I posted this over in the PCM section, but I thought maybe the truck guys might have some ideas....I am stumped

I pulled the fuel pump module on my truck and installed a dual in tank pump setup yesterday. I reused the existing gauge sender and have never had any probs. After I finished install, I fired it up and noticed the low fuel light was on and the gauge was on empty. I pulled it all apart again to check the floater, thinking I must have messed it up during the install. Anyways, all was well and I checked the output of the sender with an ohm meter and it is ok.

I put it all back together and fired it up again and the check engine light starts flashing so I hook up A-tap and it has codes 300 (random misfire) and 463 (fuel sender high voltage, or something) Service manual says that when 463 sets the gauge will default to empty and the low fuel light will come on.

My diagnostic capabilities are limited with the A-tap, but like I said the sender checks out with a dvom and if I clear the codes the gauge works fine til the codes return. What in the heck is going on here??