Cooker hood pulls weakly or not at all — how to find the cause

You are frying meat, the kitchen fills with smoke, and the hood roars at full speed — yet the air simply isn't being pulled away. Or it hums while the blades barely move. In most of the cases that reach our bench, the fault isn't the motor but the air path: clogged grease filters or saturated carbon. So we always start with the airflow, not the electronics — otherwise you can replace a part that was never broken.

This article walks through the common symptoms in the order we check them ourselves: first the easy air causes, then the motor with its capacitor, then the wiring and lamps. We work with cooker hoods from Bosch, Siemens, Elica, Faber, Cata and other makers. In Riga's older buildings two local factors get added — grease hardens faster in the damp coastal air, and during the heating season the dry heat saturates the carbon filter without you noticing.

Weak suction — the easy causes first

If the hood runs but pulls weakly, nine times out of ten the culprit is not the motor but a blocked air path. Check these in order, because each next one is harder to reach.

1. Metal grease filters. Those aluminium meshes on the underside of the hood trap the grease aerosol. After a few months of cooking they fur up with a sticky layer and the air simply can't get through. Take them out (usually a spring catch) and hold them up to the light — if you can't see a bright pinpoint through them, they're clogged. Wash them in hot soapy water or in the dishwasher; for badly caked ones, soaking in hot water with baking soda helps. That alone often restores full suction.

2. Recirculation carbon filter. This is where the most commonly forgotten mistake hides. If the hood isn't connected to a ventilation duct but blows the air back into the room (recirculation mode), it has an extra activated-carbon filter to catch odours. This carbon is not washable — once it saturates, it becomes a dense plug and the suction drops by half. Many people have never changed it since they bought the hood. The rough service life is a few months of active cooking; you replace it, you don't wash it.

3. Ducting. On ducted hoods, check the flexible pipe behind the body. A run that is too long, sharply bent or crushed cuts the suction badly. A flattened loop behind the upper cabinet is a typical culprit in kitchens rebuilt without a plan.

4. Backdraught flap. Many ducted hoods have a flap (a non-return valve) at the outlet that stops cold air from the duct flowing back in. If its hinge sticks with grease deposits and doesn't open fully, the air meets a half-closed flap. Check that the flap swings open freely by hand.

If after cleaning the filters, replacing the carbon and checking the duct the suction is still weak — then we move on to the motor.

The motor hums but barely turns — capacitor and bearings

If on switch-on you hear a hum but the fan won't spin, or it only starts spinning after you nudge the blades by hand, the start capacitor is almost always at fault. In these single-phase induction motors the capacitor creates the phase shift that gives the rotation. When the capacitor ages and loses capacitance, the motor draws current (hence the hum and the heating) but can't start turning on its own.

This is a typical bench job, not a hands-on home one: the capacitor sits under voltage and can hold a charge even after you unplug from the mains, so it is discharged before anyone touches it. We measure it and replace it with a part of the same capacitance and voltage — the correct value is critical here.

Tell this apart from worn bearings. If the motor does spin but a rising howling or grinding noise sets in and the suction slowly drops, the motor bearings are worn. A bearing with dried-out grease only howls at first, then starts to bind, and finally brakes the rotor so hard that the capacitor symptoms get added on top. The dry heating-season air in Riga flats speeds up that drying out.

Won't switch on at all — switch, board, lamps

If pressing the button does nothing — no fan, no lamps — we start with the power. Check that the group breaker in the consumer unit hasn't tripped and that the hood's plug (on many it's hidden in the upper cabinet) is seated in the socket. In Riga's older buildings, voltage dips and tired contacts in these hidden connections are common.

If the power is there but the appliance stays silent, the fault is usually in one of two units:

• Mechanical slider or push-button switch. Older hoods have a simple slider or keys. Grease vapours creep into the switch contacts over the years, they oxidise and stop conducting. This is a frequent cause on hoods older than the typical four-to-six-year ownership window.
• Control board (PCB). On models with touch buttons and electronic keys the commands run through a board. From low voltage, moisture or simply ageing, relays or supply components on it can fail. Board diagnosis and component replacement is a service-centre job.

While you're at it, check the lighting. If the lamps don't light but the fan works, the problem is local — more on that in the next sections.

Loud rattling or vibration

A new, unfamiliar noise almost always comes from a rotating part. Before you think about the motor, check the simplest thing: the layer of grease deposits on the fan blades builds up unevenly and throws the rotor out of balance. An unbalanced fan produces a rhythmic vibration and hum that rise with speed. Clean not only the filters but the blades themselves — often the vibration goes away at once.

If the blades are clean and seated firmly but a steady howl remains, we go back to the bearings from the section above — cleaning won't fix those. A separate case is a loose body or a vibrating duct joint: a metallic rattling tone that changes when you press the body by hand points to a loose screw or a flapping backdraught flap, not to the motor.

Where to draw the line: cleaning the blades and tightening screws you can safely do yourself. Replacing the bearings and the motor block needs disassembly and balancing — that is a service-centre job.

Lamps work, fan doesn't (or the other way round)

This symptom is useful because on its own it narrows the diagnosis. The lighting and the motor in a hood are usually separate circuits, so which side works tells you where to look.

• Lamps on, fan silent. There is power in the appliance (the lamps prove it), so the fault is on the motor side: a faulty start capacitor, the motor winding, or the motor switch contact. If you also hear a hum — go back to the capacitor section.
• Fan pulls, lamps dark. The motor circuit is intact. Most often the bulb itself has blown — halogen bulbs in hoods fail regularly, and you change them yourself (cooled down, with a dry cloth, not touching the glass with your fingers). If there are LED modules, their small power converter (the driver) may be faulty, and that is a service-centre replacement.

So: if only one side works, for the other look either for a simple part (a bulb) or a single unit (the capacitor, the driver) — not the whole appliance.

Worth repairing or replacing

Cooker hoods are simple by construction, and most faults are individual replaceable parts, not the end of the whole appliance. Below are the lines we judge by.

The logic is simple: if a single part has failed in a well-preserved body, repair is almost always more sensible than a new hood. If several faults coincide or the body is already damaged, the scales tip towards replacement. We see the specific part only after an inspection — we run a fast on-site diagnostic, and only then start the work. You can read more about what we repair on our home appliance repair page.