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First you want to press the "RECEIVER" button near the top of the Pioneer remote and test if the volume buttons work.
Next I would try unplugging the Pioneer unit for at least 5 minutes.
Next I would try performing a Factory Reset as described in the user manual. See page 38:
Next use a digital camera to test if the remote is SENDING any signals when pressing those buttons- point the tip of the remote into the lens of a digital camera, watch the camera screen as you press buttons on the remote. You should see a flashing light coming from the tip of the remote. If you see that then the remote is sending a signal.
If all of that fails, the last resort is replacing the remtoe. They are hard to find but can be purchased on eBay:
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