I have an 03 TL-S.

When I use my aftermarket remote start, it works every single time, however when I use my key to start the vehicle, sometimes it wont turn over.

Do our keys have a chip in them thats required to start the car?

If so, the installer put one under the dash so the remote start works, if not, I don't know what I did with one of the spares.

But why would it start first time, everytime with remote start but not manually with the key?

 

Our cars have a chip inside the key which requires to programed at a dealership.
If you made a key from a key smith, only thing you can do with that key is lock and unlock the doors.
But the dealership charges for programing a key.
Welcome to the forums.

 

All my keys are the ones I got when the car was purchased.

Since my alarm installers mounted a key under the dash or near the steering column, would that produce a problem if there are two keys in the vicinity of each other when trying to start the car. Again, when using the remote start, the car starts first time, every time, but occasionally, it will take a few attempts to start car, the engine cranks, just doesn't turn over.

 

That wouldn't produce the problem cause I too have an auto start.
What remote system do you have?
It may be the keys that's causing the problem(?).

 

Its made by Code Alarm, and the company that put it in is no longer in business.