95 Crown Vic: Check Engine light, code P0401

I have a 95 Crown Vic with 65K miles. A few days ago the CEL light came on (steady, not blinking). The repair shop read a code of P0401. After some more testing the DPEF sensor was found to be faulty and was
replaced. This was followed by a "quick test (OK)" and a "road test (OK)"
After I had picked up the car, the light came on again after about 2 miles. I did some reading on the internet and found reports indicating that these symptoms are often caused by plugged tubes that route the exhaust from the EGR to the intake manifold. I brought the car back to the shop with a print-out of the internet reports.
The code was again P0401. The following were checked: - electrical part of circuit (OK) - EGR solenoid operation (OK) - EGR valve operation, diaphragm (OK) - EGR valve effect on engine (could not notice any change) - pressure from reference tube at EGR exhaust tube (slight pressure difference)
Then the EGR tube was cleaned. The EGR operation was "checked as per manual and found to be normal". There was a "road test extended monitor DPFE sensor parameters and percentage of EGR opening: operation is normal.
The PCV (not EGR) hose was found to be collapsing at throttle body and was replaced. The PCV was also replaced.
The invoice took into account that I had already paid to have this problem fixed and that this was a "redo".
However, after I had picked up the car and driven 12 blocks the light came on again. Any idea as to what is going and any recommendation (besides the obvious)? TIA
Roger
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P0401 Insufficient EGR Flow. No vacuum to EGR solenoid......EGR valve stuck.........DPFE sense hoses damaged...........EGR solenoid although passes electrically, does not work mechanically......................

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Something was not done correctly, the light should not have come on after just 2 miles, or 12 blocks of driving, unless you had at least two start up to operating temp. followed by a cool down.

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snipped-for-privacy@compusmart.ab.ca wrote:

Thanks to all who replied - good information. I now know much better what's going on.
Roger
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