Do not driveTo the nearest safe stop
J1939 code SPN 1548. Below — how the fault is worded at the factory by International, and what the second half of the code (FMI) means in the standard.
| FMI 0 | HVAC Duct Temperature — HVAC Outlet Temp Sensor reading above normal range |
| FMI 1 | HVAC Duct Temperature — HVAC Outlet Temp Sensor reading below normal range |
| FMI 3 | HVAC Duct Temperature — Duct temperature sensor voltage high |
| FMI 4 | HVAC Duct Temperature — Duct temperature sensor voltage low |
Do not drive. Stop as soon as it’s safe to do so — don’t keep going.
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.