Originally created for use by vehicle testing companies so their work (DYNO and offroad) would not affect the vehicles covered mileage. Today, we see more CAN blockers are fitted to cars in an attempt to cover up excess distances covered by people who have leased vehicles on low mileage contracts.
Selling and installing CAN blockers/mileage shavers is not illegal in the UK, however it is illegal if the alterations are not disclosed when selling the vehicle or when presenting the car for MOT.
How does this affect you, the technician working on the car?
CAN blockers are small, but they can cause much larger issues. Known to cause programming failures if left installed and can also cause the car to present errors for systems the car does not have, such as convertible roof failures and electronic parking brake failures etc.
How can you tell if one is fitted?
This can be quite hard, a fault code read can help as strange faults such as missing or incomplete message from the instrument cluster can be stored in multiple control units. For example, a BMW may display CDA322 – No message (Units BN2020, 252.0.4) receiver DME, transmitter KOMBI (Cluster). Other manufacturers could use similar terminology.
Decide on a suitable road test route, say about 20 miles. Download/install a GPS mileage tracker app on your phone and start the application at the same time as you commence the test drive. When you reach the end, both distances recorded should be the same…you may be surprised just how many miles these devices shave off.