage at the relay (they apparently know where it is). Good.
- Measured resistance across the the O2 sensor itself. Not in spec, they said it's only 3 ohms when it should be much higher than that.
- Tried another O2 sensor, it also did not fix it, and also didn't have the resistance value they expected.

Next step:
Monday they will try to order an OEM Honda O2 sensor. If that doesn't work, then they want to look at the computer as a possible culprit, but I'm not down with that.

It's never the computer. Always buy Original (not claimed OEM, but original).

I always heard an ohm meter could damage an oxygen sensor. Maybe that's why there were expecting a high reading. Cause they always burn them out.

I have seen (and helped diagnose) a car computer where the sensor was fine and the computer input was bad. Rare though.

As for an ohm meter damaging the sensor, seems rather unlikely. About all that could happen (theoretically) is that the ohm meter might push too much current through the device, burning out a coil. In which case the resistance would then read VERY VERY high, not low.

Cheers

That is super rare, but mechanics like to blame the computer and replace them willy-nilly. I've never seen a bad computer that wasn't physically damaged.



Maybe they used a megger?

Here is an excerpt from the service manual of your car.

I took only the pages corresponding to your DTC code. At the end, you'll find some more information on how to do some tests and how to reset your ECM.

Hope it helps.

Page 11-59 is the one that would help.

I found that your O2 sensor should read 10 to 40 ohms. That 3 ohm sensor is likely one for another car that happened to have the same connector. Aftermarket parts vendors are always doing this kind of stuff.

You say so because of the image on page 11-45? In fact, 11-59 deals with DTC P0131: Primary H02S (Sensor 1) Circuit
Low Voltage. To throw a P1166, his car probably have the wide-band O2 sensors in place of a standard O2 sensor.

This is the same conclusion that my tech arrived at. I'm awaiting their word on whether they are able to order the correct part.