i got a p0401 code, and my check engine light is on.how do i fix this problem? on 1999 Toyota Avalon

i took apart the egr valve, was not cloged, cleaned the sensor it worked fo a week then the same problem returned.

Asked by for the 1999 Toyota Avalon
in your case its usually not the egr valve itself.check for restrictions in the piping heading to the egr valve.
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I had the same problem.... But After driving with the ECT PWR button on the dash, the check engine light disappeared after about 100 miles or so (maybe a little more) and never showed again. It may not be a guaranteed solution for your car but certainly a possibility if you have this ECT PWR button.