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Check the EGR selenoid valve. The vacuum will pass throught the defective selenoid valve and go the EGR valve even if it has not been activated. The ECM will not detect a defective selenoid valve - only an EGR problem.

DPFE sensorIf it's a Ford product, you may need a DPFE sensor. You should also check for a restriction in the EGR passageway.
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โˆ™ 2015-07-17 17:55:32
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Q: What can you check or replace if the check engine light comes back on and the code is P401 and you have already replaced the EGR valve?
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