I have an '02 F 150 with 4.6L engine about 60K miles.

The 'Check Engine Soon' light came on. My Scan Gauge II scanner was not working when I needed it, so I drove the truck to Advantage Auto. They scanned it for me and got the P0405 code. Clerk said it was most likely the EGR valve as his car displayed the same code, and replacing the EGR valve did the trick. Taking his advice, I bought an EGR valve, and installed it the next day. The clerk said they don't clear the codes, but said "warning light should go out on its own after you drive 75-100 miles". It didn't. The warning light stayed on for two weeks. By the way, there were no obvious drivability problems with the truck - it idled fine and drove well.

I then got my repaired Scan Gauge II back from the manufacturer. I checked for stored codes, and as expected got the P0405. I cleared it, and thought all might be well - not quite! When I restarted the truck, the 'Check Engine Soon' light came on again. If I cleared the code (the same P0405 always appeared), it would come back on after I restarted again and again.
Still no problem with drivability.

I've read about DFPE sensors going bad, and if you plan on replacing it, buy a Ford OEM part, as opposed to a generic auto parts sensor. So what would be the next logical step? I'm into about $55 thus far for the new EGR valve, but I need to get the Check Engine light to stay off if I want to sell the truck soon.

PS: I know the truck doesn't have many miles on it, but this is the very first problem I've ever had with this truck since I bought it. I've probably been lucky, but I'm quite pleased with Ford quality!!

Thanks for any ideas.