What a mileage blocker actually does
A mileage blocker freezes a vehicle's odometer so the recorded mileage stops climbing while the car keeps being driven, the longer it runs, the further the figure drifts from reality. Because the stored figure still looks internally consistent, blockers are hard to spot without a proper check.
Establishing the history
A desk Records Certificate runs a multi-point check of official vehicle records and shows whether the mileage has been altered at any point in the car's life, even if nothing is fitted today.
Confirming what's fitted now
A clean history doesn't prove a blocker isn't fitted right now. Confirming that needs a specialist on-vehicle inspection, the only check that can defensibly state no active device is fitted.
The claim boundary
A desk check can never state that no blocker is fitted, only that records show no evidence of tampering. Confirming no live device requires a physical inspection.