It can affect mileage, but it might not be the valve. EGR systems tends to get clogged by gunk. Take the valve off and see how dirty things are. If the system is clogged up, take apart as much as you can to clean it up. You can test the valve for operation with a good scanner (autozone doesn't do this--they just tell you the code). Here are instructions from autozone.com:
Linear EGR Valve Diagnosis
The linear EGR valve diagnosis procedure varies depending on the vehicle make and model year. Always follow the recommended procedure in the vehicle manufacturer's service manual. The scan tester may be used to diagnosis a linear EGR valve. The engine should be at normal operating temperature prior to EGR valve diagnosis. Since the linear EGR valve has an EVP sensor, the actual pintle position may be checked on the scan tester. The pintle position should not exceed 3% at idle speed. The scan tester may be operated to command a specific pintle position, such as 75%, and this commanded position should be achieved within two seconds. With the engine idling, select various pintle positions and check the actual pintle position. The pintle position should always be within 10% of the commanded position. When the linear EGR valve does not operate properly:
- Check the fuse in the 12 V supply wire to the EGR valve.
- Check for open circuits, grounds, and shorts in the wires connected from the EGR valve winding to the PCM.
- Use a digital voltmeter to check for 5 v on the reference wire to the evp sensor.
- Check for excessive resistance in the EVP sensor ground wire.
- Leave the wiring harness connected to the valve, and remove the valve. Connect a digital voltmeter from the pintle position wire at the EGR valve to ground, and manually push the pintle upward. The voltmeter reading should change from approximately 1 V to 4.5 V.
Linear EGR valve circuit. Courtesy of Chevrolet Motor Division-GMC.
If the EGR valve did not operate properly on the scan tester and tests 1 through 5 are satisfactory, replace the valve.
Here is what your valve looks like:
If you have more questions, please let me know.