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Yes, if you know of an issue with the vehicle that could result in an MOT Test failure, it is best to have this fixed prior to the MOT Test.
If not, you could run the risk of the issue leading to an MOT Fail. In which case you would need to get the part repaired and then pay for another test (or partial retest).
To save time and money, you would be best to have any issues fixed before you book your MOT.
Many garages offer an MOT and Service package. The benefit of this is that a service is carried out before the MOT inspection. Any issues can be highlighted and then rectified off the back of this service before your vehicle goes through the MOT test.