Q: 1998 Mazda protege: Had the EGR valve replaced, cleared the computer codes, still get the check engine light then a EGR problem co

Code P0400 does not necessarily mean that there is something wrong with the EGR valve itself. The only way to condemn the EGR valve is to directly test it. In particular Code P0400 includes a circumstance where the EGR passages are simply blocked with carbon and thus little or no exhaust gases can flow into the intake manifold. Code P0400 also includes the possibility that the EGR valve is not opening due to no vacuum opening the valve from the EGR control solenoid. If the valve is electronically controlled, there could be a fault in the circuit. If your vehicle is equipped with an EGR temperature sensor, if the sensor is plugged with carbon, it can’t register a temperature change when the EGR valve is opened, signifying flow. The point is Code P0400 can never be interpreted as the EGR valve has failed. The only way to determine if an EGR valve has failed is to directly test the valve. To resolve this, that is perform a complete system diagnostic and find the root cause for the code having set, the recommended service is an EGR system diagnostic. If you request that service, the responding certified mechanic will get this fully resolved for you. If you have further questions or concerns, do not hesitate to re-contact YourMechanic as we are always here to help you.