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?
- Yes, but fix soon
- DIY difficulty
- advanced
- Estimated cost
- $1,000-$4,500
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Common Symptoms
- NOx sensor fault lamp on dash
- Engine derate after warning period
- Increased DEF dosing to compensate
- Aftertreatment efficiency warnings
- Failed emissions testing (CARB/EPA)
Probable Causes (Ranked by Likelihood)
- 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- ECM or ACM calibration out of date Less Likely
Step-by-Step Diagnostic Procedure
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.
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.
Sources
This page is built from documented references. Verify against your own service info before repair work.