Sounds like the MAF was the issue.
Dirt even so small and fine that you can not see it can have an effect on the MAF.
They are very sensitive. Another thing that happens with them is that very fine dirt particles erode the coating on the hot wire. You can't see it with the naked eye. I have seen magnified pictures of the issue. Looks just like it had been sand blasted.

Yes Intake Air Temp. is affected by engine temp.
If your short term fuel trims stay at or near 0% they will eventually pull long term fuel trims down.

I think you stated you have 93k miles on the car.
MAF signals tend to slow down with age.
A new MAF "may" give you better throttle response and fuel mileage.

Both long term fuel trims are about -4% now, but I suspect I know why, I am running on E10 fuel, it changed from E5 a few months ago. The air/fuel ratio for petrol is 14.7/1, but E10 is A/F 14.04/1, the difference is -4.4%. The ECU in built map is almost certainly set for 14.7/1, so would indicate a negative fuel trim.

I have a new MAF sensor arriving tomorrow, so will get this installed on Saturday. My Boxster ran great on the 10 miles each way to work today, so I am not going to worry about LTFT unless it works it’s way up to -10%.

Robin

I was going to add that some of the things we may ask about or have you try might not work for you since you have a ROW car.

for LTFT I would just check it every now and then for a bit and see how it trends. Might take some good drives to see any difference.

RoW - Rest of the World?

My chassis number decodes as
WP0ZZZ98Z1U6XXXXX
W - Germany
P - Porsche
0 - Sports car
ZZZ - Europe/RoW
96...6 - 986
Z - Europe/Row
1 - 2001
U - Uusikaupunki Valmet factory (Finland), Boxster 97-2010

So strictly it is Europe/RoW, but I see that you are distinguishing between US and RoW.

Fuel Trim
The fuel in the UK changed from E5 for regular unleaded (95 RON) to E10 this summer, unfortunately for me that also coincided with the CAT and O2 sensor changes and the start of my problems. We can still get E5 premium unleaded, Shell V-Power (99 RON), I have never really used it, I did try a tank a month ago, but it made very little difference. Perhaps now it is running better, I would notice the difference.

E10
I think I have got the LTFT wrong for E10, as E10 is a higher fuel to air ratio the LTFT should be +4% not -4% as I thought in the last post. Shell say the V-power is 4% more efficient, so this ties in.

The good news for me is that my LTFT have come back up, bank 1 is now 0.0% and bank 2 is -1.5%. I somewhat agree with Stl-986, ignore the values of FT, just check it now and again, after all if it is running well, why worry?

I have learnt quite a bit about how a modern car sets it's fuel usage, it has been interesting, most of my real experience with this comes from motorbikes from the 70s and 80s, then you just looked at the plugs and fiddled with the air screw or jet on the carb.

Robin

I have never seen an automobile that ran constantly with 0% fuel trim.
There is always going to be some correction (fuel trim). To many variables to even make it possible to operate under 0% fuel trim at all times.. Things like engine condition, air mass (density) which changes with altitude, temperature, humidity. Driving style, engine load, fuel, ect,ect,ect.
14.7-1 AFR is only ideal under a very limited engine operating range.
The engine needs a rich mixture at times as well as a lean mixture at other times.
Catalytic converters need a fluctuation in AFR (fuel trim) to operate properly.
Emissions are not cleaned up with out that fluctuation in AFR's
Even in the days of carbureted engines AFR needs changed according to demand.

The only time I would start worry about fuel trim numbers is if fuel trims begin to exceed + or - 10%.
Most manufactures of the early 2000's limit is + or - 25%
The other times I look at fuel trims is to help diagnose other issues.

New MAF sensor arrived yesterday and fitted this morning, my car is still running well. To be honest, it isn’t really any different to the cleaned MAF, but when I catch the light on the new MAF sensor, it’s like a very smooth mirror and doing the same on the old sensor you can see the dirt, so don’t think the old sensor will be much use.

It looks like I am finally fixed, just in time for the bad weather and possibly snow, so I guess I will be using my 4x4 rather than my Boxster for a bit.

Robin

A positive end result even if you have to wait for decent weather to enjoy it.;)