Handwarmer error
Had the same problem and beat my head against the wall for such a simple problem... so first off you got it right, its the handwarmers, and the best way to figure out where the problem is, by just tracing the wires to see which go to the handwarmers, they will be in a pair, mine are black. Then just disconnect them and take turns pluggin each side in seperately... good chance its just one. You'll know you have it when you start up the sled and the error is gone.