Cummins fault code 633
Low #3 RB Cylinder Power: fault
What it means
Cummins' description: Low #3 RB Cylinder Power.
SPN 1145 is Low #3 RB Cylinder Power. FMI (45) means the ECM detected a fault in this system.
The ECM needs clean, steady battery voltage, and it supplies regulated 5-volt power to many sensors. Voltage problems can cause strange codes across many systems at once.
What the lamp means
An amber (yellow) warning lamp means the engine can usually keep running, but the fault should be repaired soon, ideally the same day.
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 low-voltage code after a hard winter start or after the truck sat a while, then clears.
Schedule a repair
Codes return, the truck is slow to crank, or voltage reads low while driving. Test the charging system soon.
Stop and get help
Engine dies, several systems drop out at once, or the ECM loses power. Don't run it until the power supply is fixed. It can leave you stranded.
Common causes
The sensor reading itself is out of range or the condition is real, so look at the system:
- Weak batteries
- Failing alternator
- Parasitic drain
What you can check before calling
- Check battery terminals and ground straps for corrosion or looseness
- Have the batteries and alternator tested
- Note if several unrelated codes appeared at the same time
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: Bad alternator or bad battery? How to tell.
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.
- Older Cummins engines may report this code as SPN 1329 FMI 1 (Low #3 RB Cylinder Power).
Same SPN, other failure types
Related Battery, power supply and sensor supply codes
- 632 SPN 1144 FMI (45)
Low #2 RB Cylinder Power: fault - 634 SPN 1146 FMI (45)
Low #4 RB Cylinder Power: fault - 631 SPN 1143 FMI (45)
Low #1 RB Cylinder Power: fault - 635 SPN 1147 FMI (45)
Low #5 RB Cylinder Power: fault - 636 SPN 1148 FMI (45)
Low #6 RB Cylinder Power: fault - 637 SPN 1151 FMI 18
Low #7 RB Cylinder Power: reading below normal (moderately severe)
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 633 mean?
Cummins fault code 633 is J1939 SPN 1145 FMI (45): Low #3 RB Cylinder Power. In plain terms: low #3 rb cylinder power: fault.
Can I keep driving with Cummins code 633?
Usually for a short time, if the engine runs normally and no other warnings are on. Get it diagnosed soon, because many amber codes lead to a derate if ignored. Look at the green, yellow and red examples on this page to judge your situation.
What is the SPN and FMI for Cummins code 633?
SPN 1145 identifies the part or system (Low #3 RB Cylinder Power). FMI (45) describes the type of failure: fault.
What usually causes Cummins code 633?
The most common causes are: weak batteries; failing alternator; parasitic drain.
Diesel down in the Midlands?
We work on Cummins-powered pickups, vans and medium-duty trucks across Lexington, Columbia and the Midlands, on site at your yard or roadside.