What Boxsterra asked? If the CEL is on there will be at least one active code. Even if the CEL is dark there could be a pending code.
Prolonged cranking -- even for a few seconds -- with my Boxster (and Turbo) was rare. Besides the time the Boxster fuel pump failed and the engine would crank as long as I cared to let it crank with no start the few and infrequent times for any cranking time out of the nearly instantaneous start at all other times proved to be the early warning sign of a bad battery.
An elevated idle speed suggests an intake air leak. For my Boxster the only intake air leaks have been due to a failing AOS, except two times once at around 150K miles and again at around 300K miles when the oil tube cap developed a leak. But even in the case of the bad caps the idle speed was unaffected. The only indication I had of something wrong was a CEL and at least one error code that pointed to a mixture problem at low engine speed which the first time I mistook for a failing MAF.
But while my car never experienced this I have been told the intake manifold can develop a leak.
Besides reading any codes: active (CEL on); or possibly pending; not much you can do on your driveway. If there is a throttle reset/calibration procedure for your MY car you could try that. There was one for both my 2002 Boxster and 2003 Turbo. Briefly: Key off. Foot off the gas pedal throughout the entire procedure. Turn the key to on and leave on for at least 60 seconds. At some point during this 60 seconds I could hear some mechanical activity. After 60 seconds turn the key off for at least 10 seconds. The next time the engine is started the throttle calibration is complete. If the CEL comes on -- I don't expect this but just covering the bases... --- read the codes. There's like a problem with the e-Gas/throttle system.
The only time I found the throttle body "dirty" and to the point cleaning might have helped was when the throttle body, specifically the butterfly valve, was wet with oil from what proved to be a bad AOS. Up to you if you want to pull this and clean it. If you remove it handle with care. If you drop it...
Be sure you get the TB installed with *no* intake air leaks. Clean with a residue free cleaner. An electronics component cleaner that comes in an aerosol can. CRC makes something:
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www.crcindustries.com]