Michael Thompson Key Takeaways
- F-7 and F-9 mean stop using the appliance: a welded relay passes power the control cannot switch off, and an over-temperature trip means something got too hot.
- F-1 and F-2 are the same temperature sensor failing in opposite directions — open reads too cold, shorted reads too hot.
- F-5 is the humidity sensor, so auto-cook programs become unreliable while manual power and time cooking stays fine.
- F-3 is often condensation or grease behind the touchpad rather than a failed membrane — clean, dry, and power-cycle before assuming.
- Never open a microwave cabinet: the capacitor holds a dangerous charge long after the appliance is unplugged.
The Bottom Line
Read the code for safety first. F-7 and F-9 get the breaker; the sensor codes can wait for an appointment; F-3 is worth cleaning and power-cycling before anyone is called. Sparking, buzzing, or running with the door open is not a code and is not something to work around.
LG microwave error codes are unusually readable once you know that the F numbers are grouped by what kind of thing failed rather than by where it sits. Two of them are the temperature sensor, one is the keypad, one is the humidity sensor, one is a relay, and one is over-temperature protection. That grouping answers the question owners actually have, which is not what the code means but whether the appliance is safe to keep using while they arrange something.
| Code | What it names | Safe to keep using? |
|---|---|---|
| F-1 | Temperature sensor, open circuit | Yes — book a repair |
| F-2 | Temperature sensor, shorted | Yes — book a repair |
| F-3 | Touchpad membrane shorted | Often not usable at all |
| F-5 | Humidity sensor, auto-cook functions | Yes — manual modes still work |
| F-7 | Relay stuck or welded closed | 🔴 No — switch it off |
| F-9 | Over-temperature protection tripped | 🔴 No — switch it off |
Two Codes Mean Stop
Most of the list is a component that has stopped reporting correctly, and a microwave with a failed sensor is inconvenient rather than hazardous. Two are different.
F-7 is a stuck or welded relay. A relay that has welded closed is passing power when the control has told it to stop, and that is the fault class you do not work around. Switch the appliance off at the outlet or the breaker.
F-9 is over-temperature protection. Something got hotter than it should have. The protection did its job, and the condition that caused it is still present. Both of those belong in the same category as a car's oil light: the warning is not the problem.
Where the Appliance Lives Changes What Fails
An over-the-range unit and a countertop unit of the same model do not fail the same way, and it is worth knowing which you have when reading this list. Mounted above a cooktop, the appliance takes steam, heat, and grease-laden air onto its underside every time somebody cooks below it — and the humidity sensor, the touchpad membrane, and the control all sit in that path.
That is why the sensor and keypad codes turn up disproportionately on over-the-range units, and it is also why the single most effective preventive measure is running the vent fan while the cooktop is on.
The Sensor Codes Are the Common Ones
F-1 and F-2 are the same part failing in opposite directions — an open circuit reads as impossibly cold, a short reads as impossibly hot — and both make the control do the wrong thing with total consistency. What you notice is sensor-driven programs behaving oddly while manual power-and-time cooking is fine.
F-5 is the humidity sensor, which is what the auto-cook and popcorn-style programs use to decide when food is done. When it fails, those programs are unreliable and everything else is untouched. That is worth knowing because it means the appliance remains genuinely useful while you decide, which is not true of every fault.
F-3, and the One Thing to Try First
A shorted touchpad membrane means the control believes a key is being held down. Sometimes it is right, and the cause is not the membrane at all:
- Clean the panel with a barely damp cloth and dry it completely. Grease and steam behind a membrane produce exactly this.
- Look for physical damage — a dent, a bubble, a lifted corner.
- Power-cycle once. Unplug for five minutes and restore. A membrane fault will return; a condensation problem often will not.
- On an over-the-range unit, check the steam. A microwave mounted over a cooktop takes steam onto its control panel every time somebody boils something, and a vent fan that is not being used makes that worse.
That last point matters more than people expect, and our range and oven maintenance guide covers the cooking side that produces it.
The Reset That Is Worth One Try
Before treating any of these as a fault, unplug the appliance for five minutes and plug it back in. A brief flick at the switch does not do it — the control holds charge, and it needs long enough to lose it.
What that distinguishes is genuinely useful. A code caused by a supply glitch, a brief brownout, or condensation after a steamy session will often not return. A code caused by a component that has failed comes straight back, usually within the first cycle. One power cycle is diagnosis; repeated power cycling to keep using an appliance that keeps reporting a fault is not, and on the relay and over-temperature codes it is actively unwise.
The Thing Not to Do
A microwave capacitor stores a dangerous charge after the appliance is unplugged, and it does not bleed off on its own. This is the one appliance in a domestic kitchen where opening the cabinet to look is genuinely hazardous, whatever your experience with everything else in the house.
If the unit sparks inside, buzzes loudly under load, smells of burning, or runs with the door open, switch it off at the breaker and leave it. Those are not codes, and they are not symptoms to work around.
Getting It Repaired
Diagnostic visits start from $89, and on a microwave the diagnosis is decisive — the high-voltage side and the sensing side test cleanly one way or the other. The final price is confirmed after the on-site diagnosis, with a written estimate before any work begins.
Our LG microwave repair service covers the sensing circuits, the control, and the door interlocks. If you have F-7 or F-9, book a visit and leave the appliance off at the breaker until then.