Cdj 900 slip mode tempo defect

I bought a 2nd hand pair of cdj900s, one of them is slightly faulty. When you turn slip mode on on the faulty one the tempo range increases randomly. This is also affected by moving the jog wheel plate with slip mode activated. When slip mode is activated and the jog wheel plate is pressed down the slip light flashes on and off. I have read of a few cases similar to this on this forum. If anyone has encountered this problem and/or managed to fix it I would be very grateful for any advice. If anyone has had this problem fixed by a pioneer specialist, what work was required and roughly what was the cost?
Many thanks