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EGR stand for exhaust gas recirculation and it exists to reduce nitrous oxide emissions by recirculating a small amount of exhaust gas back through the combustion chambers.
There is an exhaust tube that goes to the egr valve, the valve itself, and then passages inside the intake manifiold that distribute the exhaust evenly to each cylinder.
To control the valve there is an engine vacuum source, a computer controlled valve to control when vacuum gets to the egr valve, and then the egr valve itself.
You have a new EGR valve.
If you managed to drive 4-500 miles without the code resetting, one of a few things come to mind
A) the mechanic fixed your car 400 miles ago and now something else in the EGR system broke.
B) More likely if the egr and its controls were ok and the mechanic mis-diagnosed the problem 400 miles ago, and for it to have lasted this long without setting a code it is probably a clog impacting only one or two cylinders.
Regardless, it should be treated as a new problem.
1) visually inspect the vacuum hoses, pull them off the tubes they attach to and check inside for cracks than can bleed vacuum.
2) put a vacuum gauge on the hose that goes to the valve, and make sure there is vacuum under a slight acceleration. Depending upon how sophisticated the 97 egr control system is, you might have to put it into drive. There should not be vacuum until the engine warms up, but once warmed up there should be vacuum every time you accelerate. If there is not, then the egr control solenoid or the ecm that controls the solenoid is bad.
If there is vacuum;
3)Put the car in park and with the engine running apply a little vacuum to the egr valve and open it. The engine should stumble and stall. This proves that egr valve can open, the tube from the exhaust to the egr is clear and at least a portion of the passages in the intake manifold are clear.
If the engine does not stumble/stall then the egr did not open and it is no good, (new does not equal good) or the tube from the exhaust side is clogged or the passageways inside the intake manifold are completely clogged. If the tube or the intake had been completely clogged when the egr was replaced you almost certainly would not have driven 400 miles without the light coming on, so most likely there is a partial blockage to one or more cylinders in the intake manifold.
This video I found on the internet shows how to clean it, I do not know your car is exactly the same as the honda he is working on, but it is similar.

A big chunk of carbon may also have just randomly broken loose from and blocked the flow of exhaust through the new egr valve, and some cars use screens under the egr valve to prevent a chunk of carbon from getting into the egr valve and jamming it open. Over time the screens can dirty and clogged.

But my guess is you need to do the cleaning service as shown in the video.

Hope that helps,
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