There are two versions of this complaint and they have opposite answers. A hood that runs for a while after you finish cooking and then stops by itself is almost certainly a heat sensor doing exactly what it was designed to do — many hoods, and most over-the-range microwaves, switch the fan on automatically when the surface below gets hot and hold it until things cool. A hood that never stops at all, or that starts when nobody has touched it, has a stuck switch, a welded relay, or moisture and grease in a touch panel.
Read this first
- Most range hoods are hardwired, so the controls on the hood do not isolate it. Switch off the breaker and verify the circuit is dead with a meter before opening the housing.
- A fan you cannot switch off at the hood must be switched off at the breaker. Do not leave a hood running unattended for days while you wait for a part — grease-laden wiring running continuously above a cooktop is a fire risk.
- Working on a ladder over a cooktop is awkward. Confirm every burner is off and cool, and have someone foot the ladder.
- If you find scorched insulation or a browned connector, stop and call an electrician.
Step by step
Establish whether it eventually stops
Cook nothing for two hours and watch. A fan that runs on for a period and then shuts down on its own is an automatic heat function, not a fault, and the answer is in the manual rather than in the toolbox. A fan that is still running the next morning is a genuine fault. Note also whether it runs at one fixed speed — an auto function typically picks a specific speed, while a stuck switch holds whatever speed was last selected.
Look up the automatic fan feature for your model
Many hoods and nearly all over-the-range microwaves have a thermostat above the cooktop that starts the fan when the surface gets hot and holds it until it cools. It exists to protect the appliance, and on a hot day, on a unit above a range that has just run a long bake, it can run for a long time. Some models let you disable or adjust it and some do not. If the behaviour started after a hot cooking session, this is the first explanation to eliminate.
Cut power at the breaker and see what happens
Switch the hood's breaker off, wait a full minute, and switch it back on. If the fan comes straight back on with no input, you have either a welded relay or a switch stuck in the on position, because nothing else survives a power cycle. If the fan stays off until you touch a control, the fault is intermittent and probably in the panel or its wiring. Either way this test costs nothing and it splits the diagnosis in half.
Clean and test the speed switch
Range hood switches sit directly in the grease stream and they are the most commonly failed part on the appliance. With the breaker off and verified dead, get behind the switch, disconnect it, and test for continuity between the line terminal and each speed terminal through every position including off. A switch that shows continuity in the off position has welded contacts and needs replacing. Clean the spade terminals with contact cleaner while you have it apart.
Deal with a touch panel that is phantom-triggering
On touch-control hoods, grease film and steam behind the membrane cause the panel to register presses nobody made — a hood that changes speed on its own, or starts in the night, is the signature. Wipe the panel with a barely damp cloth and then a little isopropyl alcohol on the cloth, never sprayed at the panel and never enough to run into the seam at the edge of the film. If the behaviour has been getting steadily worse over months, the panel has taken on moisture and will need replacing.
Check the control board relay
On electronic hoods the fan is switched by a relay on a small board. A relay whose contacts have welded closed leaves the motor powered no matter what the panel says, and it is a common failure on a hood that has been switched on and off several times a day for years. With the breaker off, look at the board for a browned area around a relay, a burnt smell, or a blown fuse on the board itself — some carry a replaceable glass or blade fuse.
Check for a second control you have forgotten
Hoods are often wired through more than one control. There may be a wall switch feeding the hood, a remote handset, a downdraft or make-up air interlock, or a home automation relay someone fitted and forgot. On some installations the hood is tied to a switched circuit that another device is holding on. Walk the kitchen and check every switch, and try removing the batteries from any remote handset — a handset with a stuck button will hold a hood on indefinitely.
Make it safe while you wait for a part
If the fan cannot be switched off and a part is on order, switch the hood off at its breaker rather than leaving it running. Label the breaker so nobody restores it absent-mindedly. Do not simply pull a wire nut apart inside the housing to stop it — a disconnected live conductor in a greasy enclosure is worse than the original problem. If the hood shares a breaker with something you need, get an electrician to isolate it properly.
What this repair costs
Doing it yourself
$12–$95
Hiring a technician
$150–$400
Prices are general estimates for typical US jobs, not quotes. What you actually pay depends on your appliance's brand and age, the part required, your region, and each shop's labour rate. The DIY figure assumes you supply the labour and buy the part yourself.
Parts you may end up buying
| Part | Install | Typical price | Typical lifespan |
|---|---|---|---|
| Grease Filter | easy | $15–$65 | Wash monthly; replace every 2–5 years |
| Charcoal Filter | easy | $15–$55 | 6–12 months |
| Blower Wheel | moderate | $20–$80 | 10–15 years |
| Blower Motor | moderate | $60–$260 | 10–15 years |
| Speed Switch | easy | $12–$90 | 8–15 years |
| Light Socket and Lamp | easy | $8–$40 | 3–10 years |
| Damper Flap | easy | $10–$40 | 8–15 years |
Still not fixed
When to call a pro
Call an electrician if the hood is hardwired and you are not equipped to verify the circuit dead, or if you find scorched wiring, browned terminals or a burnt smell inside the housing — a grease-soaked, heat-damaged connection above a cooktop is a genuine fire risk and outranks the fan problem. Call an appliance technician if the switch tests correctly and the fault is on the control board, since boards run $60 to $200 and are not returnable once fitted. On hoods where the board has been discontinued, the manufacturer's only remedy may be a new hood, which is worth knowing before you spend anything.
More range hood guides
- Range Hood Fan Weak or Not Venting — Restore Airflow45–120 minutes
- Range Hood Light Not Working — Bulbs, Sockets and Switches15–45 minutes
- Range Hood Won't Turn On — Power, Switch and Control Checks30–75 minutes
- Range Hood Loud or Rattling — Quiet It Down45–120 minutes
- Range Hood Dripping Grease — Fix the Cause, Not the Drip60–150 minutes
- Downdraft Vent Won't Rise or Lower — Free the Lift45–120 minutes