... P0401...EGR flow insufficient.
....
From 1996 Ford CD,
Diagnostic Trouble Code (DTC) P0401 indicates that Continuous Memory Self-Test has detected insufficient EGR flow.
Possible causes:
-- EGR valve stuck closed or iced.
-- EGR valve diaphragm leaks.
-- EGR valve or flow path restricted.
-- EGR vacuum hose off, plugged or leaks.
-- VPWR circuit open to EGR Vacuum Regulator (EGR V.R.) solenoid.
-- EVR circuit to PCM open.
-- EVR circuit to PCM shorted to PWR.
-- Differential Pressure Feedback EGR (D.P.F. EGR) sensor pressure hoses both off.
-- D.P.F. EGR sensor pressure hoses reversed.
-- D.P.F. EGR sensor VREF circuit open.
-- Downstream pressure hose off.
-- Downstream pressure hose plugged.
-- Damaged orifice tube assembly.
-- Damaged EGR V.R. solenoid.
-- Damaged D.P.F. EGR sensor.
-- Damaged PCM.
I suggest you do the EGR suck test (vacuum test) to determine if it is the DPFE or EGR which is the problem.
Edit. From Old Forum
EGR vacuum test.
"(remove the green vacuum hose from the top, be careful as it is brittle, and apply a vacuum (attached a small hose and suck on itl) to see if the car stumbles while running. This indicates a properly operating EGR.
Does it work properly?
NO: The EGR valve is defective or the EGR passages are clogged. Check (and clean, if needed) the passages behind the Throttle Body. Purchase a TB gasket, and remove the air plenum (rubber accordion tube) then remove the TB. Clean the EGR passage (small cut-out in the outer edge of the TB's mating surface) really well, install a new gasket and re-attach the TB. If the passages are clear, the EGR valve is defective. While rare, they do fail.
YES: The EGR valve works and the passages are clear. The likely fault is the DPFE sensor or its sample hoses, followed by the EVR solenoid or the PCM. May also be caused by interconnecting vacuum lines or electrical connections."