Cummins fault code 678
Cyl #8 LB Exhaust Temp Sens Failed Low: circuit voltage low / shorted low
What it means
Cummins' description: Cyl #8 LB Exhaust Temp Sens Failed Low.
SPN 1144 is Cyl #8 LB Exhaust Temp Sens Failed Low. FMI 4 means the voltage on the circuit is lower than possible for a good sensor, usually a short to ground, a damaged harness, or a failed sensor.
Large Cummins engines can have an exhaust temperature sensor at each cylinder's exhaust port. Comparing them lets the ECM spot a cylinder that is running hot, cold, or not firing.
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
A single exhaust temperature sensor code with the engine running smoothly.
Schedule a repair
The code keeps returning or several cylinders show imbalance. Book diagnosis. It can hide a real cylinder problem.
Stop and get help
Exhaust temperature imbalance with rough running, smoke or a stop lamp. Shut it down. A cylinder running very hot can burn a piston or valve.
Common causes
FMI 4 is an electrical or signal fault, so start with the circuit:
- Exhaust temperature sensor (thermocouple) failed
- Damaged sensor wiring from heat
- Sensor module or connector fault
If the circuit checks out, the reading may be real. Other possibilities: real imbalance from a weak injector or misfire in that cylinder; exhaust leak at the manifold; valve problem in that cylinder.
What you can check before calling
- Look for heat-damaged wiring along the exhaust manifold
- Note any rough running or smoke
Clearing the code without fixing the cause usually brings it right back, and on emissions systems it can restart derate timers.
Engines and sources
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: Diagnostic Fault Codes for Cummins Engines. Always confirm with Cummins service information (QuickServe) for your engine serial number.
Same SPN, other failure types
- 628 SPN 1144 FMI 18
Low #8 LB Cylinder Power: reading below normal (moderately severe) - 632 SPN 1144 FMI (45)
Low #2 RB Cylinder Power: fault - 648 SPN 1144 FMI 0
High #8 LB Cylinder Exhaust Temperature: reading above normal (most severe) - 652 SPN 1144 FMI (30,45)
High #2 RB Cylinder Exhaust Temperature: fault - 668 SPN 1144 FMI 16
High #8 LB Cylinder Power: reading above normal (moderately severe) - 712 SPN 1144 FMI (45)
High #2 RB Cylinder Power: fault - 722 SPN 1144 FMI (30,45)
Cyl #2 RB Exhaust Temp Sens Failed Low: fault
Related Exhaust temperature sensors codes
- 677 SPN 1143 FMI 4
Cyl #7 LB Exhaust Temp Sens Failed Low: circuit voltage low / shorted low - 676 SPN 1142 FMI 4
Cyl #6 LB Exhaust Temp Sens Failed Low: circuit voltage low / shorted low - 675 SPN 1141 FMI 4
Cyl #5 LB Exhaust Temp Sens Failed Low: circuit voltage low / shorted low - 674 SPN 1140 FMI 4
Cyl #4 LB Exhaust Temp Sens Failed Low: circuit voltage low / shorted low - 673 SPN 1139 FMI 4
Cyl #3 LB Exhaust Temp Sens Failed Low: circuit voltage low / shorted low - 672 SPN 1138 FMI 4
Cyl #2 LB Exhaust Temp Sens Failed Low: circuit voltage low / shorted low
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 678 mean?
Cummins fault code 678 is J1939 SPN 1144 FMI 4: Cyl #8 LB Exhaust Temp Sens Failed Low. In plain terms: cyl #8 lb exhaust temp sens failed low: circuit voltage low / shorted low.
Can I keep driving with Cummins code 678?
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 678?
SPN 1144 identifies the part or system (Cyl #8 LB Exhaust Temp Sens Failed Low). FMI 4 describes the type of failure: circuit voltage low / shorted low.
What usually causes Cummins code 678?
The most common causes are: exhaust temperature sensor (thermocouple) failed; damaged sensor wiring from heat; sensor module or connector fault.
Diesel down in the Midlands?
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