Hi, new member here. I was on a business trip when I got the dreaded "Service DEF System See Dealer" message. I was in a remote location on the Oregon coast and the only dealer around was booked for 2 months. I had a code reader and was able to clear the codes a couple of times, but then I got the 200 mile countdown to no-start. I was still over 800 miles from home. Long story short, I made it home (ask me how) and decided that I probably needed a new DEF tank with the reductant quality module built in. That's about $3,100 plus labor at the dealership.
I decided to at least save myself some money and replace the DEF tank myself. So I drained the tank and removed the tank and harness. I almost ordered a new DEF tank but I thought I'd do a continuity check on all the pins/wires in the harness and that's when I found the root cause of all the check engine lights and DEF warnings. One of the wires to the DEF quality module wasn't passing current. I pulled back the wire loom and unwrapped the harness and that's when I found a wire that had been chafing and was corroded completely through.
The harness routing to the DEF tank is a poor design. The harness is so tight as it comes around from the outside of the frame to the tank, it rubs on the frame. I'm sure that most left this way from the factory. You should go check yours. After I repaired the wire harness with a watertight splice, I redrilled the mounting holes for the main disconnect about 1" towards the rear of the truck to give the harness more slack so that it wasn't chafing on the frame. I buttoned everything back up and filled the tank. Started the truck up and no more DEF warning. The "permanent" check engine light went away after a short drive.
I decided to at least save myself some money and replace the DEF tank myself. So I drained the tank and removed the tank and harness. I almost ordered a new DEF tank but I thought I'd do a continuity check on all the pins/wires in the harness and that's when I found the root cause of all the check engine lights and DEF warnings. One of the wires to the DEF quality module wasn't passing current. I pulled back the wire loom and unwrapped the harness and that's when I found a wire that had been chafing and was corroded completely through.
The harness routing to the DEF tank is a poor design. The harness is so tight as it comes around from the outside of the frame to the tank, it rubs on the frame. I'm sure that most left this way from the factory. You should go check yours. After I repaired the wire harness with a watertight splice, I redrilled the mounting holes for the main disconnect about 1" towards the rear of the truck to give the harness more slack so that it wasn't chafing on the frame. I buttoned everything back up and filled the tank. Started the truck up and no more DEF warning. The "permanent" check engine light went away after a short drive.
