Most common questions used to investigate
Is your car having trouble climbing up hills and/or when overtaking other vehicles in the highway?
Are there noises coming from underneath your car?
Does your clutch fail to release even after you have released the clutch pedal?
The most common reason for this is that your car is low on clutch fluid. Check the reservoir and top it up with more clutch fluid. Turn on the engine and give the clutch a couple of pumps; the pedal should feel stiffer than before, this means the clutch hydraulic fluid has already circulated into the master cylinder, and now you can shift gears as normal. Did this help?
Is the clutch pedal too hard to press?
Common conclusions
You might have a slipping clutch. The clutch plate is supposed to slip a little when you are changing gears, but when it is released then it should hold firm. If your car struggles to accelerate or if there is not enough torque to get it up somewhat steep inclines, the lining of the clutch plate might already be worn out and must be replaced.
There are many possible causes of the noises coming from your car's transmission, like a worn down input shaft bearing, misaligned release bearing, misaligned clutch plate, and many others. All of these issues are serious so you need to bring your car to the mechanic as soon as possible.
You still need to bring your car to the garage to find out why your car is leaking clutch fluid.
You need to bring your car into the garage, if you cannot shift gears even if the clutch fluid reservoir is still at a normal level, there might be something seriously wrong with your clutch, so you need to get your car towed to the garage.
There might be a binding pedal linkage, clutch cable, or other parts. There might also be a blockage in the hydraulic system that causes the pedal to be harder than usual.
There might be some air bubbles trapped in the clutch line. If you know where the master cylinder is, you can bleed the clutch lines to get the air bubbles out. If you don't know how to do this, you can bring it to your mechanic and have him/her do it for you.
References
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