When I start to drive and the car moves, it has no pick up probably due to it starts from second gear instead of the first gear.
And when it travels at 100km/hr, the engine sound is like roaring as if it is moving at low gear.
What could be the problem?
Your transmission is stuck in one gear, most likely the second gear. It could be shifting into third, but I can't say for sure. This can occur for two reasons. Either the transmission is failing or your car has gone into limp mode. Limp mode exists to keep the car from further damage and usually won't allow you to go above about 30 MPH. The speed will be different with every manufacturer. The motor is roaring because it is turning at a very high RPM (revolutions per minute). I would lean toward a transmission problem because the motor is roaring so loudly. Usually, limp mode won't allow this.
Your car needs to be diagnosed very soon to figure out if you are going to need a transmission or if something else is putting it into limp mode. This condition can cause some catastrophic problems if not taken car of sooner than later. If you need help having this checked, have a local professional, like one from YourMechanic, come to your car's location to inspect the gear issue and advise you on a repair.
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