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2004 Sienna Oxygen Sensors

My wife’s van has just shy of 99,000 miles and a few weeks ago the engine light went on. Other than the light the van drove fine. As we were to drive this van on a vacation however we thought we should check it out before getting stranded on some highway far away from home.
Took the car to the local Toyota dealership… they said both oxygen sensors, before and after the catalytic converter were bad and the fix was about $800 to fix. This also included and charged for a reboot of the computer but was not to replace the converter.
My question… is this a 4 hour job as they indicated and whats with the reboot. I asked the guy who checked us in and the guy who processed the paperwork (first one was on break) and got two different reasons for the reboot and such.
So… does this sound legit and reasonable? What would happen if I ignore them? The light is off and has not returned.
(Oh, and not really important but we rented a fun car for the trip… a brand new Chrysler 300… fun to drive and fun to put the miles on their car.)

Thanks for any guidance.

Usually the check engine light doesn’t go on and then off when either O2 sensor is bad. Usually it just stays on. You may just have a bad connection in one of them, if it was indeed the code related to the O2 sensor.
You may be able to google exactly where your O2 sensors are located (they are on the exhaust pipe somewhere) and follow the connections. They usually have a short wire and connector attached to them. The connector may have gotten fouled up, causing an intermittent connection and the light to turn on/off.

Don’t ignore it. Make sure the condition is gone. A bad O2 sensor could cause the car to run rich and that will eventually mess up your catalytic converter (ie CAT). That is a very expensive part to replace on most cars.

You could invest in a cheap OBD2 reader and read your own codes. The codes are quite cryptic (like P0420, for instance). The book that it likely comes with describes each code in mechanic’s terms but that may point you in the right direction or at least make you into an informed consumer.
You can get several that are quite inexpensive but capable that you interface to your PC by means of USB. You plug the reader in, start the program and can read all sorts of things on the health of your car.

As far as four hours for two O2 sensors: four hours seems like a lot of time.
A stubborn O2 sensor could get stuck and may require a couple of hours of just sitting after they hose it down with some penetrating fluid but it wouldn’t be right for them to charge you for that because that isn’t actual work done by a person.
In reality, one hour for actual work is a lot more reasonable for both.
Resetting the computer takes seconds.

“rebooting the computer” almost definitely means reflashing with newer software

Many manufacturers have realized that their initial software was too sensitive, so they came out with revised software. I expect labor to reflash to be in the neighborhood of 0.5hrs to 1hr

What exact fault codes did you have?

If you had P0420, which is a catalytic converter efficiency code, the mechanic is doing the right thing by recommending the oxygen sensors before replacing the cat. Degraded sensors are often the cause of those cat codes

Reflashing the pcm is also the right thing, because some cat codes are caused by outdated software.

You really need to post the exact codes, though. For example P0420, P0130, etc.

$800 to fix doesn’t sound that out of line, either, for 2 sensors, a reflash, labor and tax. Toyota uses Denso sensors, which aren’t cheap, and the labor rate is probably above $100 per billed hour.

Here is a bit of advice. Wherever you go to get this fixed, INSIST that after they do their thing and clear the codes, they let the appropriate monitors run to completion. That is the correct way to verify that the problem is indeed resolved. If they balk, or they don’t know what you’re talking about, walk away.

@Db4690, not questioning your analysis because you may very well be right but out of curiosity:
Have you seen a check engine light go on and then off at random with a bad O2 sensor? Do those things fail like that? I ask because I’ve never seen the light go back off.

Four hours does seem excessive to me.


I have indeed seen oxygen sensor codes come and go

On my brother’s car, an oxygen sensor code once caused the MIL to light up. I cleared it, and it never came back. I checked once awhile, and there wasn’t even a pending code

On the other hand, I have also experienced cases where the MIL eventually turned off on its own, without replacing the sensor, only to have it come back on at a later time.

In those cases, the sensor was intitially very marginal, meaning that it did its job occasionally. And when the MIL came back on for good, the sensor had failed 100%.

@db4690 Thanks for that clarification. I did not know O2 sensors failed like that - unless a connection was bad, of course.
Goes to show you that one learns something new every day.

Thanks everyone for the insight.
The best that Toyota did was give me a TSB from feb of 2005 that lists 4 different codes… P0031, P0051, P2238 and P2241. The dogs didn’t give me anything that stated which code tripped and really gave me nothing but the bill for the $45 diagnosis and the printed TSB. I didn’t notice this until now.
I hear Autozone can read codes… can a code still be read from memory later… like now and are they a good resource?
Does this work need to be done by mother toyota or can a good shop handle it?

Guys, this for your benefit, since I know OP already has this TSBлямбда%20зонды%20коды%200031%200051%202238%202241.pdf


I don’t think there’s any point in going to Autozone to read codes. The Toyota dealer already did that.

I urge you to let them perform the TSB. The reason I say this is because I’m a professional mechanic, who happened to work at a dealership for many years. I can tell you from my experience that those TSBs actually work.

In plain english . . . if you have the codes listed on the TSB, which you apparently do, performing the steps listed in the TSB will almost certainly resolve your issue

And again, the price doesn’t seem out of line. The labor rate is almost certainly over $100 per billed hour. Denso makes the sensors, and online you’d pay over $200 for the pair, plus tax and s/h. However, the sensors at the dealer will also have the Toyota part number on them, in addition to the Denso number. So they’ll cost more. Any sensor that doesn’t actually have the Toyota part number or logo on it and doesn’t come in the Toyota box is technically an aftermarket part.

Toyota may be charging you over $200 for each sensor, which isn’t out of line. A genuine Toyota sensor costs more than a Denso sensor, and any shop (dealer included) makes a profit on the parts.

So . . . labor plus parts, plus tax can add up to $800.

I suggest you let the dealer handle this.

Not every independent shop is set up to reflash your pcm with updated software.

I’d hate for you to get cheaper sensors installed at a shop, only to have the MIL remain on, or come back on, because you didn’t get the pcm reflashed with updated software