Cooker hood won't turn on, or hums but won't spin — bench diagnosis

When a cooker hood won't turn on, or hums but won't spin, that is a different story from a hood that simply pulls weakly through a greasy filter. A dead stop almost always means an electrical or mechanical failure — in the power supply, the switch, the start capacitor, or the motor itself. This article is the bench view: how to tell these faults apart, what you can safely check yourself, and where the service work begins.

We're talking about ordinary kitchen cooker hoods here — wall-mounted, island, built-in, and downdraft units with a normal AC motor or a simple DC motor. If your hood does run but pulls weakly, that's a completely different repair; read about it separately in cooker hood weak suction.

Weak suction versus a complete stop

First, be honest with yourself and separate two completely different situations, because they need a different approach and a different repair.

Weak suction means the motor is turning — you hear a normal hum, the fan is spinning, but almost no air moves. That is nearly always a blockage: a clogged grease filter, a saturated carbon filter, a blocked duct, or a stuck check valve. That's maintenance, not an electronics repair.

A complete stop is what this article is about. Here the motor does not turn at all. It has three characteristic scenarios, and each points to a different part:

• Completely dead — you press the button and nothing happens: no lights, no motor, no beep, no display. The problem is most likely in the power supply or the control panel.
• Hums but won't spin — you press the motor button and hear a low buzz or hum, maybe feel a faint vibration, but the fan stands still. That's the classic symptom of a seized motor or a failed start capacitor.
• Lights work, but the motor is silent — the lamps come on normally, but the motor doesn't respond at any speed. That narrows the search to the motor circuit: the switch, the capacitor, the winding, or the control board.

If you read correctly which of these three is your case, you're already halfway to the diagnosis.

Power supply, switch, and control panel

If the hood is completely dead, start not with the motor but with the power supply — that's exactly how we do it on the bench too.

1. Check the distribution board. Many kitchen hoods are fed from the same line as a socket or the lighting. Look for a tripped breaker or a tripped RCD. In older Riga buildings with tired wiring, this happens more often than you'd like.
2. Check the socket or the connection itself. Built-in hoods are often hard-wired behind the cabinet, where there can be a loose terminal or a burnt contact. If the hood is on a plug, try another appliance in that socket.
3. Inspect the power lead at the body. Above the cooktop, the cable suffers constantly from heat and grease. A broken lead at the cable entry is a textbook cause of a "completely dead" hood.

If the power is fine but the panel stays silent, the control comes next:

• Mechanical switches (clicky buttons / sliders). Older hoods use simple push-buttons. Grease and steam oxidise the contacts until they stop conducting. A typical and rewarding failure — the switch is replaceable.
• The sensor / touch panel. Modern hoods use an electronic board with touch buttons. If the display is blank or the buttons don't respond, the fault may be in the board itself, the ribbon connector, or the power section on it.

Self-help ends here, at the casing. The breaker, the socket, and an obviously damaged cable you can check safely. But inside there is mains voltage and — pay attention — a start capacitor that can stay charged even after the hood is unplugged. Do not touch it.

A seized motor and a failed start capacitor

This is the most common "hums but won't spin" fault, and it's worth understanding in detail, because this is where checking most often leads to a specific part.

Most cooker hoods use a single-phase induction motor with a start capacitor. That kind of motor can't start turning on its own — the capacitor creates a phase shift that gives the motor a "push" in the right direction. If the capacitor is dead, the motor gets voltage, the winding buzzes and heats up, but it never starts turning — which is exactly why you hear a hum without rotation.

A very telling bench test: if the capacitor is the culprit, after a small push (in the service, with the unit unplugged and discharged) the motor picks up speed and keeps spinning — until the next switch-on. That's a near-certain sign the capacitor needs changing, not the motor.

A seized motor is a different story. Over years, greasy steam settles on the shaft and bearings; the lubrication thickens or dries out, and the rotor stops turning freely. At first the motor starts heavily and hums, later it won't start at all. With a seized rotor, the thermal protection (if fitted) cuts the motor after a few seconds to keep the winding from burning — which is why some hoods "hum and go quiet".

Safety warning: if you smell burning or the motor hums hot, unplug the hood immediately and stop using it. Switching it on again with a seized rotor burns the winding, and then a local repair turns into a motor swap.

Lights work, but the motor is silent

This symptom has real diagnostic value, so let's stop on it separately.

If the lights come on normally, that proves two important things: the hood is receiving mains voltage, and the overall power supply is intact. So the fault is not "before" the hood, but in the motor circuit inside. That rules out half the possible causes and focuses on:

• The motor switch or speed selector — the lighting and the motor often have separate buttons. The light button works, the motor button doesn't. Oxidised or burnt contacts right in the motor circuit.
• The start capacitor — as described above; voltage reaches the motor, but without the start push it won't turn.
• A break in the motor winding — rarer, but the winding can fail from overheating; then the motor gets no current at all.
• The control board — on an electronic hood, the board can power the lighting channel while the motor relay or triac on it is faulty.

In practice this means: if the lights work, the problem is local and often rewarding to repair — a switch, a capacitor, or a relay on the board. That's good news.

What you can safely check yourself

Before bringing the hood in, a few safe checks can save time. Do everything that follows with the hood disconnected from the mains (pull the plug or trip the breaker).

1. Breaker and RCD. Look at the distribution board; if it's tripped, switch it back on once. If it trips again, don't insist — that points to a short circuit, and that's for the service or an electrician to assess.
2. Filters and fan. Remove the grease filter and try to turn the fan with your finger (on an unplugged hood!). If it spins freely and easily — the rotor isn't seized, so think capacitor or switch. If it stands fast or turns heavily — a seized motor.
3. Unplug and leave it for a minute. Fully disconnect the hood from the mains for 1–2 minutes and switch it on — this clears a short-lived electronics "freeze" on modern panels.
4. The lights test. Check whether the lights come on. If they do — the power is good, so look in the motor circuit (see the previous section). That's a valuable observation to tell the technician.
5. Smell and heat. If you smell burning — stop any attempts, unplug it, and bring it in for inspection. Repeated switch-ons will only make the damage worse.

The line is clear: everything you can do without opening the casing is yours to do. The moment it comes to checking the capacitor, the switch, or the motor inside — that's where mains voltage and a charged capacitor are, so leave the casing closed. This is where the bench begins.

If it turns out the motor does spin and the suction is still weak — not a stop at all — the cause is often in the filters; which one to wash and which to replace, read in cooker hood filters: clean or replace.

Capacitor or motor replacement in the service

On the bench, diagnosing a "hums but won't spin" hood usually goes in this order: we check the power right up to the casing, measure the switch contacts, discharge and test the start capacitor with a meter (its capacitance is usually down or zero), and only then assess the motor itself — its bearings, shaft, and winding resistance.

The repair-versus-replace decision, in our experience, breaks down like this:

The simple principle is the same as for any kitchen appliance: if one local part is damaged — capacitor, switch, or bearings — and the rest of the body is sound, a repair is almost always sensible. If the winding is burnt out on an old, already worn hood, the honest thing is to say so plainly. Inspection decides it.