If you really want it fixed right you need to take it to a competent Volvo technician that is also a licensed smog technician. I do this stuff for a living and we can guess about this one until the cows come home and maybe even get lucky and fix it but if you want it done right you need to have it looked at by someone who 1) knows what they are doing and 2) has the proper tools to diagnose and verify repairs on your car, i.e. the smog machine.

Probably not a vacuum leak. Typical vacuum leak results in a lean condition and a lean condition will have low to no CO. If it is so lean it goes into a lean miss you will usually have elevated levels of HC to go with moderately elevated levels of CO (not 4X the amount).

Bad cat, not likely either. This will usually result in elevated numbers across the board, not in just one catagory and not 4X in one catagory.

Bad MAF or ECT, now that is a definite possibility along with a faulty O2 sensor. However should it be a faulty MAF sensor this should be an LH2.4 system which means that there will be no mixture screw to turn on the MAF and in the same respect if it is posting codes that again means that it has self diagnostics and is an LH 2.4 system (no misture screw).

You also want to consider the possibility of a faulty FPR (fuel pressure regulator). Elevated fuel pressure caused by a bad FPR will cause high CO for sure.

In the state of California it is illegal for a regular technician no matter how good he is to do repairs on a vehicle in an attempt to make it pass smog unless he is duely certified by the state to do so and is employed by a smog test and repair facility. At first it sounds like a bunch of bullshit but honestly I have seen this in action and can agree with the state on it. When you have a technician doing repairs on a car and no way to really verify his repairs because he does not have a dyno and a five gas analyzer it becomes an expensive and time consuming game of ping pong with the customer stuck in the middle. When the customer decides to seek assistance from the state in the form of a waiver only money spent at a certified smog test and repair facility qualifies towards the minimum amount necessary to qualify for a cost waiver. It is at this point that the non licensed repair facility finds themselves with their tit in a wringer with the customer and the state Bureau of Automotive Repair.

A customer can do all of the repairs to his own car that he wants but the problem is that you only know that you fixed it when you take the car back and have it put on the analyzer. This costs money everytime you do it and unless you fix it right the first time it gets old and expensive fast as well.

I know this is probably way more information than you want or need but it is reality.

Good luck with you CO problem and if you could, post all of your readings as it would help with the diagnosis.

Mark