Cummins fault code 1668
DEF Tank Level Sensor Circuit: circuit voltage low / shorted low
What it means
Cummins' description: Aftertreatment 1 Diesel Exhaust Fluid Tank Level Sensor Circuit - Voltage below normal, or shorted to low source.
SPN 1761 is Aftertreatment 1 Diesel Exhaust Fluid Tank Level. FMI 4 means the voltage on the circuit is lower than possible for a good sensor, usually a short to ground, a damaged harness, 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 4 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), 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
- 1669 SPN 1761 FMI 3
DEF Tank Level Sensor Circuit: circuit voltage high / shorted high - 1673 SPN 1761 FMI 1
DEF Tank Level: reading below normal (most severe) - 1699 SPN 1761 FMI 2
DEF Tank Level Sensor: erratic or intermittent signal - 3497 SPN 1761 FMI 17
DEF Tank Level: reading below normal (least severe) - 3498 SPN 1761 FMI 18
DEF Tank Level: reading below normal (moderately severe) - 4677 SPN 1761 FMI 9
SAE J1939 Multiplexing PGN Timeout Error: abnormal update rate - 4679 SPN 1761 FMI 5
DEF Tank Level Sensor Circuit: circuit open / current low - 4732 SPN 1761 FMI 13
DEF Tank Level Sensor: out of calibration
Related DEF codes
- 1677 SPN 3031 FMI 4
DEF Tank Temperature Sensor: circuit voltage low / shorted low - 1678 SPN 3031 FMI 3
DEF Tank Temperature Sensor: circuit voltage high / shorted high - 1679 SPN 3031 FMI 2
DEF Tank Temperature: erratic or intermittent signal - 1682 SPN 3362 FMI 31
DEF Dosing Unit Input Lines: condition exists - 1683 SPN 3363 FMI 3
DEF Tank Heater: circuit voltage high / shorted high - 1684 SPN 3363 FMI 4
DEF Tank Heater: 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 1668 mean?
Cummins fault code 1668 is J1939 SPN 1761 FMI 4: Aftertreatment 1 Diesel Exhaust Fluid Tank Level Sensor Circuit - Voltage below normal, or shorted to low source. In plain terms: def tank level sensor circuit: circuit voltage low / shorted low.
Can I keep driving with Cummins code 1668?
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 1668?
SPN 1761 identifies the part or system (Aftertreatment 1 Diesel Exhaust Fluid Tank Level). FMI 4 describes the type of failure: circuit voltage low / shorted low.
What usually causes Cummins code 1668?
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.