Last updated: 2026-10-30 · By Fix It or Replace editorial team

Whirlpool Microwave Not Heating? Demo Mode, Control Lock (LOC/L), Power Level, Shipping Spacers, Door — and Why the Magnetron, Diode and Capacitor Are Pro-Only (2026)

Verdict at a glance

Safety check before anything else (read all of it)

Microwave repair is different from every other appliance on this site. Stop and don't go further than the outside checks if any of these is true:

• Do not open the outer case — ever. A microwave's high-voltage capacitor can hold a charge even after it's unplugged (GE Appliances' support article says so, and HomeGuide warns about it too). We don't publish capacitor "discharge" steps, and you shouldn't follow anyone else's. Unplug it and leave it to a trained technician.
• The magnetron, high-voltage capacitor, high-voltage diode, transformer, and door switches are pro-only. Whirlpool's user guide says the microwave should be serviced only by qualified service personnel, and Whirlpool's not-heating article says to contact a professional for the door switch, magnetron, and diode.
• Never defeat or tamper with the safety interlocks, and don't use it if the door is bent, the hinges or latches are broken or loose, or the door seals are damaged (Whirlpool user guide).
• Arcing, sparks, smoke, or a burning smell: stop, unplug, and don't use it again until it's inspected. Two Whirlpool microwave-hood recalls involved fire (see "Recalls").
• Grounded 3-prong outlet only. Whirlpool's guide says: don't remove the ground prong, don't use an adapter, and don't use an extension cord.
• In year 1? Whirlpool's warranty covers parts and repair labor. Call before anyone opens it.

Fix order at a glance (cheapest first)

DIY tiers: Easy (outside checks, case closed) · Pro only (anything inside the case). On a microwave there's no "Moderate" or "Advanced" tier: if it needs a screwdriver, it needs a technician.

Which Whirlpool microwave do you have?

Whirlpool's user guide says the model and serial number are on the front facing of the microwave opening, behind the door. CPSC's 2016 recall notice puts them inside, above the oven cavity on the left.

Five-minute checks before you call anyone

1. Water test: 1 cup of water, HI, 1 minute. Warm = heating.
2. Display: "LOC", "L", or padlock → Control Lock. "dOn" or "DEMO" → Demo Mode. Flashing ":" or "PF" → power failure; reset the clock. A letter followed by a number → an error; Whirlpool's guide says call for assistance.
3. Settings: Cook Time, then Cook Power if not 100%, then Start.
4. Door message: Whirlpool's guide says if the door has been closed 5 minutes or more without starting, a door message appears; open and close the door, then start.
5. Spacers: door spacer and the 2 vent-grille spacers removed.
6. Power: a grounded wall outlet; breaker not tripped.

How to know it's fixed: the water is clearly warmer after the test.

What Whirlpool's own pages say vs. each other

Our take: use the 2-minute cold-water test; it's the stricter of the two. For Demo Mode, follow the steps for your model in its manual. If the "3"-pad hold doesn't show "dOff" or clear "DEMO", try the other method. If neither works and it still won't heat, book service.

Whirlpool microwave codes and signs

Which part each symptom points at (for your technician's quote, not for DIY):

Checks and codes from Whirlpool product-help articles, Whirlpool's not-heating article, and the WMH31017 user guide (sources 1–6). The part mapping is our own from these sources, HomeGuide's symptom table (G2), and the Sears PartsDirect catalog.

Diagnose by symptom → likely part (for your technician, not for DIY)

1. Runs, turntable turns, water stays cold

Likely cause: a failed high-voltage diode or magnetron. Whirlpool's article names both; HomeGuide says technicians often replace them together.

Who fixes it: Pro only.

2. No heat with a loud hum

Likely cause: the high-voltage capacitor, diode, or magnetron. Only a technician with the right equipment should test the capacitor.

Who fixes it: Pro only.

