I have a 1995 850 Turbo Wagon with a little over 200,000 miles on it. Last August after a fill up my "Check Engine" light came on and stayed on.
I checked all the fluid levels and everything was fine, the car ran just fine. A couple of months later I took the car to Advance Auto Parts for a free scan,(even though my car is 1995, it has the OBDII plug under the coin holder.)
The scan showed code #:PO133, O2 sensor. the strange thing is that the car does not have any of the symptoms a bad sensor is suppose to cause. It runs great, I am getting good milage and the plugs look good.
Last week I got a Launch scanner from Harbor Freight, plugged it in, followed the instructions and the scanner showed no codes, just a couple of dashes followed by the word LAUNCH. After four scans the check engine light went out but came back on after about a 150 miles. I of course re-scanned until the light went out again, the scanner still showing no codes.
My question is, will the check engine light keep coming back on every 150 miles or so and do I need to worry about the O2 sensor?

Sorry about the long post, Peter

There are several codes for specific problems with the oxygen sensor (this is in addition to the suggestions on the other reply). If your car is a 1995 you can use the built-in reader to reveal which ones. Details about reading codes and a list of them can be found on volvospeed.com Bay13.

One code for the oxygen sensor is for slow response - I got this repeatedly. It returned each time a couple of weeks or months after clearing the code. Replacing the oxygen sensor cured it. There were no performance symptoms.

A small vacuum leak can also trigger a persistently returning code, again without obvious symptoms.

Car could seem to run fine but burning lean (typical of an 02 sensor going). May not be 02 sensor either, but some vacuum hose leaks that are registering on the 02 sensor. Just keep an eye on things....How's the exhaust? Is there a sooty surface on the tail pipe? Are the plugs turning black? Just a few things to check out. Most 02 sensors go around 100K+ miles and so many routinely replace them at 100K miles.
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1998 S70 T5 Emarald Green Metallic, 2004 V70 2.5T Ruby Red, Previous Owner of Black '94 850 Turbo Wagon. My cars have been running so well lately they've got me worried.