1996 Tercel Code PO401

Hi my check engine light goes on for a couple days no sign of reduced engine performance, then goes out on its own. I checked the codes and got code P0401- EGR flow insufficient. Visually there is no sign of a problem at the EGR. The Tercel is in good shape about 87k miles and is used primarily for shopping trips maybe 35 to 50 mph operating speeds. Im guessing it may be some carbon blockage in one of the EGR passageways. If anyone as insight into this before I take here apart I would appreciate knowing more before I dig in.

I checked the passages for carbon it was relatively clean, no sign of carbon buildup. ERG vacuum hoses appear in good shape and ERG seems to be functioning normally. The check engine light has changed it comes on now and stays on until reset. The engine runs perfect with the light on or off but still sets PO401 code. I reset the code and took it in for a smog check it passed. Does anyone know what sensor originates the PO 401 code? It appears to only go on when the engine is warm operating a mid range RPM.

This is going to sound a little strange but take the egr valve off and start the car up. If it runs fine then you have carbon blocking off the passages. If it surges like crazy while the egr is off then everything is clear. Been there, done that with my 92 tercel.

Thanks, Im out of town right now when I get home Ill give it a try and see. Ive tried everything without luck.