3. Won't start, or the light doesn't follow the door

Likely cause: a door interlock switch. Whirlpool says replacing one can mean taking apart the control panel and wiring, so contact a professional.

Who fixes it: Pro only.

4. Sparks, smoke, or burning smell

Likely cause: the high-voltage transformer, magnetron, or diode. Fixr says a microwave that sparks, smokes, or smells like burning is no longer safe and should be replaced.

Who fixes it: Pro only, and a strong replace signal.

5. Pads dead, or an error code that returns

Likely cause: the control board (electronic, power, relay, or inverter board, depending on model).

Who fixes it: Pro only.

Recalls

We checked the CPSC site on 2026-10-30. Two Whirlpool microwave-hood recalls involved fire:

• August 25, 2016 — microwave hood combinations (recall 16-253): about 15,200 Whirlpool microwave hood combinations, models WMH53520CS, WMH53520CW, WMH53520CB, WMH53520CE, WMH53520CH, WMH73521CS, WMH73521CW, WMH73521CB, WMH73521CE, WMH73521CH with serial numbers TR33500000 to TR34899999, sold January 2014 through April 2016. Internal arcing can ignite an internal plastic part. Remedy: stop using it, unplug it, and contact Whirlpool at 800-990-6254 for a free replacement.
• October 16, 2001 — microwave-hood combinations (recall 02-018): about 1.8 million over-the-range units sold under Whirlpool, KitchenAid, and Kenmore names, serial numbers beginning XC, sold January 1998 through September 2001. They could overheat and catch fire. Remedy: stop using and unplug; free repair; Whirlpool (800) 785-8897. Countertop microwaves weren't included.

If your microwave matches, use the recall remedy instead of paying for a repair. Search your exact model at cpsc.gov/Recalls.

Prevent it next time

• Keep the cavity, inlet cover, rack supports, and door frame clean. Whirlpool's guide says soil buildup there can cause arcing.
• Don't run it empty (HomeGuide).
• Use a grounded wall outlet, ideally a separate circuit (Whirlpool recommends one for its microwave hood combination), with no adapter or extension cord.
• Learn your Demo and Control Lock buttons so they don't get set by accident.

Related guides: Whirlpool range oven not heating · Whirlpool range burner not working · GE microwave not heating · Whirlpool dishwasher not starting

What Whirlpool's warranty covers on a microwave

Per the One Year Limited Warranty in the Whirlpool WMH31017 user guide (W10918338A, retailer-hosted copy; see Sources); terms vary by model and year, so check your own:

What this means for a microwave that won't heat: in year 1, a failed magnetron, diode, capacitor, switch, transformer, or board is covered when the microwave was installed, operated, and maintained per the instructions. Whirlpool doesn't cover in-home instruction, improper installation or maintenance, correction of household electrical (house wiring, fuses), misuse, or removal or reinstallation of inaccessible built-in fixtures. Proof of the original purchase date is required. Older Whirlpool microwaves may have different terms; check your own manual.

→ Whirlpool service (per the user guide): 1-800-253-1301

Before you pay for any repair: a 4-point check

1. Warranty: year 1; retailer or credit-card extended protection.
2. Recalls: check the 2016 and 2001 microwave-hood recalls, then search cpsc.gov/Recalls.
3. Free fixes done: water test, Demo Mode, Control Lock, settings, spacers, power source, breaker.
4. Written quote: ask for part, labor, and trip fee separately, and which high-voltage part tested bad.

What it costs: repair parts and pro service (2026)

Part prices checked 2026-10-30 on Sears PartsDirect public search pages (S1–S7). Pro-cost cells tagged [GUIDE] from G1 (Fixr) and G2 (HomeGuide), re-read 2026-10-30; national averages, not quotes. Prices change; confirm with your model number.

Tip: tell the tech your model number, whether the light, fan, and turntable run, any code, any hum, sparks, or smell, and the water test result.

