Aftertreatment SCR Catalyst Conversion Efficiency - Below Threshold
The CUMMINS-3791 (Cummins) diesel fault code means: Aftertreatment SCR Catalyst Conversion Efficiency - Below Threshold. This is a moderate severity code.
- Keep driving?
- Reduced power (derate)
- DIY difficulty
- advanced
- Estimated cost
- $1,000-$4,500
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What Are the Symptoms of CUMMINS-3791?
- NOx sensor fault lamp on dash
- Engine derate after warning period
- Increased DEF dosing to compensate
- Aftertreatment efficiency warnings
- Failed emissions testing (CARB/EPA)
What Causes CUMMINS-3791? (Most Likely First)
- Failed NOx sensor (typical 150K-300K mile life) Very Likely
- Corroded NOx sensor harness or connector Likely
- NOx sensor contamination from DEF crystallization Likely
- SCR catalyst aged or damaged by overheat Possible
- Exhaust leak upstream of sensor Possible
- ECM or ACM calibration out of date Less Likely
Can I Still Drive with CUMMINS-3791?
Reduced power (derate). CUMMINS-3791 is rated moderate severity. Keep trips short and get it diagnosed soon rather than waiting for symptoms to worsen.
Frequently Asked Questions
What is Cummins fault code 3791?
Cummins fault code 3791 indicates: Aftertreatment SCR Catalyst Conversion Efficiency - Below Threshold. It is a moderate-severity fault in the aftertreatment or emissions system.
Can I drive with Cummins fault code 3791?
Short-term, yes, but the engine will enter derate (typically starting at 55 mph and progressing to 5 mph for emissions faults). Drive directly to a repair facility. Do not defer this fault more than a few operating hours.
How much does it cost to fix Cummins fault 3791?
Repair cost is approximately $1,000-$4,500 including parts, labor, and any dealer programming required. DIY repairs (sensor replacement) fall at the low end; pump, aftertreatment, or ECM work falls at the high end and usually requires INSITE diagnostic software.
What actually makes the computer set this NOx or SCR code?
California's heavy-duty OBD regulation requires the system to flag a NOx-converting catalyst when its conversion capability drops enough to push NOx emissions more than 0.4 g/bhp-hr above the applicable standard (Title 13 CCR 1971.1, section (e)(6.2.1)) — so on a 0.2 g/bhp-hr engine the trip point is around 0.6. Catalysts in series may be monitored individually or as a group, which is why a code can point at the aftertreatment as a whole rather than at one brick. This threshold comes from the heavy-duty OBD regulation, which covers 2010 and later heavy-duty engines. If your engine is in a pickup at or under 14,000 lb GVWR it is certified under the separate medium-duty rule instead, so treat this as the design intent rather than your engine's exact limit.
Mechanics Guide Step-by-step diagnosis
How Do You Diagnose CUMMINS-3791?
Connect INSITE and monitor both inlet and outlet NOx sensor live readings to identify the faulty sensor for 3791.
Inspect NOx sensor harness routing for damage from exhaust heat, chafing, or crystallized DEF contamination.
Measure NOx sensor heater circuit resistance and supply voltage at the module connector.
If sensor is more than 200,000 miles old with this fault, replacement is typically required -- NOx sensors have a finite service life.
Check SCR catalyst efficiency via INSITE active test; a failed SCR can mimic NOx sensor faults.
Verify exhaust leaks upstream of the sensor are repaired, as a leak will skew readings and trigger this code.
Sources 2
This page is built from documented references. Verify against your own service info before repair work.
- California Code of Regulations, Title 13 section 1971.1 (Heavy-Duty OBD)malfunction criteria, section (e)
- SAE J1939-73Application Layer: Diagnostics (defines the SPN/FMI fault-identifier framework)
Which Vehicles Commonly Get CUMMINS-3791?
- Peterbilt 579, 567 with Cummins ISX/X15
- Kenworth T680, T880 with Cummins X15
- Freightliner Cascadia with Cummins ISX15
- Ram 2500/3500 with 6.7L Cummins
- Freightliner M2 106 with Cummins ISL9/L9
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