What does SPN 1781 FMI 1 (1781/1) mean?

Retarder Minimum Continuous Speed: data below normal range (most severe level).

What does SPN 1781 FMI 2 (1781/2) mean?

Retarder Minimum Continuous Speed: data erratic or intermittent.

Do not drive. Stop as soon as it’s safe to do so — don’t keep going.

Do not driveTo the nearest safe stop

J1939 code SPN 1781. Below — how the fault is worded at the factory by DAF, and what the second half of the code (FMI) means in the standard.

FMI 1	AdBlue level F723 level too low (extended period)
FMI 2	AdBlue level F723 implausible consumption
FMI 3	AdBlue level F723 voltage too high at ECU pin B49 (extended period)
FMI 4	AdBlue level F723 voltage too low at ECU pin B49 (extended period)
FMI 18	AdBlue level F723 low level

SPN 1781 FMI 11781/1 · 1781.1	data below normal range (most severe level)
SPN 1781 FMI 21781/2 · 1781.2	data erratic or intermittent
SPN 1781 FMI 31781/3 · 1781.3	voltage above normal or shorted high
SPN 1781 FMI 41781/4 · 1781.4	voltage below normal or shorted to ground
SPN 1781 FMI 181781/18 · 1781.18	below normal — moderate severity

Do not drive. Stop as soon as it’s safe to do so — don’t keep going.

Time you have: To the nearest safe stop

The code is flagged critical: the manufacturer considers it unsafe to keep driving without checking.

This code alone cannot say what will fail next, so stop at the first safe opportunity and look at it on the spot.

That said, the manufacturer flags this code as critical — if in doubt, it is safer to stop and check on the spot.

Scanner threw several codes at once? Paste the whole list — we will show which one is the root cause and which ones simply followed.