schweiger

I just had my 2005 honda accord 4 cylnder car 90000 mile maintenance done and the next day the light goes on in the dashboard. I take it back and they hook it up to the computer and I think it is error code 0171 too lean. They say it is the o2 sensors and need to replace front and back with the front costing $175 and back like $375 plus labor or over $600. I looked up o2 sensors and their cost is like $45 probably not OEM parts. How do they know they need both and also how do they know it is o2 sensors not something else entirely?

Stuff

You are right in questioning. If that is the code it is more likely they knocked a vacuum hose loose while they were doing something else. Read this for the basics http://www.samarins.com/diagnose/p0171.html

Now that being said your O2 sensors could be on the way out after all these years but most likely not related to the code. There are different O2 codes that should come up

gwiffer

If it is the O2 sensor, I would only replace the upstream/primary sensor; it is responsible for your AFR. The downstream/secondary sensor just monitors the state of the catalytic converter. I'm surprised that the secondary sensor is significantly more expensive; is that exactly what the dealer told you?

DCFIVER

The secondary O2 does more than monitor cat efficiency, this is a common misconception. It also responsible for rear fuel trim. (Just an FYI)

DCFIVER

Ask if they will guarantee that replacing the O2's will keep the light off....

Crashmaster

Too often after maintenance the technician will clean the throttle with various cleaners and that will cause the O2 sensor to go out. Did the shop clean the throttle bore or do a fuel injector cleaning? You might try and drive the vehicle for a bit longer to see if the O2 sensors will clean up on their own. Often this can happen. If not, replacing the O2 sensors are not that hard a job and you can purchase them (aftermarket) relatively cheap and do the job yourself. You can also call around some independent shops and check to see what they would charge you for installing O2 sensors.

There are also some YouTube videos showing how to clean your O2 sensors to get rid of the CEL O2 code. There are also very helpful videos on how to replace your O2 sensors. It is not that difficult of a job and you should be able to do it yourself.

gwiffer

Why is the primary sensor a LAF/wideband sensor, while the secondary sensor is a narrowband oxygen sensor?

What do you mean by rear fuel trim? The fuel trim after the cat?

The only generic OBD parameters that I've seen on a Honda are ST and LT fuel trims (my V6 has Banks 1&2). Is this secondary O2 sensor feedback (rear fuel trim adjustment), Honda-specific?

DCFIVER

Because the rear O2 is exposed to exhaust gas that has already been equilibrated. It does not need to be as accurate as a AF sensor.

Yes.
It is not Honda specific. Most if not all late model vehicles utilize rear fuel trim. It has been around for years, though the subject matter information was scarce intialy, much more info can be found now days. The amount of authority rear trim has varies. With Honda it has always been very low.