Dials to the top on start up

[Fozzy]

I know i've seen people with this problem before, but can't remember what the problem is.

Just started up my HDi and the rev and mph dial shot up and stayed at top for a few seconds then went to idle. Haven't driven the car for a few days, and there was a lot of rain yesterday so hope this is a one off.

Any ideas what may be the cause?

Thanks
John