Ford f250 2008 6.4 diesel, truck has codes and won't pass the SMOG check due at the end of this month. Code P041D complained about egr temperature sensor too high. After replaced the egr cooler and egr, and code came and disappeared, but the code P0684 persists after a few deletes. Considering I never replaced the glow plug, and possibility the control module did get damaged during a quick battery positive and negative accidental touch, I decided to replace the glow plugs, wire harness and the control module.
***The plan:
Replace: 1) locate control module, wire harness and plugs; 2) disconnect negative battery, find old cable and red rubber seal, put it on. Remove wheel liners, remove driver side wheel. 3) remove air box to gain access. 4) drive side wire harness and plugs, find tubing, testing plug for ohms, put new one in. Start 8, then 6, 4, and 2. 5) passenger side wire harness, 1,3,5,7. 6) control module replacement.
Put back air box, put back battery. Test drive. Put back wheel liner.
***The do:
Start at 10 am Saturday, truck parked half way in garage half in drive away. first one is #8 on driver side. w wheel off, reaching is at ease. bungee cord to open up more space for manuvoring w socket and turning. finding the plug and first unscrew turn was easy. inserting tubing and pulling out was quite nervous fearing the plug would drop inside. ohms testing of old plug and new plug doesn't show parts is bad.
completed driver side then had a good lunch. 
passenger side was little more involved as I was warned due to tight space near the transmission tub and ac line.
was able to unscrew and open up a few inch space just enough to insert the 10 mm socket. #5 took more than 1 hour to replace as I kept missing feeling the plug. when putting back new plug, feel the same in finding the right hole.
#3 and #1 ought to be a breeze. it turned out a nightmare, the plug was inserted and stuck inside the valve cover. it was about 2 inch in, I tried needle pry, screw driver, anything I can think of and have access, it just stuck inside. 5:30 in the afternoon w truck half way inside garage. my options are to remove the valve cover, to keep trying to take out the plug, or to leave it until the next day. I wish I didn't start this. Considering parts to remove valve cover will be several hours plus the risk of damaging things, it appears the only option is find a means to take out the plug. I thought about fishing line, then the masonary line came to mind. I tested knotting and pulling by the plug just below the connector. inserting the loop using a metal wire that I tie rebar, the loop was closed, pull, and there it came out. a moment of joy and thus the shot of that picture. 
took less than half an hour to install the plugs, putting wire harness, then replacing the control module.
by 8 pm I drove out the truck.
a day of 10 hours wrestling. scary I may have lost 3 lbs of weight.
***The testing drive:
cleared code this morning and drove to work about 30 miles, no cel yet. Truck drives normal, I conclude the sanding sand is from the new egr cooler.
It seems we constantly facing confined space, pressing for time and how much we are willing to put to it. After the plug change, I sure know a little more about my truck. Takes so long to be a master of anything.
