LG Microwave Error F-1: What It Means and What to Do

What error F-1 means

Error F-1 on an LG microwave means "PCB thermistor short". That is the exact wording in LG's service manual for the LMH2235ST over-the-range microwave, in the section "6-3 Self diagnosis (humidity Sensor and PCB thermistor check) & Error code", where it sits next to F-2 "PCB thermistor open" and F-4 "Humidity sensor open or short". In plain terms, a thermistor is a small temperature sensor, and this one is on the oven's circuit board (PCB); the board is reading it as a short circuit, so it cannot trust its temperature reading and shows F-1 instead. The source covers the LMH2235ST; other LG models may use their own code list, so check the service manual for your model if it differs.

Why error F-1 happens on a LG Microwave

On a LG Microwave, error F-1typically 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-1. 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-1 appears and your model number

   Write down whether F-1 shows the moment the oven gets power, or only after it has been heating for a while. Then find the model number on the label inside the door frame or on the back of the oven. LG's code list is model-specific, and the service centre will ask for both.

   Pro tip: Take a photo of the display showing F-1 and of the model label.

2. 2

   Step 2

   Power-cycle the microwave

   Unplug the microwave from the wall socket (or switch it off at the MCB if the plug is hard to reach), wait at least one minute, then plug it back in. A code logged during a brief supply disturbance can clear on a clean restart.

   Caution: Do not keep unplugging and restarting if F-1 returns each time. That does not repair a sensor fault.

3. 3

   Step 3

   Rule out the socket and supply

   Plug the oven directly into a different 3-pin socket, with no extension board or multi-plug adapter, and avoid sharing the circuit with a kettle, induction cooktop or geyser while you test. A loose, scorched or overloaded socket can upset the electronics. If you have a socket tester or a multimeter and know how to use it, check that the socket is wired and earthed correctly.

4. 4

   Step 4

   Let the oven cool and keep the vents clear

   If F-1 appeared after a long cooking session, leave the oven unplugged for 30 minutes so it cools fully, and make sure nothing is blocking the vents on the back, top or sides. Then plug it in and heat a cup of water for one minute to see whether the code returns.

5. 5

   Step 5

   Book LG service if F-1 returns

   If F-1 comes back after the restart and the socket change, stop using the microwave. A shorted thermistor on the circuit board needs a technician to test and, if needed, replace the board or sensor. Contact LG India customer care with the model number and the code F-1. If the oven is under warranty, use LG authorised service so the warranty stays intact.

   Caution: Do not open the microwave cabinet to look at the board. 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-1

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-1 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-1 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-1 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-1-precursor symptoms early turns a major repair into a routine maintenance visit.

If error F-1 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.