Cummins fault code 4789
Fan Speed: reading above normal (most severe)
What it means
Cummins' description: Fan Speed - Data Valid but Above Normal Operational Range - Most Severe Level.
SPN 1639 is Fan Speed. FMI 0 means the sensor is working, but the value it reports is far above where it should be. This is the most severe 'high' level.
The cooling fan pulls air through the radiator and charge air cooler. On most trucks it's controlled by a fan clutch the ECM switches on and off, and some have speed sensors or variable pitch blades.
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 fan code that set once with normal engine temperatures.
Schedule a repair
Temperatures creep up at idle or with A/C on, or the code keeps returning. Get it checked before summer heat finds it for you.
Stop and get help
Engine overheating because the fan isn't engaging. Shut it down and let it cool before driving.
Common causes
The sensor reading itself is out of range or the condition is real, so look at the system:
- Fan clutch worn and slipping
- Air leak to an air-operated fan clutch
- Fan stuck on or off
What you can check before calling
- Listen for the fan engaging when the engine is hot or the A/C is on
- Look for damaged wiring or air lines at the fan clutch
- Watch whether temperature climbs at idle or in traffic
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: Car overheating: what to do right now, and what's causing it.
Engines and sources
Listed by Cummins for: QSB 6.7, QSL 9, QSX 15, QSB 4.5, QSF 3.8, 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
Related Cooling fan codes
- 5183 SPN 6799 FMI 3
Fan Blade Pitch Position Sensor Circuit: circuit voltage high / shorted high - 5184 SPN 6799 FMI 4
Fan Blade Pitch Position Sensor Circuit: circuit voltage low / shorted low - 5185 SPN 6799 FMI 7
Fan Blade Pitch: not responding / out of adjustment - 3634 SPN 5484 FMI 4
Engine Fan Clutch 2 Control Circuit: circuit voltage low / shorted low - 3633 SPN 5484 FMI 3
Engine Fan Clutch 2 Control Circuit: circuit voltage high / shorted high - 6263 SPN 647 FMI 3
Fan Control Circuit: 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 4789 mean?
Cummins fault code 4789 is J1939 SPN 1639 FMI 0: Fan Speed - Data Valid but Above Normal Operational Range - Most Severe Level. In plain terms: fan speed: reading above normal (most severe).
Can I keep driving with Cummins code 4789?
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 4789?
SPN 1639 identifies the part or system (Fan Speed). FMI 0 describes the type of failure: reading above normal (most severe).
What usually causes Cummins code 4789?
The most common causes are: fan clutch worn and slipping; air leak to an air-operated fan clutch; fan stuck on or off.
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.