Diagnostic Overview
Cummins fault code 3134 is commonly associated with SCR conversion efficiency. The ECM has determined that the SCR catalyst is not reducing NOx emissions to the expected level under monitored conditions. Common causes include contaminated or diluted DEF, improper DEF dosing, a restricted or crystallized doser, faulty inlet or outlet NOx sensors, exhaust leaks, or a degraded SCR catalyst. Diagnosis should begin with DEF quality, DEF dosing quantity, NOx sensor comparison, and exhaust leak inspection before replacing the catalyst.