The answer first
A LG Signature that turns, lights and does not heat is usually either on a reduced power setting or has a fault on its high-voltage side. The ranking below is ordered for this brand; the water test is the first thing to do and it takes a minute.
What you observe
- Heating is weaker than it used to be, not absent — That usually means a power level, a programme, or a much larger load than the oven is being credited for.
- A buzz or a hum while it runs, and no heat — The oven is trying. That points at the high-voltage side rather than at a setting.
- Food comes out exactly as cold as it went in — Confirm it with a mug of water rather than with food, which can mislead.
- NEAR MISS — it will not start at all — That is a starting fault, and the page for an oven that will not start ranks those causes.
The ranking that matters
- The supply the oven is running on — A microwave draws heavily. On an undersized extension lead, or a circuit shared with other heavy appliances, the voltage at the oven sags and the heating suffers while everything else appears to work. First on this brand: this oven is plugged in wherever there is a socket, and an undersized extension lead starves it while the display carries on as normal. LG Signature is the built-in end of the LG range: the same platform lineage, but installed into cabinetry, which changes what can go wrong and what it takes to get at it.
- The power level or the programme — Ovens remember a reduced power level from the last time it was used, and defrost programmes run at low power by design. The oven then behaves perfectly and heats very little, which is read as a failure. Next in order: it takes one full-power water test to eliminate.
- A door switch that has half-failed — The interlock switches do not only stop the oven: one of them controls whether the high-voltage side is allowed to run. A switch that has worn can leave the oven turning, lighting and never producing heat, which is exactly the symptom on this page. Third on this list: it is real but it is behind the panel, which is where the owner's part ends.
- The load and how it is being heated — A large, dense or frozen load takes far longer than people expect, and a cold dish pulls heat out of the food as fast as the oven puts it in. Neither is a fault, and both produce food that comes out disappointingly cool. Further down: it explains weak heating rather than no heating.
Check in this order, and why
- Plug the oven directly into a wall socket and repeat the water test — An oven that heats properly off the extension lead has told you the supply was the problem, not the oven.
- Set full power explicitly and run the water test — Select the power level rather than trusting the default, then a mug of cold water for a short run.
- Note whether the oven has ever needed the door pressing to run — A history of the door needing pressure points at the interlock, and that is worth telling a technician before they arrive.
What you can do yourself
- Take the oven off any extension lead and test again — Plug it straight into a wall socket with nothing else heavy on the same circuit.
About 10 min
- Run a mug of cold water at full power and feel it — Use a plain ceramic mug and stand by the oven. Warmer water means the oven heats and the fault was a setting.
Tools: plain ceramic mugAbout 10 min
- STOP — never remove the cabinet of a microwave oven — The capacitor inside can hold a dangerous charge long after the oven is unplugged. There is no owner check behind that case.
Before you open anything
Safety
Unplug the oven at the wall before cleaning anything inside it, and leave the cabinet on. A microwave capacitor can hold a dangerous charge long after the oven is unplugged, which is why no owner check on this page involves removing the outer case. Stop using the oven immediately if it arcs, smells of burning, or the door no longer closes square.
The line between a fix and a call
Call — or replace the oven — once the power level is confirmed at full, the supply is proven and a mug of water still comes out cold. That points at the high-voltage side, which is licensed work. Never run the oven empty to test it.
Nearby pages
The limits of this page
Generalized information. Written for LG Signature microwave ovens as a class rather than one family or model line; a family-specific page, where one exists, is the more precise answer and is linked above. Not covered here: internal component part numbers and figures that vary between families.
Lead Appliance Repair Technician · 12 years experience
Experienced technician with 12 years specializing in washing machine and dryer repairs across all major brands.