Repair vs. replace calculator

1. What's the all-in repair cost? → R = trip/diagnostic fee + part + labor + tax, minus anything Whirlpool's warranty (or a recall) covers
2. What does a comparable new microwave cost, installed? → N = unit + installation (over-the-range and built-in) + haul-away
3. How old is your microwave, and how long should it last? → Age and Life (HomeGuide says 7–10 years; Fixr says 8–10; use 9 if you don't know)
4. How much have you spent on repairs in the last 2 years? → Past

Step A — Cost ratio: R ÷ N
Step B — Life used: Age ÷ Life
Step C — Repeat-repair check: (R + Past) ÷ N

Repeat-repair rule: if Step C is over 50%, lean toward replacing. HomeGuide says repair if it costs less than half of a replacement, and lists multiple failed parts, more than 4–6 years old, sparks and smoke, a damaged magnetron, and a budget microwave as replace signals. Fixr says a microwave more than six years old is probably due for replacement.

Worked example 1 (illustrative numbers): A new Whirlpool microwave hood combination runs but won't heat. The shipping spacer was still inside the door, so it wasn't fully closed. R = $0.
→ Not a repair.

Worked example 2 (illustrative numbers): A 3-year-old Whirlpool over-the-range microwave runs with no heat. A tech replaces the high-voltage diode: R = $140; N = $700 installed.
140 ÷ 700 = 20%; life used 3 ÷ 9 = 33% → Repair.

Worked example 3 (illustrative numbers): A 6-year-old Whirlpool over-the-range microwave needs a magnetron and a capacitor: R = $360; N = $650 installed.
360 ÷ 650 = 55% (inside the 50–60% second-quote zone); life used 6 ÷ 9 = 67% → Replace.

Tips toward replacing: sparks or smoke, a countertop model, two or more high-voltage parts, an older unit, or parts no longer sold.
Tips toward repairing: it's in year 1; it's a single part such as a diode or door switch; it's an over-the-range or built-in model that's a few years old.

Why 50%? It's the common rule of thumb for appliances, and HomeGuide uses "less than half" for microwaves. Some technician checklists draw the line a little higher, at about 60%, so treat a repair that costs 50–60% of new as a second-quote zone, not an automatic replace.

Who services Whirlpool microwaves near you

1. Whirlpool (warranty). Whirlpool's user guide says all warranty service is provided by authorized Whirlpool Service Providers; call 1-800-253-1301 or start at whirlpool.com/product_help. Have the model and serial number, a description of the problem, and proof of purchase ready.

2. A partner appliance pro (out of warranty).
Our call line connects you with an independent appliance repair provider in your area. Tell them your model number, the type, any code, and the water test result.

Questions to ask any technician:

• Which high-voltage part tested bad, and how did you test it?
• If it's the magnetron, did you also check the diode and capacitor?
• Will you check the door interlock switches before you close it up?
• Is my microwave on either Whirlpool microwave-hood recall?

Optional: is an appliance warranty worth it?

A home warranty (service contract) can cover repairs for a yearly fee plus a per-visit service fee. Read the exclusions carefully: plans often exclude misuse (metal or foil), cosmetic parts, and pre-existing problems.

FAQ

Why does my Whirlpool microwave run but not heat?
Whirlpool lists Demo Mode, Control Lock, a low power level, Start not pressed, Delay Start, a door that isn't fully closed (including a shipping spacer left in), and an inverter or generator power source. If none apply, it names a faulty door switch, magnetron, or high-voltage diode; those need a professional.

How do I turn off Demo Mode on a Whirlpool microwave?
On the over-the-range models in Whirlpool's help article, open the door, hold "3" for 5 seconds until a "d" icon lights, then press Start within 5 seconds; "dOff" shows. The WMH31017 guide says hold "3" about 3 seconds until "DEMO" changes. Check your own manual.

