What this code means
Motion sensor, i.e. the motion sensor, is fitted to the gearbox and transmits speed pulses to the tachograph vehicle unit (VU). Code 21, motion sensor fault, means that this communication is disrupted — the vehicle unit is not receiving correct motion data from the motion sensor.
Code 16, a sensor fault following a self-test, means that the sensor itself has reported an internal problem during its own diagnostics. Code 21, on the other hand, concerns communication between the sensor and the vehicle unit — wiring, power supply or pairing — and does not necessarily indicate a fault in the sensor itself.
What to do now
- The message must be confirmed, by pressing OK on the display.
- Drive to a safe stopping place.
- You must make a printout from the tachograph for the current day.
- Make manual entries all further activities on the reverse of this printout — until the fault is repaired.
- Inform the fleet manager and reach an approved workshop as soon as possible.
Continuing to drive without manual records when you already know about the motion sensor fault is a serious infringement in all EU countries.
If the same fault keeps recurring across several vehicles, tachograph analysis software shows it for the entire fleet in one place.
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- Do not attempt to replace or repair it yourself of the motion sensor — unauthorised interference is one of the possible causes of this code and may be treated as an attempted manipulation during an inspection.
- Do not confuse it with code 16 (motion sensor fault after the self-test) — this is a different problem, within the motion sensor itself.
- Do not continue driving without making manual entries — unlike a low-priority fault, code 21 requires an immediate response.
- Do not ignore a recurring message on the same vehicle — this indicates a persistent problem with the sensor or wiring.
Why does it appear?
According to the manual, the cause of code 21 may be:
- interrupted power supply of the motion sensor;
- damaged or loose wiring between the motion sensor and the vehicle unit;
- missing pulses from the motion sensor;
- odometer mismatch — discrepancy between the data from the motion sensor and another source of motion data; or
- unauthorised replacement of the sensor — pairing of a new motion sensor with the vehicle unit does not match.
Such as EventFaultType 0x09 (“motion data error”) according to Annex I C to Regulation (EU) 2016/799, code 21 belongs to the same group as codes 020, 022 and 087 — the fault is stored in the vehicle unit’s mass memory and on the driver card.
Can you continue driving?
In short: yes, but only to reach the nearest safe stopping place. Code 21 has high priority, because the motion sensor is the source of data on speed and distance travelled, so its fault affects the reliability of the entire driving time record. From the moment you become aware of the fault, you must continue driving using only manual records of activities on the back of the day’s printout — until the vehicle is repaired at an approved workshop.
Formally, Article 37 of Regulation 165/2014 applies: repair as soon as circumstances permit, and if the vehicle cannot return to base within one week of detecting the fault, the repair must be carried out en route.
Consequences for the company
The motion sensor supplies data on vehicle movement, on which the entire recording of driving time, breaks and rest periods is based. Enforcement authorities take a motion sensor fault seriously — it is one of the components most frequently checked when tachograph tampering is suspected, because unauthorised replacement of the sensor is one of the possible causes of code 21.
If the driver continues driving without the required manual records, or the company cannot document the repair within a reasonable time, the roadside check may result in an infringement being recorded for failure to comply with the repair obligation under Article 37 — and, if tampering is suspected, also in further investigative proceedings.
Also displayed as
- Motion sensor fault 21 — VDO DTCO 4.1, short display text, code 021
- EventFaultType 0x09 „motion data error” — official wording, Annex I C, Regulation (EU) 2016/799, covers codes 020, 021, 022, 087
Related codes
Frequently asked questions
I can see “motion sensor fault” on the VDO display — can I continue driving?
In short, yes — to reach a safe stopping place. You must then: (1) press OK to confirm, (2) make a printout from the tachograph for the relevant day, (3) make handwritten records of all subsequent activities on the back of this printout, (4) inform the fleet manager, and (5) reach an approved workshop as soon as circumstances permit. Continuing to drive without handwritten records after you become aware of the fault is a serious infringement in all EU countries.
What causes code 21?
According to the manual, the cause may be an interrupted power supply to the motion sensor, damaged or loose wiring between the sensor and the vehicle unit, missing pulses from the sensor, a mismatch in the counter, or an unauthorised replacement of the sensor.
What is the difference between code 21 and code 16?
Code 16 means that the sensor itself has reported an internal problem after its own self-test. Code 21 concerns communication between the sensor and the vehicle unit — wiring, power supply or pairing — and does not necessarily indicate a fault in the sensor itself.
Can an unauthorised replacement of the motion sensor trigger code 21?
Yes. If the new motion sensor is not correctly paired with the vehicle unit at an approved workshop, the tachograph will report a motion sensor fault. This is one of the reasons why code 21 has a high priority and requires a workshop inspection rather than an attempted repair by you.