Volvo D13 SPN 5246 FMI 31 �?SCR Conversion Efficiency Low

Code Details

Plain-English Meaning

The Volvo D13 SCR system calculates conversion efficiency by comparing NOx levels at the catalyst inlet and outlet. When the downstream sensor shows too much NOx exiting the catalyst relative to what enters, the EMS logs low efficiency and starts the inducement countdown. Long-haul Volvo trucks can accumulate inducement distance quickly at highway speeds. On Volvo VNL and VHD trucks with the D13, the SCR efficiency calculation in VCADS Pro uses NOx sensor data from before and after the SCR catalyst substrate. A D13 SCR catalyst that is physically intact but has aged past its service life may pass a visual inspection but fail the efficiency threshold — only the NOx sensor ratio test in VCADS Pro can confirm this. Volvo recommends evaluating D13 SCR catalyst replacement on engines with more than 600,000 miles that are failing efficiency-based fault codes despite good DEF quality and dosing system function.

SPN 5246 is Aftertreatment 1 SCR Conversion Efficiency in J1939. FMI 31 indicates the condition exists. On Volvo D13, the EMS monitors both upstream and downstream NOx sensors and calculates a conversion ratio over a defined window. Causes of low efficiency include catalyst substrate degradation, poisoning from oil or coolant contamination, chronic DEF quality issues, dosing system faults, and NOx sensor drift. VCADS Pro provides the diagnostic data needed to identify the most likely cause.

Common Symptoms

• Amber check engine lamp active
• SCR system or aftertreatment warning on the Volvo instrument cluster
• Inducement counter running in VCADS or Remote Diagnostics
• No immediate driveability change at initial fault set

Possible Causes

Possible causes may include the items below. The list is not a parts diagnosis.

• SCR catalyst substrate degraded from high-mileage operation or a thermal event
• Catalyst poisoning from oil or coolant contamination in the exhaust stream
• Chronic DEF quality issues reducing conversion performance over time
• DEF dosing system fault causing insufficient reagent injection
• NOx sensor drift on either the upstream or downstream sensor

First Checks

• Connect Volvo VCADS Pro and review all active codes, NOx sensor live data, and DEF quality and dosing history
• Compare upstream and downstream NOx sensor readings at normal operating temperature
• Test DEF dosing module function via VCADS service routines
• Inspect exhaust system for oil burning or coolant contamination sources
• Compare upstream and downstream NOx sensor readings in VCADS Pro to calculate the actual SCR efficiency ratio
• If catalyst age and mileage suggest end of life, evaluate catalyst replacement before further diagnostic steps

Can I Keep Driving?

The Volvo D13 can typically continue operating with this fault active, but the inducement timer is running. At long-haul highway speeds, the distance accumulates quickly. Diagnose and resolve before the derate threshold is reached to avoid disruption during a trip.

Related Lookup Pages

• SPN 5246 topic page
• FMI 31 explanation
• Aftertreatment fault codes
• Search this code