What does LOC mean on a Whirlpool microwave?
Control Lock is on. On the WMH31017 guide's models, hold Cancel about 3 seconds to unlock.

How do I test if my Whirlpool microwave is heating?
Whirlpool's help articles use 1 cup of water on HI for 1 minute. The user guide uses 1 cup of cold water for 2 minutes at 100%.

Can I replace a Whirlpool microwave magnetron or diode myself?
We don't recommend it. Whirlpool says the microwave should be serviced only by qualified service personnel, and the high-voltage capacitor can stay charged after unplugging. Leave it to a trained technician.

How much does it cost to fix a Whirlpool microwave that won't heat?
HomeGuide puts a magnetron at $100 to $300 and a diode at $80 to $170. When we checked Sears PartsDirect on 2026-10-30, Whirlpool-format magnetrons listed at about $149 to $299.

Is my Whirlpool microwave recalled?
Check two recalls: 2016 (WMH53520 and WMH73521 models, serials TR33500000–TR34899999) and 2001 (over-the-range units with serials beginning XC). Search your model at cpsc.gov/Recalls.

About the author

Fix It or Replace editorial team — We research appliance repairs using manufacturer documentation, parts-supplier data, and current US cost guides, and we cite every source below. We are not appliance technicians. Microwave repairs inside the case involve high voltage and must be done by a qualified technician.
Last updated: 2026-10-30.

Sources

1. Whirlpool product help: "Microwave is Not Heating - Countertop Microwave" (1 cup / 250 mL water, HI, 1 minute; inverter or generator may power lights and controls but not the heating source; check power level; "LOC" or "L" = control lock; Demo Mode runs light, fan, turntable without the magnetron). https://producthelp.whirlpool.com/Cooking/Microwaves/Countertop_Microwave/Operation/Not_Heating/Microwave_is_Not_Heating_-_Countertop_Microwave (accessed 2026-10-30)
2. Whirlpool product help: "Not Heating - Has Power - Over-the-Range Microwave" (Control Lock "LOC", "L", or padlock; Demo Mode off; press Start; Cook Time and Cook Power steps; first use: remove the shipping spacer inside the door; 1 cup water, HI, 1 minute). https://producthelp.whirlpool.com/Cooking/Microwaves/Over-the-Range_Microwave/Operation/Not_Heating_or_Operating/Not_Heating_-_Has_Power_-_Over-the-Range_Microwave (accessed 2026-10-30)
3. Whirlpool product help: "Demo Mode - Over-the-Range Microwave" ("dOn"/"dOff"; open door, hold "3" for 5 seconds, press Start within 5 seconds; repeat to deactivate). https://producthelp.whirlpool.com/Cooking/Microwaves/Over-the-Range_Microwave/Error_Code/Other_Error_Codes/Demo_Mode_-_Over-the-Range_Microwave · "Not Heating - Built-In Microwave" (Cook Time, Cook Power, Start; water test). https://producthelp.whirlpool.com/Cooking/Microwaves/Built-In_Microwave/Operation/Not_Heating/Not_Heating_-_Built-In_Microwave (accessed 2026-10-30)
