My new to me 99 S70 had the check engine light right after I bought it. I took it to the dealer. At 1st they thought it was the catalytic converter. Since that's under warranty, they took their time and decided instead to replace the Mass Airflow Sensor (302 of my $). Thereceipt says "The car's mass airflow sensor faulty and causing o2 sensors to read out of range."
Right after I picked up my car, the CEL came back on. Now the dealership is saying "THE CHECK ENGINE LIGHT IS ON AS A RESULT OF A FAILED OXYGEN SENSOR FRONT. FAIL CODE ECM4801 THREE WAY CAT. ALL TEST SHOW THAT FRONT OXYGEN SENSOR IS NOT OPERATING CORRECTLY, IT IS NOT SHOWING THAT IT IS READING ANY MEASUREMENTS COST 445.00"
The service writer says that the O2 sensor was compensating for the mass airflow sensor, but now it's out of that range and "gave up (not reading any voltage)" As far as I know, O2 sensors don't exhibit learned behavior...am I being hosed?
I'd appreciate any alternate theories.
Thanks

Thanks both for your inputs.
Here's how it ended up:
I called the service manager after the car had been in three days (just for this 2nd visit) and was told they were replacing the catalytic converter. The service manager looked further into my car's record and discovered that the O2 sensor had been replaced last April by the previous owner.
In the end, I got the catalytic converter and O2 sensor replaced for free, plus an alignment, 2 floor mats and replacements hubcaps for my troubles.
I am all in favor of independent shops and doing some work myself (thought about changing the sensor myself) but, on this go around I would have spent a whole lot of money on parts if I wasn't going through a (honest) dealer and getting the whole shebang done under warrantee.

as most other dealerships would have you pay for a new O2 sensor, then finally replace the cat converter under warranty.

I take back my "degrading comments" on this dealership. It appears that they corrected their mistake and did some "extra stuff" for you. Definitely I would patronize this dealership and recommend it to friends.

Nice to know that once in awhile, things are done the right way.

Cheers
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'88 240, '92 745, '98 v70 -John, Tampa Bay

And if anyone is wondering- the service was done by Don Beyer Volvo in Northern Virginia :)

Dealerships are in the business of replacing components one by one, at your expense, until the dreaded CEL goes off.

Probably there was nothing wrong with MAF, and you're out of $300. Most likely the front O2 sensor is bad, maybe the cat converter is bad, but first the dealer makes you pay good money before they say is the cat converter. Get the codes from the dealer.

Stop patronizing that dealership and find yourself a trusty indie volvo repair shop.

If you're handy with tools and don't mind getting dirty, you could replace the O2 sensor yourself, they run about $150 so you'll save a bundle in labor.
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'88 240, '92 745, '98 v70 -John, Tampa Bay

I believe you are being hosed, but then again I could be wrong. I'm with you that the O2 is only a sensor that sends signals to the ECU. The sensor does not compensate, it reports. It is the ECU that does the controlling i.e. timming, lean mixture etc. It is possible that the MAF sensor and the O2 gone bad very close in time to each other. $445 is too much to replace one O2 sensor.