Code Details
| Display code | SPN 3563 FMI 15 |
|---|---|
| SPN | 3563 |
| FMI | 15 |
| OEM code | None listed |
| Manufacturer | Cummins |
| System | Aftertreatment �?DEF system |
| Component | DEF tank level sensor |
| Source address | Unknown or not applicable |
| Severity | low |
| Review status | source backed |
| Source confidence | medium |
| Last reviewed | 2026-06-11 |
Plain-English Meaning
The DEF level sensor in the tank continuously monitors how much fluid is available for the SCR system. When the level falls to the warning threshold �?typically around 10 to 15 percent of tank capacity �?the ECM logs this code and alerts the driver. If the tank runs out completely, the inducement sequence activates and the engine will eventually derate.
SPN 3563 is Aftertreatment 1 DEF Tank Level in J1939. FMI 15 indicates the value is valid and at or below the low-level warning threshold. On Cummins CM2350 ISX15/X15 engines, the DEF level is reported continuously by the tank level sensor. The warning threshold is calibrated to give the operator advance notice before the critical level that triggers inducement is reached. FMI 17 (below critical) or FMI 1 (quality fault) can follow if the issue is not addressed.
Common Symptoms
- DEF low-level warning lamp or message on the dash
- Inducement counter may begin in Cummins Insite if the level has already dropped below the inducement trigger
- No immediate power loss at the low-level warning stage
Possible Causes
Possible causes may include the items below. The list is not a parts diagnosis.
- DEF tank has been depleted through normal system consumption and has not been refilled
- DEF level sensor fault reading low when the actual level is acceptable (less common)
- DEF leak from a supply line, module, or dosing injector causing faster-than-expected consumption
First Checks
- Refill the DEF tank with ISO 22241 compliant DEF �?this is the standard response when the truck is genuinely low on DEF
- Confirm the DEF fill quantity matches the expected tank capacity to verify the level sensor is reading correctly
- If DEF consumption seems higher than expected, inspect the DEF supply lines, module, and dosing injector area for leaks
- Connect Cummins Insite and monitor the tank level live data to confirm the sensor reading increases after filling
Can I Keep Driving?
At the low-level warning, the vehicle can continue operating normally. The warning is advance notice to refill DEF before the level triggers inducement and a potential derate. On a long haul, the driver should refill DEF at the next available opportunity.
Related Codes
Related Lookup Pages
Sources
- Cummins INSITE Service Tool — Public Reference Documentation Cummins Inc. · oem · accessed 2026-06-11 · confidence medium
Source: Cummins Inc., Cummins INSITE Service Tool — Public Reference Documentation. This page paraphrases factual fields only and is not a substitute for the original document.
Open source - QuickServe Online Cummins Inc. · oem · accessed 2026-05-05 · confidence medium
Source: Cummins Inc., QuickServe Online. This page paraphrases factual fields only and is not a substitute for the original document.
Open source - SAE J1939 Standards Collection SAE International · official · accessed 2026-05-05 · confidence medium
Source: SAE International, SAE J1939 Standards Collection. This page paraphrases factual fields only and is not a substitute for the original document.
Open source
FAQ
How much DEF does a typical Class 8 truck with a Cummins ISX15 use per mile?
DEF consumption is typically 2 to 5 percent of diesel fuel consumption by volume. For a truck consuming 7 miles per gallon, that translates to roughly 0.3 to 0.7 gallons of DEF per 100 miles. A typical 20 to 30-gallon DEF tank may last 3,000 to 10,000 miles depending on the application, ambient temperature, and engine load.
How long after SPN 3563 FMI 15 sets on the ISX15 before the engine starts to derate?
The timing depends on how quickly the remaining DEF is consumed. At normal consumption rates, a truck might have 500 to 2,000 miles remaining after the low-level warning. If the driver continues without refilling, the level will reach the critical threshold that triggers the inducement sequence. The distance varies by tank size and application.
Can the DEF level sensor give a false low-level reading on the Cummins ISX15?
Yes. A stuck or failed DEF level sensor can report a low level when the tank is actually full. If the low-level warning appears immediately after a known full tank fill, or if the truck refill quantity does not match the expected deficit, the level sensor should be inspected using Insite live data to verify accurate operation.