Cummins fault code 4242
DEF Temperature 2: erratic or intermittent signal
What it means
Cummins' description: Aftertreatment 1 Diesel Exhaust Fluid Temperature 2 - Data erratic, intermittent or incorrect.
SPN 3515 is Aftertreatment 1 Diesel Exhaust Fluid Temperature 2. FMI 2 means the signal is jumping around, dropping in and out, or doesn't make sense compared to other readings.
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
The sensor reading itself is out of range or the condition is real, so look at the system:
- 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
- Weak pump or blocked DEF filter
- Frozen DEF with a heater not working
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: QSX 15, 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
- 4233 SPN 3515 FMI 3
DEF Temperature 2 Sensor Circuit: circuit voltage high / shorted high - 4234 SPN 3515 FMI 4
DEF Temperature 2 Sensor Circuit: circuit voltage low / shorted low - 4243 SPN 3515 FMI 10
DEF Temperature 2 Abnormal Rate of Change: abnormal rate of change - 4743 SPN 3515 FMI 5
DEF Temperature 2 Sensor Circuit: circuit open / current low - 4744 SPN 3515 FMI 6
DEF Temperature 2 Sensor Circuit: circuit grounded / current high - 4745 SPN 3515 FMI 11
DEF Temperature 2: root cause not known - 6619 SPN 3515 FMI 10
DEF Temperature 2: abnormal rate of change
Related DEF codes
- 4244 SPN 4337 FMI 2
DEF Dosing Temperature: erratic or intermittent signal - 4245 SPN 5798 FMI 2
DEF Dosing Unit Heater Temperature: erratic or intermittent signal - 4235 SPN 3521 FMI 31
DEF Property: condition exists - 4249 SPN 4337 FMI 10
DEF Dosing Temperature: abnormal rate of change - 4251 SPN 5798 FMI 10
DEF Dosing Unit Heater Temperature: abnormal rate of change - 4277 SPN 3364 FMI 10
DEF Quality: abnormal rate of change
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 4242 mean?
Cummins fault code 4242 is J1939 SPN 3515 FMI 2: Aftertreatment 1 Diesel Exhaust Fluid Temperature 2 - Data erratic, intermittent or incorrect. In plain terms: def temperature 2: erratic or intermittent signal.
Can I keep driving with Cummins code 4242?
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 4242?
SPN 3515 identifies the part or system (Aftertreatment 1 Diesel Exhaust Fluid Temperature 2). FMI 2 describes the type of failure: erratic or intermittent signal.
What usually causes Cummins code 4242?
The most common causes are: 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.
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.