Cummins fault code 4155
DEF Dosing Unit Heater Relay: circuit voltage high / shorted high
What it means
Cummins' description: Aftertreatment 1 Diesel Exhaust Fluid Dosing Unit Heater Relay - Voltage Above Normal, or Shorted to high source.
SPN 5746 is Aftertreatment 1 Diesel Exhaust Fluid Dosing Unit Heater Relay. 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: QSB 6.7, QSL 9, QSX 15, QSB 4.5, QSF 3.8 (With SCR). 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
- 4157 SPN 4376 FMI 7
DEF Return Valve: not responding / out of adjustment - 4168 SPN 5745 FMI 3
DEF Dosing Unit Heater: circuit voltage high / shorted high - 4169 SPN 5745 FMI 4
DEF Dosing Unit Heater: circuit voltage low / shorted low - 4171 SPN 5745 FMI 18
DEF Dosing Unit Heater: reading below normal (moderately severe) - 4174 SPN 4337 FMI 3
DEF Dosing Temperature Sensor: circuit voltage high / shorted high - 4175 SPN 4337 FMI 4
DEF Dosing Temperature Sensor: 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 4155 mean?
Cummins fault code 4155 is J1939 SPN 5746 FMI 3: Aftertreatment 1 Diesel Exhaust Fluid Dosing Unit Heater Relay - Voltage Above Normal, or Shorted to high source. In plain terms: def dosing unit heater relay: circuit voltage high / shorted high.
Can I keep driving with Cummins code 4155?
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 4155?
SPN 5746 identifies the part or system (Aftertreatment 1 Diesel Exhaust Fluid Dosing Unit Heater Relay). FMI 3 describes the type of failure: circuit voltage high / shorted high.
What usually causes Cummins code 4155?
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.