Compensation is due. A mechanical problem is the airline's own responsibility.
Check your flightThis is the single most common reason airlines give, and the most commonly misunderstood. A technical or mechanical problem with the aircraft is not an extraordinary circumstance. The Court of Justice settled this in Wallentin-Hermann: technical faults are inherent in the normal activity of running an airline, so the airline carries the cost.
It does not matter that the fault was unexpected, or that fixing it was the safe decision. Safety is not in dispute. The question is only who pays for the disruption, and the answer is the airline.
The narrow exception is a hidden manufacturing defect revealed by the manufacturer, or damage caused by an outside act such as sabotage. Those are rare and the airline has to prove one applies. A routine component failure, a warning light, or a part that was not in stock does not qualify.
If the airline has told you the delay was technical, you are in a strong position. Write and claim.
Compensation is a flat amount set by distance, not by ticket price: €250 up to 1,500 km, €400 for longer flights inside the EU and for 1,500–3,500 km, and €600 beyond 3,500 km. The UK equivalents are £220, £350 and £520.
Distance is measured in a straight line from your original departure airport to your final destination, which on a connecting itinerary often lands you in a higher band than you would expect.
Duty of care is separate from compensation and does not depend on the cause. Once you are delayed two hours or more the airline owes you meals and drinks, two phone calls, and a hotel plus transfers if you are stuck overnight. This applies in bad weather, during ATC strikes, and in every other situation where no compensation is payable.
Airlines frequently provide none of this during major disruption. Buy your own, keep every receipt, and claim the cost back afterwards.
Work out what you are owed