Cummins fault code 5866
DEF Dosing Unit Relay Feedback- Voltage Above Normal or Shorted to High Source.: circuit voltage high / shorted high
What it means
Cummins' description: Aftertreatment Diesel Exhaust Fluid Dosing Unit Relay Feedback- Voltage Above Normal or Shorted to High Source..
SPN 520953 is DEF Dosing Unit Relay Feedback- Voltage Above Normal or Shorted to High Source.. FMI 3 means the voltage on the circuit is higher than possible for a good sensor, usually an open signal wire, a short to a power wire, or a failed sensor.
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
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
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 3 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: 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 DEF codes
- 5278 SPN 6802 FMI 31
DEF Dosing System Frozen: condition exists - 6475 SPN 3363 FMI 7
DEF Tank Heater: not responding / out of adjustment - 6476 SPN 3363 FMI 18
DEF Tank Heater: reading below normal (moderately severe) - 6477 SPN 5491 FMI 3
DEF Line Heater Relay: circuit voltage high / shorted high - 6478 SPN 5491 FMI 4
DEF Line Heater Relay: circuit voltage low / shorted low - 6479 SPN 3363 FMI 3
DEF Tank Heater: 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 5866 mean?
Cummins fault code 5866 is J1939 SPN 520953 FMI 3: Aftertreatment Diesel Exhaust Fluid Dosing Unit Relay Feedback- Voltage Above Normal or Shorted to High Source.. In plain terms: def dosing unit relay feedback- voltage above normal or shorted to high source.: circuit voltage high / shorted high.
Can I keep driving with Cummins code 5866?
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 5866?
SPN 520953 identifies the part or system (DEF Dosing Unit Relay Feedback- Voltage Above Normal or Shorted to High Source.). FMI 3 describes the type of failure: circuit voltage high / shorted high.
What usually causes Cummins code 5866?
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?
We work on Cummins-powered pickups, vans and medium-duty trucks across Lexington, Columbia and the Midlands, on site at your yard or roadside.