Latest update;
I've changed the flex pipe / rear catalytic because the flex pipe was broken;
Cleaned the MAF sensor;
Checked visually and with propane for vacuum leaks; Didn't get time to build a smoke machine yet.

Here's some new questions...

1. Can I check the fuel pressure simply with a tire gauge? There is a Schrader valve at the end of the fuel rail (under the plastic cover).

2. I changed the "Volvo" spark plugs to NGK BKR6ES. The NGK plugs were listed for a S40. However, the electrode tips are like you would find on an older car. The Volvo plug electrodes pretty much came to a point. Could that be the problem?

3. I've noticed some hesitation at wide open throttle (WOT). When driving normally (which means no WOT) there is no hesitation, actually runs very well and is returning 28 mpg. At WOT, it does seem like it's running lean - just not getting enough gas.

4. I'm trying another air filter because the CEL first showed up after changing the old air filter.

5. I do have to change the alternator (noisy bearings), but I can't believe that would have anything to do with it.

I was hoping that after 3-4 weeks now another code would be shown, but it is still only the P0171.

Maybe I'll put the old plugs back in this weekend.

Thanks as always.