Need help with Spec Boxster no-start issue

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If the motor cranks ok, the battery is fine. Eliminate that from your possibilities. When the battery is toast it will not turn over with the starter. Period.

Now to the issue. Several possibilities:
1. Failed/intermittent ign switch. A very likely possibility
2. Failed crank position sensor. Maybe but usually once they die they will never start again.
3. Failed/intermittent fuel pump.
4. Failed/intermittent ECU

Change out the ign. switch first if you can't clearly isolate the problem. Cheap and reasonably easy.