Not the dipstick. Tech checked it today and saId it is not loose, the o-ring's intact, and he's never seen one leak.
To recap most of the the time the tech told me the cause of a P1128 error is a dirty MAF, dirty because of a failure of the air filter -- the early Boxster air filter element is notoriously a difficult item to replace and often it gets torn/damaged when installed. There's no way to check it once in place. The bad filter lets dust through and this over time settles on the MAF and its hot film surface. This surface is because of its size relatively immune to some contamination but at some point the bad filter and the dust it lets through takes its toll.
However, when I replaced the MAF the other day the old MAF looked just fine. No signs of any dust build up.
Another possible explanation can be when the old engine air filter is removed some trash that has collected on top of the filter falls down into the air box and if not removed can end up against some plastic screen/mesh located further down the intake just ahead of the MAF. This trash -- most likely a leaf -- can disturb the air flow and affect the MAF's performance which can trigger a P1128.
I am well aware of the trash falling off the filter as this has happened almost every time I have changed the air filter. The filter element top surface collects a lot of trash. Last time the top had a solid layer of leaves on it. What a mess. But I was careful to remove all the stuff that fell into the air box.
What the tech said then if none of the above that a common problem is the intake develops a leak. He said the intake o-rings dry out and leak. He also mentioned there are a number of vacuum hoses and once in a while one of these cracks/splits and leaks.
Every time the P1128 has appeared it was not accompanied by any other error code. Also, I always check for any pending codes and there haven't been any. I've never gotten an error code related to the gas tank ventilation or vapor recovery system or the secondary air injection system, so I sort of kind of think the hoses are ok.
The tech said what is done is a smoke test is used to determine where the leaks are and then that determines what is done. He said often the intake manifold ends up getting removed and re-sealed -- new o-rings installed. 'course, if the leak is from a hose the hose and whatever else may be contributing the leak is replaced.
While I was hoping it was the dipstick that appears to be not the case.
I kind of like the intake o-rings for the reason. I know the spark plug tube o-rings failed -- they just deteriorated due to age -- and had to be replaced. So it would not surprise me at least one intake o-ring was leaking due to old age.
So, I have tentatively made an appointment to bring the car in next Thursday and have this looked into. I was thinking of waiting until the car's next service but with the lower number of miles I'm putting on the car now -- just 10K a year -- the next oil service is 4/5 months out. So next Thursday is as soon as the service department can get the car in.
Since after leaving the dealer on the way to work the CEL came on again today I am most assuredly going to keep the appointment. And after I get home tonight after work probably park the car and use the other car in the meantime.