Cummins fault code 1621
Auxiliary Pressure Sensor Input 1 Circuit: circuit voltage low / shorted low
What it means
Cummins' description: Auxiliary Pressure Sensor Input 1 Circuit - Voltage below normal, or shorted to low source.
SPN 1387 is Auxiliary Pressure #1. 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.
These codes cover extra inputs and outputs the ECM controls or monitors: auxiliary temperature and pressure sensors, relays, lamps, and generator or equipment controls on off-highway and generator engines.
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
An auxiliary code for an accessory that isn't used on your truck.
Schedule a repair
An accessory, gauge or lamp not working with the code set. Fix the circuit when convenient.
Stop and get help
The code is tied to a shutdown or protection relay. Have it checked before relying on that protection.
Common causes
FMI 4 is an electrical or signal fault, so start with the circuit:
- Relay or driver circuit open or shorted
- Auxiliary sensor or wiring fault
- Equipment-side switch or gauge wiring problem
If the circuit checks out, the reading may be real. Other possibilities: real reading from the equipment the input is monitoring.
What you can check before calling
- Find out what the auxiliary input is wired to on your truck or equipment
- Check connectors and relays in that circuit
Clearing the code without fixing the cause usually brings it right back, and on emissions systems it can restart derate timers.
Engines and sources
Listed by Cummins for: QSB 6.7, QSL 9, QSX 15, 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 Auxiliary inputs, outputs and generator controls codes
- 1432 SPN 4223 FMI 31
Pre-High Engine Temperature Warning Relay Driver Diagnostic has detected an error: condition exists - 1429 SPN 4187 FMI 31
High Engine Temperature (HET) Shutdown Relay Driver Diagnostic has detected an error: condition exists - 1427 SPN 4185 FMI 31
Overspeed Shutdown Relay Driver Diagnostic has detected an error: condition exists - 2195 SPN 703 FMI 14
Auxiliary Equipment Sensor Input # 3 (OEM Switch) Engine Protection: special instructions - 779 SPN 703 FMI 11
Auxiliary Equipment Sensor Input # 3 (OEM Switch): root cause not known - 2557 SPN 697 FMI 3
Auxiliary PWM Driver 1 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 1621 mean?
Cummins fault code 1621 is J1939 SPN 1387 FMI 4: Auxiliary Pressure Sensor Input 1 Circuit - Voltage below normal, or shorted to low source. In plain terms: auxiliary pressure sensor input 1 circuit: circuit voltage low / shorted low.
Can I keep driving with Cummins code 1621?
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 1621?
SPN 1387 identifies the part or system (Auxiliary Pressure #1). FMI 4 describes the type of failure: circuit voltage low / shorted low.
What usually causes Cummins code 1621?
The most common causes are: relay or driver circuit open or shorted; auxiliary sensor or wiring fault; equipment-side switch or gauge wiring problem.
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.