Check the switched supply and exact model rating before resistance diagnosis.
Check that the switch or lighting circuit controlling the defogger is on. Identify the exact defogger model and voltage from its label; standard products and custom heaters must not be assumed to share every rating.
Electrical access and testing are for a qualified electrician. Isolate all involved circuits and verify they are de-energized before opening boxes, disconnecting conductors or measuring resistance. Energized voltage measurements require appropriate electrical safety precautions.
The electrician should distinguish missing supply, a switch/wiring fault and a heater fault. Resistance measurements require the defogger leads to be disconnected from the supply circuit.
For the nominal resistance of each standard defogger, see the technical article “Mirror defogger resistance values and EMF”. The installation manual and the current sell sheet agree on these values; the 30 × 40-inch FFM-3040, for example, is 86 Ω (141 W, 1.28 A at 110 V). Custom defoggers can differ, so compare against the exact model on the label.
Do not cut, fold or repair the heating film, bypass protection, or apply a voltage different from the unit’s rating. Retain the label, installation details and measured results for WarmlyYours technical support at (800) 875-5285.
Sources: mirror-defogger installation manual and current mirror-defogger sell sheet.