4. Whirlpool: "Microwave not heating: possible causes and fixes" (power and breaker; Control Lock "LOC" or "L"; Demo Mode; low power level; Delay Start; faulty door switch, light behavior, contact a professional; defective magnetron or broken high-voltage diode, contact a professional). https://www.whirlpool.com/blog/kitchen/microwave-not-heating.html (accessed 2026-10-30)
5. Whirlpool Microwave Hood Combination User Guide W10918338A (WMH31017), retailer-hosted copy (abt.com, not a Whirlpool-hosted file) (serviced only by qualified service personnel; don't defeat or tamper with safety interlocks; door, hinge, latch, and seal damage; grounded 3-prong outlet, no adapter, no extension cord; 120 V, 15- or 20-amp supply, separate circuit recommended; Control Lock: hold Cancel about 3 seconds, padlock icon; Demo Mode: hold "3" about 3 seconds, "DEMO" icon; troubleshooting: fuse or breaker, call an electrician if it continues; 1 cup cold water for 2 minutes at 100%; door message after 5 minutes; packaging spacer; 2 packing spacers on the vent grille; arcing from soil buildup; flashing ":" or "PF" = power failure; letter + number = error, call for assistance; model and serial behind the door; One Year Limited Warranty: Factory Specified Replacement Parts and repair labor; exclusions incl. in-home instruction, improper installation or maintenance, correction of household electrical (house wiring, fuses), misuse, removal or reinstallation of inaccessible built-ins; proof of purchase; 1-800-253-1301). https://content.abt.com/documents/140132/UserGuide-WMH31017X-1.pdf (accessed 2026-10-30)
6. GE Appliances US support: "Microwave - Runs But Does Not Heat" (cited only for the general warning that a microwave's internal high-voltage capacitor can hold a charge even after unplugging). https://products.geappliances.com/appliance/gea-support-search-content?contentId=110 (accessed 2026-10-30)
7. U.S. Consumer Product Safety Commission: "Whirlpool Recalls Microwaves Due to Fire Hazard" (August 25, 2016; recall 16-253; about 15,200 microwave hood combinations; WMH53520C*/WMH73521C* serials TR33500000–TR34899999; internal arcing can ignite an internal plastic part; stop using, unplug, free replacement; 800-990-6254; sold January 2014–April 2016). https://www.cpsc.gov/Recalls/2016/whirlpool-recalls-microwaves (accessed 2026-10-30)
8. U.S. Consumer Product Safety Commission: "CPSC, Whirlpool Announce Recall of Microwave-Hood Combinations" (October 16, 2001; recall 02-018; about 1.8 million; Whirlpool, KitchenAid, and Kenmore; serials beginning XC; sold January 1998–September 2001; can overheat and catch fire; free repair; (800) 785-8897; countertop microwaves not included). https://www.cpsc.gov/Recalls/2001/cpsc-whirlpool-announce-recall-of-microwave-hood-combinations (accessed 2026-10-30) · recalls search https://www.cpsc.gov/Recalls
9. Federal Trade Commission: endorsement and affiliate disclosure guidance. https://www.ftc.gov/business-guidance/resources/ftcs-endorsement-guides-what-people-are-asking
10. Part prices (Sears PartsDirect, S-list) and repair cost guides (Fixr G1, HomeGuide G2), see lists below (accessed 2026-10-30).

