How to Tell If a Car Has Had a Mileage Blocker: UK Buyer’s Guide (2026)

By Paul · Updated

Short answer: you usually won’t find out by plugging in a diagnostic scanner. You find out by checking whether the car’s story adds up. Compare the odometer with the MOT history, service records and the car’s condition, look behind the dashboard for an aftermarket module, and ask the seller directly. In the UK, a seller who hides unrecorded miles is breaking the law, so you have real protection if you’re misled.

Table of Contents

What a mileage blocker does

A mileage blocker is a plug-and-play module fitted behind the instrument cluster. While it’s switched on, it stops new distance being recorded. It doesn’t wind back existing mileage, which is what “clocking” does. Blockers are legal for dyno testing, track days and off-road use, but a seller must disclose any distance that wasn’t recorded. See Mileage Blocker vs Mileage Correction.

Seven checks to make before you buy

  1. Check the MOT history. Enter the registration on GOV.UK’s free “Check MOT history” service. The mileage is recorded at every MOT, so you can see whether the car’s use has been steady, or has slowed down suspiciously while it was still being driven.
  2. Compare the service records. Service stamps, invoices and the manufacturer’s digital service history should show the mileage increasing in line with the dates.
  3. Run a vehicle history check. Paid UK checks pull mileage readings from several sources and flag inconsistencies.
  4. Inspect the car’s condition. Worn steering-wheel leather, pedal rubbers, seat bolsters, gear knob, tyres and brakes should match the mileage shown.
  5. Look for an aftermarket module. A blocker sits behind the instrument cluster. A technician removing the trim can check for one, and on some models the speedometer stops or warning lights come on while a blocker is active.
  6. Check connected-car data. On newer cars, the manufacturer app or a franchised dealer may be able to see recorded mileage history.
  7. Ask the seller. Ask directly whether a mileage blocker has ever been fitted or used, and get the answer in writing.

Warning signs

  • Very little mileage added between MOTs on a car that has clearly been in regular use
  • Gaps or inconsistencies in the service history
  • Wear that doesn’t fit the mileage
  • A seller who won’t answer questions about the car’s history

If you think you’ve been misled

Selling a car without disclosing distance that wasn’t recorded can be a criminal offence under consumer protection law and the Fraud Act 2006. If you’ve bought from a dealer, you also have rights under the Consumer Rights Act 2015. Contact the seller first, then Citizens Advice, which can refer cases to Trading Standards. See Are Mileage Blockers Legal in the UK?

FAQs

Can a diagnostic scanner detect a mileage blocker? Scanners read fault codes and stored data. They aren’t designed to look for aftermarket modules, which is why the checks above matter more. See Can You Detect a Mileage Blocker?

Can a dealer tell? Dealers compare the car’s mileage with its MOT and service history and its condition. See Can Dealerships Detect Mileage Blockers?

Is a car with a blocker always a problem? No. Track and dyno cars may have used one legitimately. What matters is that the seller tells you, and can explain the history.