This code means that the VDO tachograph has detected that the additional, independent motion signal is missing or not available.
Code 81 is generated when the tachograph is unable to receive a valid secondary motion signal. This signal is used as a cross-check to confirm vehicle movement data and its absence is treated as a system fault rather than a driver issue. The tachograph itself is not necessarily the source of the problem.
Common causes include issues on the vehicle CAN bus, interruptions to the data connection between the tachograph and the vehicle systems, or power supply irregularities affecting CAN bus components. In some cases, external factors can also contribute — for example, the vehicle operating in an area where supporting signals (such as GPS-based inputs) are temporarily unavailable.
Issues that cause code 81 to appear are likely to cause the recorded data to be inaccurate, which is why it’s important to resolve them quickly.
If Code 81 is repeatedly displayed, the vehicle should be referred to an approved tachograph centre for investigation. As the fault may originate outside of the tachograph head unit itself, authorised diagnostic checks are required before the vehicle is returned to normal operation.