Re: how do I fix 2001 grand marquis showing obd code 401?
Insufficient EGR flow? Sounds like a stuck/sticking EGR valve. In the old days the EGR valve was attached to a spacer that was sandwiched between the carburetor and the intake manifold. At the back of the spacer was a vacuum-controlled diaphragm - it wasn't tiny, and easy to spot. This controlled a valve.
But now we're talking about much newer technology. I haven't seen a 2001, but you can bet the EGR valve will be found somewhere on the intake manifold, since the purpose of the EGR valve (Exhaust Gas Recirculation) is to re-route some of the exhaust emissions back into the intake to be re-burned. It could still potentially be vacuum-controlled, but I would suspect it could also be electronic, too. Consult with a shop manual, or a knowledgeable person who owns a 2001...
Regardless of the technology, if something is telling the computer there is insufficient EGR flow, you will very likely be faced with replacing the EGR valve. It's also possible to have a blocked passageway in the EGR system, but to have a faulty EGR valve isn't uncommon (and this isn't restricted to Crown Vics).
You'll be happy to know that replacement of this valve is not a difficult procedure.