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Cummins ISX Emissions Faults Explained

An active emissions fault on a Cummins ISX derates the engine or counts down to a shutdown. Either way, the machine stops working. The difficulty is that the ISX aftertreatment system has four fault-generating circuits: the EGR valve, NOx sensor, DPF, and DEF injection, and their fault codes overlap in ways that consistently produce misdiagnosis. Replacing an expensive NOx sensor when the actual fault is an EGR pressure differential issue is common and avoidable. This guide explains what each circuit does, what each component looks like when it fails, and the diagnostic order that identifies the correct fault before anything is replaced. For EGR valve replacements compatible with Cummins ISX engines, visit our EGR valve replacement for your machine page.