Most gear failures, up to 75% in automatic transmission is cause by driver's error.

Up to 80% of vehicles on Nigerian roads don't have active hand brakes, they make use of the "P" and that's why they always have gear failures.

Let's not forget:
"P" = parking mode
"N" = neutral mode
"" = drive mode.

Common mistakes drivers of automatic transmission make is when the car is stopped many drivers push the gear handle directly from "" to "P" then before applying hand brake (if the vehicle have any functional hand brake at all).

This is very wrong

Correct way is when the car is stopped, first push the gear handle to "N", apply the handbrake so the whole force and weight of the car rests on the handbrake then push the gear handle to "P" then switch off the engine.

What Happens Inside The Gearbox Each Time You Push Your Gear Handle To "P"?

Your automatic transmission has a pin like device called PAWL. It's function is to lock the transmission shaft and block it from moving front or back.

There's a circular notched ring that looks like flywheel at the base of your output shaft inside the gear.

Once you engage your gear to "P" the PAWL (pin) locks into the notched ring just like door locks in your rooms or the way your steering locks when you off the engine and remove your key from the ignition.

Once this happens the drive shaft is locked and can't move front or backward.

When you park your vehicle on a sloppy or inclined surface a lot of stress is placed on your vehicle parking pawl especially if you engaged the "P" before the handbrake, infact the whole weight of the vehicle rests on the parking PAWL.

Doing this all the time will force your parking pawl to worn out and eventually damage the gear.

Your transmission is affected more if you constantly park at sloppy surface without engaging the hand brakes first before the "P" and that's why if you want to move from the "P" to "N" or "" the gear handle become very difficult to push or make heavy disengagement sound.

That's sound is forceful disengagement of the pawl from the notched ring due to vehicle weight resting on it which is very dangerous to your gear.

If you engage the handbrake first before the "P" then the weight of the car will rest on the rear brake linings of the wheel thereby removing any pressure on the parking pawl.

Please, the parking PAWL is too small to rest your whole vehicle weight on, infact your not suppose to do it AT ALL.
If you don't have an active handbrake better go and repair it now.

It's cheaper for you to repair your handbrake than to buy new gear but if you still continue and damage your gear I can help you buy and install a new gear at high cost. What are friends for?

Thanks and enjoy your week.

Educative.

Nice one OP