getting rid of check engine light(P0401)

I have 2001 Windstar 3.8L of which check engin light has been on for a month. I went to Autozone and had the engine code read. It was 'P0401 EGR flow is insufficient'.
So, I reseted PCM and the light was turned off but turned on again when I accelerate a little bit more on the uphill road.
So, I made some test. I disconnected the vacuum tube from the EGR valve and applied some vaccum to EGR valve by my mouth. The engine stumbled. I thought the valve was OK. Then I repeated to open and close the throttle while putting my finger on the end of vacuum tube. I definitely felt the difference of the vacuum strength.
I could not find the reason. So, I decided to take the valve apart.
I loosened the bolt on the orifice tube at the EGR side. But, I could not loosen the other bolts. Because I didn't have right tool. I decided to tighten the bolt on the orifice tube. But, problem was there. I cannot make it tighten easily. Starting the tightening the bolt, it wouldn't get into the right positioin. Anyway I barely tighten the bolt and made it sure that there were no leakage from them. And, I reseted the PCM again.

Questions are followings:
1. I'm not sure the bolt was properly. I didn't apply my full muscle to tighten it because it could wear the thread if the bolt were not tighten properly. Is it OK, if there is a little leakage from the joint between EGR valve and the orifice tube? At least I cannot feel it with my hand.

2. Anyway the light has been turned off so far. But, I drove only 10 minute after resetting the PCM. If it turn on later, what could be a reason?

3. How much is the cost for engine diagnostics in usual auto service? I heard it is about $50 in this area.

A common cause of the P0401 code is a defective DPFE (Differential Pressure FEedback) sensor.
In english....this is what measures the EGR flow....and is what is telling the PCM that the flow is insufficient. The DPFE has 2 hoses going to the metal tube the runs from the front exhaust manifold to the EGR valve. This is an easy part to change.

The other likely cause is insufficient EGR flow....as is being indicated.....which is most often caused by clogged EGR ports in the lower intake manifold. You have to remove the upper intake manifold in order to get to them to clean them.

Our Windstar threw this code a couple weekends ago. At the same time, fuel mileage plummeted. I replaced the DPFE later that week and the CE light went off two days afterwards (about 4 drive cycles).
The DPFE is cheap at the dealer compared to somewhere like Autozone. I paid $30.07+tax for part number 4U7Z-9J460-AA. AZ wanted around $80 for it.
There are no tools required to replace it. It's staring right at you when you open the hood. Unplug it, pull it off (pay attention if the hoses come with it, they are different sizes) and plug the new one in.

Depending on your mileage you may be able to get Ford to replace the part for you. I can't remember the exact website (I think it was the Ford 4X4 truck forums) but they show you how to do a quick voltage check on the DPFE sensor to prove it faulty or not. The sensor on my '01 Windstar failed last year (and put out the P0401 code) and I went to the dealer, showed the service advisor the code on my ODBII scanner, told him I ran the voltage checks on the sensor, and they replaced it for free. I think this sensor may be covered under the emissions warranty.

Thanks for kind answers...
After resetting the PCM, I drove for about 20 minute. The engine warmed up enough. And then the check engine light was turned on again when I accelerated on uphill road... Any paticular guess for the reason from this symptom?

Any paticular guess for the reason from this symptom?

Ummm, I thought we gave you a couple reasons for the behavior you are experiencing.

The EGR system only activates under certain conditions. It does NOT activate at idle.....but does activate under certain drive cycle conditions....
The PCM has to be calling for a certain amount of EGR flow......and then not see a reading indicating a lack of the requested flow.....in order to determine that there is not enough flow.
In other words.....it is not an on/off switch....but a request for a specific amount of flow through the EGR system that the PCM is looking for.

I described the test procedure a while ago. It will allow you to measure the voltages from the DPFE. Do a search under DPFE.

It is a commonly failed item and it is almost worth your while to just replace it without spending too much time testing but that is your call. As Wiswind suggested it could also be partially clogged EGR ports restricting the flow but more than likely it is the DPFE