LG F4 fault codewhat it means and how to fix it
A humidity sensor error in LG’s words. It affects the automatic sensor programmes rather than ordinary timed cooking.
Take care
Get a qualified appliance engineer to look at your LG F4
We do not pass enquiries to a repair firm, so there is no form here. When you book a qualified appliance engineer, tell them the make, the model and the exact code or message on the display, and ask whether they carry parts for this make — that is what turns one visit into a repair rather than a quote.
What F4 means
LG publish F4 as "a Humidity Sensor Error" and say it requires service. What the code costs you is narrower than most, and that is genuinely useful to know before you decide what to do about it. A humidity sensor is how a microwave runs its automatic programmes.
When you press reheat or a popcorn or potato setting rather than dialling in a time, the appliance is not counting minutes — it is watching the moisture leaving the food. Heat something wet and steam builds in the cavity until it starts escaping past the door and out of the vents, and the moment that curve changes shape tells the appliance the food has reached temperature. It then cooks on for a calculated extra period and stops. That is the whole mechanism, and it is why sensor programmes ask for a covered dish and why they behave badly on food that is already dry.
When the sensor fails, the appliance loses that feedback. It cannot run the automatic settings honestly, so it reports the fault rather than guessing, and on most models the timed power levels still work perfectly. So an F4 microwave is frequently a usable microwave with its convenience features gone, which changes the repair-or-live-with-it calculation. The sensor itself sits in the exhaust path, which is exactly where grease and steam pass, and contamination is a normal ageing failure rather than a defect. It is also behind the cover, along with everything else in this appliance that an owner should not be near, so LG’s answer is an engineer even though the consequence is mild.
What causes it
| Cause | What is actually happening | How likely |
|---|---|---|
| Sensor contaminated by grease and steam | It sits in the exhaust path by design, so a film builds on it over years and blunts its response. | Common |
| Sensor element failed | These devices age. A sensor that no longer changes with humidity leaves the appliance with nothing to measure. | Common |
| Wiring or connector to the sensor | An open or intermittent connection reports the same way as a failed element and is much cheaper to put right. | Occasional |
| Blocked or restricted vent path | If steam cannot reach the sensor it cannot be measured, and an obstructed exhaust can present as a sensor fault. | Occasional |
Can you fix it yourself?
Stop — this needs an engineer
What it costs to fix
Typical repair cost
£70–£160
Time on site
1 hour
Sensor plus visit, roughly £70 to £160. Worth weighing against how much you use the automatic programmes, since timed cooking usually still works.
Typical UK prices including parts and labour, updated September 2026. Your quote may differ.
Own this appliance and had this code? . Nothing is published until a person has read it.
Related LG fault codes
- F3Get it checkedLG document this one twice and differently. One article says wipe the touch panel, the other says the control panel is faulty and needs service.Related: same brand
- LOCUsually simple to fixNot a fault. LOC is LG’s child lock, and it releases by holding STOP/CLEAR until the display drops it.Related: same brand
- DOORGet it checkedThe word DOOR rather than a number. LG say the door is not closed or the latch is not engaged, and they include transit packing as a cause.Related: same brand
- F16Get it checkedLG say a heating issue was detected. They publish F19 with word-for-word identical text, so both codes land here.Related: same brand
- E10Get it checkedLG say an internal short was detected in the control board, and then give you something to try anyway: unplug it for a full minute.Related: same brand
- F1Get it checkedLG publish this as a problem with the mainboard and say it requires service. There is no owner remedy, and the useful question is whether the appliance is worth it.Related: same brand
Own a LG microwave? Save it and its codes come first next time.
Related guides
Common questions about LG F4
Can I still use the microwave?
On most models yes, using the timed power levels rather than the automatic programmes. The sensor settings are what stop working. Check that ordinary cooking heats properly before relying on it, and if the appliance refuses to run at all, treat it as a fault rather than a lost feature.
Which settings actually use the sensor?
The ones where you choose a food rather than a time — reheat, popcorn, jacket potato, some defrost programmes. Anything where you set minutes and a power level is not using it.
Is it worth repairing?
That depends on whether you use the automatic settings. Plenty of people never touch them, in which case a visit to restore them is hard to justify. If you use reheat constantly, the sensor is the appliance working the way you bought it.
Would cleaning it help?
Possibly, and an engineer will try that before quoting for a part, because contamination is the commonest cause. It is not an owner job — the sensor is inside the outer cover, and this is the appliance where that cover stays on.
Technical review
Not reviewed by an engineer
No engineer has read this page. Severity follows what the manufacturer publishes, and we would rather tell you that than let a byline imply a sign-off.
Sources and how we verified this
We establish the facts from the sources below and then write the explanation ourselves. We do not copy manufacturer wording and we do not host their manuals.
- Manufacturer support portal
This page has not been reviewed by an engineer, and we would rather tell you that than imply otherwise. The severity rating follows what the manufacturer publishes: where they say isolate the appliance and call someone, so do we.