ISX15 / X15
Cummins ISX15/X15: DPF pressure and fault codes 1921/1922
Does your Cummins show FC 1921 or 1922? Both relate to high differential pressure in the documented ISX15 table, but a high reading alone does not identify the part that needs repair. Save the SPN, FMI and other present codes before deciding on the next step.
How to read SPN 3251 on a Cummins ISX15
Scroll the table to see all columns.
| SPN / FMI - FC | What it indicates | Where to focus the inspection |
|---|---|---|
| 3251 / 3 FC 1879 | Differential pressure electrical signal too high. | Circuit / supply [1] |
| 3251 / 4 FC 1881 | Differential pressure electrical signal too low. | Circuit / return [1] |
| 3251 / 2 FC 1883 | Erratic or inconsistent differential pressure data. | Measurement reliability [1] |
| 3251 / 16 FC 1921 | High differential pressure; moderate severity. | Pressure and loading context [1] |
| 3251 / 0 FC 1922 | High differential pressure; highest severity. | Follow the exact OEM diagnostic branch [1] |
X15: when the 1883/5715 combination with 1921/1922 matters
Cummins bulletin TSB220250 describes moisture in the sensor that can cause false faults 1921, 1922 or 1981. Its criteria combine those faults with 1883 or 5715, either active or with more than one inactive count in the last 25 operating hours. It covers the listed X15 CM2350 X114B/X116B and CM2450 X124B/X134B applications, not all X15 engines. Source 2.
That is why a photo showing only “1922” leaves out useful information. Preserve the complete report before clearing codes. The bulletin also addresses damaged, obstructed, bent or leaking tubes and loose connections; checks and repairs must follow the applicable procedure.
What to save before asking for help
Prepare a report that identifies your vehicle and captures the fault context. If the equipment shows several codes, save the complete list; an isolated number can hide the information needed to select the procedure.
- Engine model, serial number and emissions generation.
- Complete code, description, module and active or inactive status.
- Name of each pressure parameter, value, unit and engine condition.
- Observed symptom and when it occurs: startup, idle or operation under load.
- Recent repairs and what changed after they were performed.
Do not compare two screenshots if the units or measurement conditions differ. This list helps prepare your inquiry; limits and tests come from the applicable service manual.
Frequently asked questions
Do FC 1921 and FC 1922 mean the same thing?
The 2013 ISX15 table associates both with high differential pressure but distinguishes severity: FMI 16 and FMI 0, respectively. That difference alone is not enough to choose a repair.
Does FC 1879 mean the DPF is plugged?
In the cited reference, it indicates high circuit voltage. A high electrical signal and high physical pressure are different findings.
Can I apply this table to any X15?
No. Confirm the ESN, CM identification and calibration. The ISX15 table and X15 bulletin in this article have separate scopes.
Sources and scope
Accessed October 6, 2026. These references identify specific applications; confirm the current engine-specific procedure before making repairs.
Torque Edge · Service video
Explore Torque Edge diagnostic support
A presentation of less than a minute, in English, about supporting shops with difficult repairs. It complements an inquiry; it is not a repair procedure for FC 1921 or FC 1922.
Initial phone guidance
Need help with your DPF code?
Request a free initial call. Tell us what your truck is showing so we can help identify the next step. Any additional diagnosis or repair is quoted separately.
By submitting, you ask Torque Edge to contact you about this inquiry.
You can also call (800) 697-7626.
More DPF pressure guides
Detroit DD13/DD15/DD16: SPN 3251 and DPF pressure
Detroit SPN 3251 FMI 0, 16, 2 and 20: EPA10, GHG14 and GHG17 differences, DOC/DPF pressure and information to save before diagnosis.
PACCAR MX-13: DPF pressure, P3762, P3770 and P3794
P3762, P3770 and P3794 on the PACCAR MX-13 EPA2013: electrical signals, outlet pressure and DPF differential pressure. What to check before applying them to an MX-11.
What SPN 3251 identifies · SPN/FMI code finder · FMI 0–31 meanings