I was told by my mechanic that the left headlight of my Toyota Estima 2002 can't be fixed by replace a light bulb but the whole headlight have to be replace . is that true ?

Must have a bad wire. Most auto parts store sell the whole fixture as a replacement. But you need to make sure it's not in your wiring before you buy that part. Only way to do that is to have a small voltage tester and test each wire with power on. This type of stuff is better left to the mechanic that has the tools to go through the process of elimination.

the whole thing? or just bulb access.
no country stated, and LIGHT VARIES by COUNTRY.
ask a question
how to?
how to remove it? the whole assembly?
its in the operators guide in the bulb swap.
the FSM covers the assembly.
try alldata.com if car us USA spec car.

most likely the fuse, have you checked that? Do the other outside lights (other than headlights) work? (side markers, parking lights) If they are also out, it's almost certainly the fuse.
If that's ok, check the bulbs themselves. Sometimes they burn out one at a time, if you don't pay attention, they can all burn out before you notice.

The bulb is replaced not the whole headlight.

My right side of this center dash window light that displays the temp.reading also went out on my 2003 4Runner. The letters & number were readable, but the back light was out to see everything at night. My mechanic said the bulb was out and he had to pull the section off surrounding the whole center dash portion that encompasses the radio, mileage, a/c & radio section. I left it up to them to do, since they said it would take an hour. Now the whole section lights w/o any problems.

replace only the bulb

Headlights are one of the most important safety features on your car, but they can only do their job if they're properly maintained. It's not just burned out headlights that cause problems; headlights that are dim or incorrectly aligned can also lead to accidents. Fortunately, learning how to repair headlights is something even the least mechanically inclined person can do. When you know how to repair headlights yourself, you can save up to several hundred dollars on mechanic's fees. Here are some tips on how to repair headlights with common problems.

Headlights that don't turn on
If you're headlights aren't working at all, the most likely cause is a burned out bulb. All you need to do to repair the headlights is replace the bulb. If you have plastic lenses, you can take out the old bulb by removing the lens and unscrewing the bulb from the back of the lens. With glass headlights, though, you'll need to replace the whole glass. Since you're buying new bulbs anyway, consider picking up one of the new, brighter types of bulbs, such as the halogen or xenon types.
When you've got your replacement bulb, install it and turn on the lights. If the headlights still don't work, it's a sign of an electrical problem. Because electrical problems are relatively complicated to repair, take your car to a mechanic before you try to do anything else with the headlights. This will help. Thanks please keep updated. please please do rate the solution positively .thank you for using fixya

Inside your trunk on either side of your tailgate are panels that allow you access to your taillights....I'll add more later.

In the Fuse Relay Box under the Hood there is a Headlight Relay. Replace this Relay along with a New Fuse and it should work. Let me Know. Please Rate my Response, I need all the Help That I can Get! Thanks!

I have the same issue on my 2002 A4. According to my Audi dealer mechanic, there is a sensor on the front suspension that controls the angle of the headlight. Replacing this may resolve the issue.