Modern off-road machinery depends on a complex network of emission control components to stay compliant, fuel-efficient, and protected against costly breakdowns. When a “Check Emission System” warning appears, it signals that a key part—such as the EGR valve, DPF, SCR/DEF system, or NOx sensor—is no longer operating within safe parameters. This guide breaks down what the warning truly means, the most common causes behind it, and how mechanical failures, sensor malfunctions, or maintenance issues can trigger derates, excess fuel use, or smoke.