The glow bulb on my heater not working, only had heater about 3 weeks.

Notify seller about return replace policy

Since there is no model, serial, or manufacturer listed, let's look at generic process. When call for heat, there are usually a few processes, like a combustion fan and a proving switch that the fan is working. Once that is satisfied, a current is sent to the glow plug to cause it to heat. This is when you check for the correct amount of power being sent to it. And it is providing that the proving process is complete. If there is current and no heat, check the amp draw (if you have the meter to do so.) If no amp draw or if you do not have a meter that measures amps, this would be the time to Kill power to the unit, unplug or disconnect the wires to the glow plug and check for continuity or ohms resistance. If resistance is present, you need to know what is the proper amount to have. Trane usually has about 80-100 ohms on their commercial roof top units. If there is no resistance or continuity, the glow plug is bad and needs replaced.