I have a 2001 S40 that loves to throw P0133. I've replaced both O2 sensors and replaced them both twice to make sure I did not buy bad O2 sensors.
P0133 never fails to come back. Sometimes It doesn't come back for 400 miles, but once it comes back, it comes back right after I clear it and nothing fixes it. I cleaned the MAF, checked for leaks, tried leaving the battery disconnected for a while, etc but nothing works.
I do have the adapter harness and read that the next step is update the ECM to the latest software so it is not as sensitive as it is now.

Was wondering how much the update runs for because I can get a used ECM on eBay for ~$100 from a 2004 which will have the updated software already.

Thanks!

 

Had the same problem.Try this it solved mine.Install the adapter cable then take the cables off of the battery and short them together.Keep it like that for two days at least.The sytem needs a hard reset.After I did this the code never came back.
You can not just buy an ECM from anywhere it is paired with the car.If you would do that volvo dealer still would need to reprogram it to your car costing a lot more than just if you go and see the dealer with your current issue.

 

Thanks!
I drive the car way too much to not be able to drive it for two days. Guess I'll have to find a way to not drive it! It's really irritating seeing the engine light, I clear it as soon it comes on so I basically have to clear it every day.
I thought the ECMs were only paired in the newer cars?

 

Guess I will figure out at some point this week.. A nearby local accepted my best offer on an ECM on eBay this weekend. Let's see what happens!

 

So the ECM was a no go, had to return it. The problem did go away though! I cleared the check engine light and for some reason, felt like switching the car from a cone filter back to the stock air box.
P0133 has not come back at all in the past week. Not complaining, the cone filter did not do much anyway.

Now that the car is problem free, I can sell it to make room for the 2005 S60R I'm getting

 

So the code popped back up shortly after my last post.
The adapter cable helped for a week but it came back after that.
I went to the dealer and they updated the software. It has never come back and car runs better and gets better gas mileage.
I average 26 mpg with 60% highway and 40% city.
It was much worse before the update. ~350 miles per tank. However, @ 350 miles only 13 gallons goes in the tank so I'm sure I could get more miles to a tank.