need help with refrigerator

I have just replaced a burnt out heating strip for my freezer on my GSH25SGPA SS model, the fan still works but the heating strip will not come on to defrost the coils so we have to do it manually with hot water, the heating did come on once since it's been replaced, but not since. What could it be?

I would check defrost thermostat and evaporator thermistor. I see on your model there is a Thermistor on the evaporator. that runs to your pc board. It should open and close a relay to your defrost thermostat. The defrost thermostat goes to your defrost heater. They all work in combination. Either one could be bad. usually an ohm meter should be O.K. to check thermistor and defrost thermostat, you might check the ohm reading on the heater also, it should read higher in range. The thermistor will have varying Ohm readings with ambient temperature. The defrost thermostat should be closed so a low ohm reading should be noticed. I would always check simple things first. Make sure all connections, thermistor is in place, defrost thermostat is hooked up good, since you did some work on it, you might double check and make sure you hooked everything back up O.K.