Part price sources (Sears PartsDirect public search results for "whirlpool microwave …", USD, accessed 2026-10-30; Whirlpool-format numbers only; shown for checking quotes, not for DIY; prices change, confirm with your model number):

• S1. Search "whirlpool microwave magnetron": WPW10693025 $149.01, W11708328 $158.84, W10754299 $162.57, W11346197 $187.88, W10245183 $198.66, 8206317 $211.46, W10844213 $299.14 (covers, thermostats, thermistors, IKEA-labeled 8206079, and LG-format 6324W1A001L excluded). https://www.searspartsdirect.com/search?q=whirlpool%20microwave%20magnetron
• S2. Search "whirlpool microwave diode": WPW10687786 $22.72, WPR9800486 $23.11, W11256462 $27.65, WP8205489 $45.00 (Crosley-labeled, LG, Samsung, and Bosch-format diodes excluded). https://www.searspartsdirect.com/search?q=whirlpool%20microwave%20diode
• S3. Search "whirlpool microwave high voltage capacitor": W10561770 $38.45, 8206080 $45.58, 8206562 $109.26, W10850446 $119.86, WP4375020 $143.25 (GE, Goldstar, Samsung, and Frigidaire/Kenmore-format capacitors excluded). https://www.searspartsdirect.com/search?q=whirlpool%20microwave%20high%20voltage%20capacitor
• S4. Search "whirlpool microwave door switch" (interlock switches): W10727360 $5.33, W10269458 $22.87, W10211972 $37.39, W10211974 $38.26, W10269460 $45.21, 4392027 (primary) $84.43, W10727408 $86.90 (actuator, supports, dryer and refrigerator switches, and other-brand switches excluded). https://www.searspartsdirect.com/search?q=whirlpool%20microwave%20door%20switch
• S5. Search "whirlpool microwave transformer" (high-voltage): W11371266 $177.09, W11371718 $218.47, WP4375286 $285.86 (low-voltage, wall-oven, and unlabeled transformers excluded). https://www.searspartsdirect.com/search?q=whirlpool%20microwave%20transformer
• S6. Search "whirlpool microwave control board": WP8206602 (relay) $92.80, W11129648 $125.26, W10893407 (assembly) $167.15, W11342846 $175.44, W10893404 (board and panel) $198.20, W11182110 $207.13, WPW10486188 (power) $215.09, W10915648 (power) $225.96, W10217711 (inverter board) $230.60 (wall-oven and range boards excluded). https://www.searspartsdirect.com/search?q=whirlpool%20microwave%20control%20board
• S7. Search "whirlpool microwave thermal fuse": microwave fuse WPW10017410 $13.19 (dryer, wall-oven, warming-drawer, dishwasher, and range fuses excluded). https://www.searspartsdirect.com/search?q=whirlpool%20microwave%20thermal%20fuse

Pro-cost guide sources [GUIDE] (US national averages, parts + labor, not brand-specific quotes):

• G1. Fixr, "Microwave Repair Cost" (updated January 31, 2025): national average $100–$200, average $150; replacement costs: fuse $50–$100, door switch $50–$100, diode $50–$125, magnetron $100–$300, transformer $100–$300, control board $100–$500; not heating $50–$500; replacement $150–$400; microwaves last 8–10 years; more than six years old probably time for a new one; sparks, smoke, or burning smell means no longer safe and should be replaced; magnetron replacement dangerous, hire an experienced, certified repairer. https://www.fixr.com/costs/microwave-repair (accessed 2026-10-30)
• G2. HomeGuide, "Microwave Repair Costs": average $100–$250; magnetron $100–$300; diode $80–$170 (often replaced with the magnetron); control board $120–$380; door switch $80–$170; fuse $75–$165; transformer $100–$300; built-in / over-the-range $130–$330; countertop $80–$180; labor $50–$125 per hour, minimum fees $70–$130; repair if less than half of replacement; new microwave $100–$600, built-in installation $100–$400 extra; replace signals incl. multiple failed parts, more than 4–6 years old, sparks and smoke, damaged magnetron, budget model; hire a professional for all electrical repairs and warns internal capacitor parts can stay charged after unplugging; microwaves last 7–10 years; don't run it empty. https://homeguide.com/costs/microwave-repair-cost (accessed 2026-10-30)

Disclosure

Fix It or Replace is an independent site. We are not affiliated with, endorsed by, or sponsored by Whirlpool Corporation. "Whirlpool" is a trademark of its owner and is used here only to describe the products we cover.

• Affiliate links: Some part links go to retailers such as PartSelect and RepairClinic. If you buy through them, we may earn a commission at no extra cost to you. Microwave high-voltage parts should be installed by a qualified technician.
• Call line: Phone numbers labeled "partner pro" are operated by a third-party lead service that routes your call to an independent repair provider in your area. We may be paid for calls. We don't employ the technicians, and we don't guarantee their work, pricing, or availability. Confirm licensing, insurance, and quotes directly with the provider.
• Prices are estimates and change often. Always get a written quote.
• Safety: Never open a microwave's outer case. The high-voltage capacitor can stay charged after the microwave is unplugged. Never defeat the door interlocks.

We don't publish fake reviews or testimonials, and we never present a tracking phone number as a specific local business.