Cummins Fault Code 3251 is a critical aftertreatment code that signals your Diesel Oxidation Catalyst (DOC) is not operating efficiently. This usually appears with SPN 3251 FMI 0, which translates to a DOC efficiency issue that can lead to regen failure and eventual engine derate.

What Does Fault Code 3251 Mean?

• Fault Code: 3251

• SPN (Suspect Parameter Number): 3251

• FMI (Failure Mode Identifier): 0 – Above Normal Operating Range

• Description: Diesel Oxidation Catalyst Efficiency Below Threshold

Your ECM has determined that the DOC isn't reducing emissions as expected, which directly impacts the DPF’s ability to regenerate properly.

Symptoms of SPN 3251 FMI 0

• Check engine light or MIL

• Failed or incomplete regen attempts

• Frequent DPF regen requests

• Engine derate (if left unresolved)

• Poor DEF system performance

What Causes Cummins Code 3251?

1. Excess soot or ash in the DOC or DPF

2. Failed temperature or pressure sensors in the exhaust stream

3. Fuel injector or dosing valve issues during regen

4. Contaminated DOC (e.g., from coolant or oil intrusion)

5. Poor quality diesel fuel or long idle times

How to Fix Fault Code 3251 on Cummins

1. Run a Full Diagnostic Scan

   • Use diesel software like Cummins Insite or Universal Diag

   • Look for related codes like 1921, 3242, 3375, or 3563

2. Inspect DOC Temp Sensors & Pressure Sensors

   • Verify temperature rise during regen

   • Check for faulty wiring or out-of-range sensor readings

3. Perform a Forced Regen

   • A successful regen with proper DOC temperature rise confirms sensor and injector function

   • If DOC doesn’t reach required temp, efficiency is likely too low

4. Check for Ash Loading

   • Use your diagnostic software to read soot and ash levels

   • Excess ash = cleaning or replacement required

5. Replace DOC if Needed

   • If all systems are functioning but the DOC still reads inefficient, the catalyst is likely spent or damaged

Recommended Tools

• RP1210 adapter (Nexiq USB-Link 2, Inline 6)

• Diagnostic laptop with Cummins Insite or Universal Diag

• Infrared thermometer or thermal imaging for DOC temps

• Regeneration-capable software

Pro Tip:

If your DOC is contaminated by oil or coolant, no regen or cleaning will restore its performance—you’ll need to replace the DOC unit to clear this code long term.

Final Thoughts

Cummins Fault Code 3251 (SPN 3251 FMI 0) means your Diesel Oxidation Catalyst is underperforming. Whether due to soot, ash, sensors, or injector issues, diagnosing the true cause early can prevent costly downtime and expensive SCR damage. Use the proper tools, verify temps during regen, and act quickly if the DOC is beyond recovery.