2003 Boxster - p0335 crankshaft position sensor a code

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Hi everyone...I'm hoping someone can help. I've searched the forum, but can't find anything specific to this code.

I was driving home last night and the check engine light came on...when I got home and hooked up my OBDII the code p0335 crankshaft position sensor a came up. A few questions -

- What would cause this?
- How hard is it to replace and can anyone point me to some how to articles?
- Is it safe to drive until I can get this resolved?...the car runs fine when cold, but gets a little finicky (hard to start) when at normal operating temps for a while

I've recently replaced the Transmission Cooler Control Valve (Tiptronic) and wondering if that would have anything to do with this as it just started happening after the Transmission Cooler Control Valve was swapped out.

Any help would be greatly appreciated.

Thanks!