LG Microwave Error F-4: What It Means and What to Do

What error F-4 means

Error F-4 on an LG microwave is a humidity sensor error. LG's service manual for the LMC1575 countertop microwaves (LMC1575SB, LMC1575ST, LMC1575SW, LMC1575BD) lists F-4 as "Humidity Sensor Error", and LG's service manual for the LMH2235ST over-the-range microwave lists it as "Humidity sensor open or short". The humidity sensor detects the steam that food gives off as it heats, which is what sensor-cooking programmes rely on; the oven's board is reading that sensor as an open or short circuit, so it shows F-4. Both manuals use the same meaning; other LG models may use their own code list.

Why error F-4 happens on a LG Microwave

On a LG Microwave, error F-4typically resolves to one of three root-cause categories. They’re ordered cheapest-to-verify first — start at the top and only escalate if the first cause is ruled out.

• Mechanical: blockage, obstruction, or worn moving part. A common cause across LG Microwaves in India — door latches and interlock switches wear and the waveguide cover degrades. Our step-by-step fix below targets this category first because it’s the cheapest to verify and resolvereports.
• Electrical: voltage spike, sensor fault, or PCB anomaly. India’s grid carries more voltage fluctuation than many appliance design tolerances assume, and a routine surge or sag can be enough to throw F-4. If you’ve had recent voltage events (lights flickering, AC tripping), start your investigation here. A working stabilizer prevents this entire category.
• Software / configuration: stuck child-lock, demo-mode, or pending firmware reset.Less common but the cheapest fix when it applies — a 60-second factory reset clears it. We list this last because it’s rarely the actual cause, but check it before disassembling anything.

Safety first

Step-by-step fix

1. 1

   Step 1

   Note when F-4 appears and your model number

   Write down whether F-4 appeared during a sensor-cooking programme (such as Sensor Cook or Sensor Reheat), during a plain timed cook, or the moment the oven was switched on. Then find the model number on the label inside the door frame or on the back of the oven.

   Pro tip: Take a photo of the display showing F-4 and of the model label for the service booking.

2. 2

   Step 2

   Power-cycle the microwave

   Unplug the microwave from the wall socket, wait at least one minute and plug it back in. A sensor reading upset by a brief supply disturbance can clear on a clean restart.

   Caution: If F-4 returns straight away, do not keep restarting the oven. That will not repair a sensor or wiring fault.

3. 3

   Step 3

   Dry out the cavity and keep vents clear

   With the oven unplugged, open the door and wipe any condensation, splashes and food residue from the cavity walls and ceiling with a soft, slightly damp cloth, then dry it and leave the door open for 15 to 20 minutes. Check that nothing is blocking the vents on the back, top or sides of the oven.

   Pro tip: Cover food that spatters. Grease and steam that build up inside the cavity over months of cooking are easy to prevent and slow to clean.

4. 4

   Step 4

   Test with a plain timed cook

   Plug the oven back in, put a cup of water inside and run a plain timed cook for one minute rather than a sensor programme. Then try a sensor programme with a covered bowl of food. Note which, if either, brings F-4 back; the technician will want to know.

5. 5

   Step 5

   Book LG service if F-4 returns

   If F-4 comes back, the humidity sensor or its connection to the board needs testing, which LG's service manuals describe as a technician job (checking the sensor's harness at the connector and testing the sensor module). Contact LG India customer care with the model number and the code F-4.

   Caution: Do not open the microwave cabinet to reach the sensor. The high-voltage capacitor can hold a dangerous charge even after the oven is unplugged.

Fixed it? Protect it next time. See an extended warranty plan for your Microwave →

Common mistakes LG Microwave owners make with error F-4

These six anti-patterns turn a routine 30-minute fix into a costly repair or warranty void. Read before starting.

• Forcing a stuck door, lid, or panel. A LG Microwave uses interlocked sensors that hold the unit shut while it is in a fault state. Forcing it usually breaks the latch or sensor — turning a small part replacement into a full service call. If the door won’t open, run the safety-disconnect step first, then try again.
• Repeated unplug-and-replug as a “reset” ritual. Cycling power three or four times without diagnosing the underlying cause stresses the PCB and can convert a soft fault into a permanent firmware-corruption code. Reset once, observe whether the error returns immediately, then move to actual diagnosis if it does.
• Pouring water (or any liquid) into electronics-adjacent areas to flush a blockage.Even a small amount near the PCB or main wiring harness can cause permanent damage that voids warranty. The unit’s drainage paths exist for a reason; if a blockage isn’t cleared by the manual procedure, it isn’t getting cleared by improvisation either.
• Skipping the safety-disconnect step.“I’ll just check quickly” is the most expensive sentence in appliance repair. Working live on a 230V circuit (especially with a hot or wet appliance) carries real shock risk and instantly voids any warranty claim. Disconnect, wait two minutes for capacitor drain, then proceed.
• Buying counterfeit replacement parts on Amazon.in. Red flags: price below 60% of LG authorised price, generic packaging without a model-compatibility list, seller name that doesn’t match a known LG parts distributor, listings dated within the last 30 days with no reviews. Counterfeit parts often work for 2-3 weeks then fail with a different error, costing you double.
• Calling an “independent” technician for a warranty-covered unit. Indian appliances under LG warranty must be serviced by authorised technicians or the warranty voids permanently. Even if the warranty is expired, third-party local technicians often replace working parts to inflate the bill — verify each part swap by asking to see the failure on the old part before they install the new one.

Preventing future F-4 on your LG Microwave

The fix above resolves the current instance. These five maintenance habits prevent it from coming back, specific to LG Microwaves in Indian operating conditions (hard water, voltage variability, monsoon humidity).

• Monthly: clean the cavity and check the door latch alignment. Hard-water deposits and everyday debris are a frequent cause of recurring F-4 in India, and a few minutes each month is the cheapest way to avoid a repeat.
• Quarterly: descale water-touching components. Use food-grade citric acid or a LG approved descaler for hard-water regions (Bangalore, Hyderabad, large parts of Maharashtra and Tamil Nadu). Skipping this in a hard-water zone shortens the life of the heating and water-touching parts.
• Run the unit through a working stabilizer. A mainline stabilizer sized for Microwaves is the cheapest protection against voltage-induced F-4 occurrences. The MCB on your distribution board is not a substitute — it trips on overload, not on under-voltage or surge.
• Decide AMC vs DIY honestly. Compare the LGAMC quote against what you have actually spent on service. If your unit is >5 years old and you’ve had two service calls in the last 18 months, an AMC usually pays for itself. Younger units with no service history: DIY plus stabilizer is cheaper.
• Watch monthly for early-warning signs. Unusual noise during a specific cycle phase, water spotting, mild burning smell — any of these means a service call within a week, not a wait-and-see month. Catching F-4-precursor symptoms early turns a major repair into a routine maintenance visit.

If error F-4 returns within 30 days of completing the fix above, escalate directly to LGauthorised service — repeat patterns within a month indicate a deeper fault (worn bearing, failing PCB, leak that wasn’t fully identified) that surface-level repair won’t resolve. Document the dates and circumstances of each occurrence; the service centre will use this to prioritize root-cause investigation.