I have a coleman 3115b-750 that runs but does not get very warm. It has power at all 6 element terminals.I removed the elements and checked with an ohm meter. All 3 read 12 ohm. With the furnace running and looking up at the elements there are only a few red spots,not the complete elements.Don't know what to check next.

If you have voltage 230v not 115v. You need to check amp draw on each element to make sure its working. Voltage going to the elements is broke on one leg of power going through the sequencer and limit switch. The sequencer is what times the blower and elements on one at a time and on shut down the sequence is reversed.