I have a coleman evcon gas furnace mod.7970c856. I have problems with gas valve staying on after thermostat kicks out. It also over heats without limit switch kicking it out.how can the solenoid in the valve still have power? Its not the valve or the thermostat because both have been replaced 3 times.?

Valve sticking? Voltage still being applied to keep valve open? Wiring wrong?

You need to call a professional. What you have here is a very dangerous situation that you are not qualified to check out or repair.

Anyone trying to give you information on this unless at your home would be a fool. Turn off the gas and call for service.

There is a relay that could also be sticking. Check it with a meter, don't just guess like you have been doing.

Below is a common parts list for the following furnace models:
7956C859, 7970C859, 7975C859, 7995C859 (Presidential, propane)
7956C856, 7970C856, 7975C856, 7995C856 (Presidential, natural gas)
7956B856, 7966B856, 7970B856, 7975B856, 7995B856 (Presidential, natural gas)
7956A856, 7966A856, 7970A856, 7975A856, 7995A856 (Presidential, natural gas)
7956-856, 7966-856, 7970-856, 7975-856, 7995-856 (Presidential, natural gas)
PART NUMBER DESCRIPTION USED ON MODELS USED ON SERIES
7956A5851 Burner Assembly All All
2631-3481 Thermocouple All All
7945-1671/B Pilot Tube All All
7956A3771 Blower Control All B
7995-3169 Booster Motor 7995 All
7945-3551 Transformer 115-24V, 15VA All All
7966-311P Blower Motor 7956, 7970, 7975 All
1468-212P Blower Motor 7995 All
8665-5701 Booster Relay 7995 All
7624A3591 Upper Limit Switch (manual) All All
7975-3281 Fan Switch All All
7956-5771 Damper Assembly All C
7956-336P Gas Valve All All
7945-3281/A Lower Limit Switch (0-140, C-110) 7956 All
7970-3281/A Lower Limit Switch (0-145, C-115) 7966, 7979, 7975 All
7995-3281/A Lower Limit Switch (0-150, C-120) 7995 All

Here is a parts list for your furnace just in case you neeed one.

Pull the wire off the gas valve to see if the furnace shuts down.