NO HEAT

disassembled and tested heated, high-limit thermostat, thermal cut-off, okay and no shorts to ground. tested duct thermal cut-off okay, thermistor 10 kilohms. reassembled and checked main control board, no visible burn outs checked voltage across heater relay on main board. 240 volts about 1 second after unit starts(indicating that once motor starts, relay is open, getting voltage through heater and stats) I thought the relay was not closing, so I changed control board, same problem, I don’t have wiring diagram or service manual ,
got service manual (thank you) no error codes, test mode and service mode did not help, software version mode flashes 9L & 12 . high limit and regulating thermostat are okay, don’t know what membrane switch is, but I replaced user interface board and thermistor , still no heat, heater relay on control board never closes checked thermistor at control board wiring harness – okay checked moisture sensor at control board; no reading without clothes 1 ohms with nickle taped to sensor(to make sure no wires are broken).
I’ve replaced control board, user interface board and thermistor still no heat, heater relay on control board not closing????? WHY?????