EGR stands for exhaust gas recirculation, a vehicle emissions control concept used in both gasoline and diesel engines. The EGR valve — which works differently depending on how old the car is and whether it uses gasoline or diesel fuel — is a key component to a car’s exhaust system and engine health. What Are the Symptoms of a Bad EGR Valve? What happens when the valve goes bad? Here’s what you need to know about your vehicle’s EGR valve: EGR Valves Take a Lickin’ EGR valves fail over time because of normal wear and tear since they’re in constant use. A buildup of carbon particles, which could clog the components, also causes them to fail over time. The Telltale Knock When the EGR valve fails, you may hear knocking noises in the engine and notice reduced fuel economy from your vehicle. Delphi says owners may also feel reduced engine performance that will show up in ways like poor acceleration and rough idle. Check-Engine Light On? Check Engine As with most engine component issues, a faulty EGR valve will also likely trigger the vehicle’s check-engine light. Obviously, the check-engine light illuminates for a long list of issues, so the next step in diagnosing the issue is to have your vehicle’s computer scanned for trouble codes.