Cummins fault code 6484
DEF Line Heater 3 Circuit: circuit open / current low
What it means
Cummins' description: Aftertreatment Diesel Exhaust Fluid Line Heater 3 Circuit - Current below normal or open circuit.
SPN 4344 is Aftertreatment 1 Diesel Exhaust Fluid Line Heater 3 State. FMI 5 means the ECM is driving the circuit but almost no current flows, which usually means an open circuit: a broken wire, unplugged connector, or failed coil.
DEF is sprayed into the exhaust ahead of the SCR catalyst to break down NOx. The system includes the tank, level and temperature sensors, a quality sensor, heaters for cold weather, a pump or dosing unit, lines and the doser.
What the lamp means
This code normally doesn't turn on a dash lamp. It's logged in the ECM for a technician to see, and often explains another active code.
Green, yellow, red: three ways this can look
The same code can be a quick fix or a reason to shut down. Match your situation to the closest example.
Keep an eye on it
The DEF level warning is on and the truck runs normally. Topping off with fresh DEF clears it.
Schedule a repair
Amber lamp or a DEF quality or dosing code that comes back after filling. Get it checked soon. If left alone, the system will start counting down toward a derate.
Stop and get help
Blinking lamp with a derate countdown, or the truck is already limited to low speed. That's an EPA inducement. It won't fix itself and it can leave a truck stuck at 5 mph.
Common causes
FMI 5 is an electrical or signal fault, so start with the circuit:
- Corroded or loose connector at the DEF tank or dosing unit
- Failed DEF level, temperature or quality sensor (often built into one tank header)
- Open or shorted heater circuit or heater relay
- Harness damage near the tank or frame rail
If the circuit checks out, the reading may be real. Other possibilities: dEF tank low or empty; contaminated, diluted or old DEF (water, coolant, diesel, or DEF older than about a year); crystallized DEF clogging the doser, lines or filter.
What you can check before calling
- Check the DEF level and top off with fresh, sealed DEF
- Look for white crystal buildup around the doser, lines and tank
- If DEF was recently added, think about whether it could have been mixed with water or something else
- Note whether the code shows up only in cold weather
Clearing the code without fixing the cause usually brings it right back, and on emissions systems it can restart derate timers. For the bigger picture, read our guide: DEF warning lights and derate: what diesel drivers need to know.
Engines and sources
Listed by Cummins for: QSB 6.7, QSL 9, QSX 15, QSB 4.5, QSF 3.8 (With SCR), QSG 12. Cummins uses the same fault code numbers across its electronic engines, including on-highway models like the ISB/B6.7, ISL/L9 and ISX/X15, though not every code applies to every engine or software level. Source: Cummins Tier 4 Fault Codes. Always confirm with Cummins service information (QuickServe) for your engine serial number.
Same SPN, other failure types
- 3422 SPN 4344 FMI 3
DEF Line Heater 3 Circuit: circuit voltage high / shorted high - 3423 SPN 4344 FMI 4
DEF Line Heater 3 Circuit: circuit voltage low / shorted low - 3425 SPN 4344 FMI 5
DEF Line Heater 3 Circuit: circuit open / current low - 6535 SPN 4344 FMI 3
DEF Line Heater 3 Circuit: circuit voltage high / shorted high - 6536 SPN 4344 FMI 4
DEF Line Heater 3 Circuit: circuit voltage low / shorted low
Related DEF codes
- 6483 SPN 4342 FMI 5
DEF Line Heater 2 Circuit: circuit open / current low - 6482 SPN 4340 FMI 5
DEF Line Heater 1 Circuit: circuit open / current low - 6481 SPN 3363 FMI 4
DEF Tank Heater: circuit voltage low / shorted low - 6479 SPN 3363 FMI 3
DEF Tank Heater: circuit voltage high / shorted high - 6478 SPN 5491 FMI 4
DEF Line Heater Relay: circuit voltage low / shorted low - 6477 SPN 5491 FMI 3
DEF Line Heater Relay: circuit voltage high / shorted high
Look up another code
Cummins fault code (like 1922), J1939 SPN and FMI (like SPN 3251 FMI 0), or an OBD-II code (like P0420).
Questions
What does Cummins fault code 6484 mean?
Cummins fault code 6484 is J1939 SPN 4344 FMI 5: Aftertreatment Diesel Exhaust Fluid Line Heater 3 Circuit - Current below normal or open circuit. In plain terms: def line heater 3 circuit: circuit open / current low.
Can I keep driving with Cummins code 6484?
Usually, yes. This code doesn't normally turn on a lamp by itself, but check for other active codes that might be more serious. Look at the green, yellow and red examples on this page to judge your situation.
What is the SPN and FMI for Cummins code 6484?
SPN 4344 identifies the part or system (Aftertreatment 1 Diesel Exhaust Fluid Line Heater 3 State). FMI 5 describes the type of failure: circuit open / current low.
What usually causes Cummins code 6484?
The most common causes are: corroded or loose connector at the DEF tank or dosing unit; failed DEF level, temperature or quality sensor (often built into one tank header); open or shorted heater circuit or heater relay.
Diesel down in the Midlands?
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