Can Carly Detect Mileage Blockers? (2026 Guide)

By Paul · Updated

Short answer: Carly is a diagnostic and coding app. It reads fault codes, live data and stored values from your car’s control units, and it isn’t designed to search for aftermarket modules such as a mileage blocker. But a Carly scan is only one check. A car’s real use still shows up in its MOT history, service records and physical condition, and the module itself can be found behind the dashboard. And whether or not a scanner finds one, hiding unrecorded miles from a buyer is illegal in the UK.

Table of Contents

What Carly does

Carly connects to your car’s OBD2 port through an adapter and a smartphone app. Depending on the brand, it can read and clear fault codes, show live data, carry out some service resets, and code certain features. Some versions also include a used-car check that compares mileage values stored in different modules.

What a Carly scan can and can’t show

  • It can show fault codes, warning-light causes and, on supported models, mileage values stored in several control units.
  • It isn’t built to search for aftermarket hardware. A mileage blocker pauses recording rather than rewriting the stored figure, so the values in different modules generally stay consistent with each other.
  • It can’t tell you whether the recorded mileage matches how the car has really been used. That needs the checks below.

On some models, the speedometer stops working or warning lights come on while a blocker is active, and a scan or a technician may notice either. See Can You Detect a Mileage Blocker? for the full picture.

Checking a used car properly

  1. Check the free MOT mileage history on GOV.UK.
  2. Compare the service records with the odometer.
  3. Run a vehicle history check.
  4. Look at the wear on the seats, steering wheel, pedals, tyres and brakes.
  5. Ask the seller directly whether a mileage blocker has ever been fitted.

Our UK buyer’s guide explains each step.

Using a mileage blocker legally

Mileage blockers are legal in the UK for dyno testing, diagnostics, track days and off-road use. They aren’t a way to make a road car look less used. If a car has covered distance that wasn’t recorded, keep a log and tell the next owner. See Are Mileage Blockers Legal in the UK?

FAQs

Can Carly detect mileage tampering? Carly’s used-car check can flag differences between mileage values stored in different modules, which is a common sign of rollback. It can’t confirm a car’s true use on its own.

Can dealers detect mileage blockers? They look at the whole history of the car, not just a scan. See Can Dealerships Detect Mileage Blockers?

Do I have to tell a buyer if I’ve used a blocker? Yes. Unrecorded miles must be disclosed when you sell.