Tunze 6255 always at max speed

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I have a Tunze 6255, which always runs at maximum speed. I tried replacing the apex cable, but it still stays at full power all the time. What is most likely to be the issue? The pump? The powerbrick? The little control box?

 

[rvitko Verified Sponsor]

Are you sure the cable is fully inserted in the controller port? A common problem it seems with the Apex cable is the rubber boot prevents it from being pushed in fair enough, I have never used one so I only know what users report, and you typically have to pull back or trim the boot to be able to push it in far enough to make contact. The controller also must be in external mode. If it is neither of these causes, I would suspect the controller.