Recently i had a valve job done on it. They resurfaced the upper head as well. After getting everything back together, It ran. The engine light was still on, but i figured it was because i never reset it. About 800-1000 miles later I checked the codes and recieved "missing or bad o2, missing or bad MAF". I reset the ecu (A2) and drove it around, then cheked for faults in A2. I found none. When I took it in for smog I failed Hydrocarbon(max=85 meas=102) at 15mph, but passed at 25mph. I've read that O2 sensors can go bad and be undetected by the ecu. It has seemed to lose power on acceleration, but i thought maybe 'cause it had 140k mi on the engine. Should i try to clean the MAF and O2 sensors? How can I be sure they're in working order? What else should I look for?

Look for simple things. You sensors could be fine if there is air coming in from even a small crack. With the work you had done perhaps a connection along the air intake path isn't tight or a vacuum connection could be off or cracked.

I got MAF codes after replacing flame trap and in the process must have broke the nipple off a vacuum hose connection.

well if its saying your o2 is bad or missing then it probably is.
Your car is running lean and since there is no front 02 (according to the computer) it runs the engine as if it was just started in the morning. Lean
Is it the original o2? How has your gas mileage been? If the o2 was replaced did you put the proper one in? There are bout 5 different ones for the 850 depending on engine/pump.
Check your maf wiring harness. Then can disconnect from vibration.

The car us running rich, not lean. HC is unburned fuel. A faulty sensor could still be the problem, but please, let's get the name of the problem right.

I keep hearing that there are 2 O2 sensors. 1 before the cat and one after. Would you be able to clarify this for me?

I can neither confirm nor deny the rumor...I'd have to crawl under and around your car first. The part that I do understand is combustion. My comment was intended only to correct an allegation that high tailpipe HC indicates a lean condition. It is indicitive of incomplete combustion, or rich condition.

Okay here it goes. The front O2 sensor does it's job by notifying the ecu that either the exhaust is too rich or too lean correct? The exhaust continues on to the cat and assuming everything goes according to plan the final product is cleaner air. If the O2 sensor however does NOT pick up the fact that the air is rich/lean then the ecu makes no adjustments and the cat works even harder and ultimitley fails. Am I on the right track? Now since I'm not getting an indication from the ecu that there is indeed a problem with the O2 sensor, would it be safe to assume that the sensor MAY be "stuck" in an open state indicating that all's well and that there is most probably a failure within the cat as well.

The function of the catalyst is to convert NOx to other stuff, so that is probably not the first place to look for a problem. You can check the O2 sensor signal with a multimeter, but once again, I am not able to give you the technical information that you need. If you have no fault codes, it may be possible that there is some mechanical/electrical problem at the throttle body that is causing air starvation at low revs. Another thought...if the engine was not hot when the low speed test was performed the test results might be normal. If so, try a retest with a hot engine. Run a bottle of Techron fuel treatment then change the oil. This is not the most informed advice that you can get, but these are the things that I would look at if it was my car.

So based on your deduction there COULD be a vacuum problem. Now if that is the case woudnt the ecu catch it at the O2 sensor? Or even better, woudnt the ecu give me some kind of fault at "A-2" socket(fuel system)? I know I'm reaching, but I just want to be clear before I decide to take it to Volvo for a diagnostic. They're telling me they're gonna charge 150 wether or not they find the problem.

Well now that you mentioned it. I do notice a sort of humming noise coming from the throttle body, when the car is in the on position. Is that normal?

This is where you will have to find a book...there may be some generic throttle body information in the 700/900 FAQ. Good luck.

Well, at one point it said that the sensors were missing or faulty. But I figured since I had to disconnect everything to take the upper head off, that's what the ecu was seeing. Yes it is the original O2 and gas mileage has been kinda low. I did manage to find an O2 sensor at Kragen for 80.00. The two he mentioned to me were described as "to 1993" and "from 1993". I found that my car was built in 1992, so I'm hoping the sensor I'll probably get will be the correct one. This is a non-turbo. Are there two O2 sensors?

yes there is more than one sensor for the car.
I did the mistake too without checking exactly what sensor was designed for my car. I relied on the seller and obviously didnt know what he was doing and kept saying yet. After research on what he sold me it ended up for a 2.4L with airpump. And my car is a 2.3L t5. I put it in and I got the code after 50km. Bad mileage and CE light. I ended up putting my old sensor bad in with a 435 code until I found the right one.. (plus the damn thing was alot and I already had one that didnt work) I finally bought the proper one and mileage in proved abit and no longer resetting the computer.

BTW I had an emissions done last month and the guy who did the test came up to me and said that my readings were incrediable. My exhaust was like air.
I looked and and almost every reading was like Zero!!
I think he wanted my autograph or something.

So what do you guys think? If I replace both O2 sensors could that reduce my hydrocarbon count? I mean like i said initially, the max HC was 85 and the measured was 102. These measurments were takin at 15mph. The fact is that I may need to replace both O2 sensors (pre cat and post cat) and its should cost around 2hundredsumthin.00, so as you can imagine, I'm gonna wanna be confident in this replacement. Is there a tool I can maybe use to determine wether or not my problem was resolved? California emissions cost aroun 90.00 each time.

are you getting a code for the rear o2? If not then just replace the front. Do it yourself... easy to replace.

Yeah, that's the thing y'know. I no longer get fault codes. After I cleared them out, I drove her around for a few miles and checked for faults and got nothing. I figured they only showed up due to me disconnecting everything to get the valve work done. Again, I no longer get fault codes.