Re: PO401 OBD-II trouble code
There are many possibilities here.
1st check vacuum lines to ERG from EGR solenoid and then to manifold vacuum. Look for holes cracks or loose fittings.
2nd check hoses to DPFE valve.
3rd clean EGR tube
4th check flow thro DPFE and solenoid use manual to see how
5th check the solenoid for continuity
6th pull vac line from EGR while idling engine should stall. If not consider replacing 1 egr control solenoid 2 DPFE sensor 3 egr valve itself.
It took me 2 years to find out the check engine cause was the EGR solenoid
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