My 2000 Nissan Xterra won't start. It seems to be a drain on the battery since disconnecting the battery cable while the car is parked seems to work. I've taken it to a mechanic several times.

Might be a valve problem. Disconnect the battery to reset the computer for about 5 min. Reconnect the battery cable and see if there is any difference. Sounds like 'fuel-rich' mixture causing BLACK smoke.

Check the cleanliness and tightness of the battery terminals. Loosen, twist and retighten them. Also check the cleanliness and tightness of the black earth cable where it attaches to the engine block/bodywork. A bad earth can cause the symptoms you describe.

A quick check you can do is to connect a jump cable to the battery negative terminal - the earth - and connect the other end of the cable to somewhere suitable on the engine/bodywork.

If the car starts no problem with the jumper cable - it's pointing towards a bad earth/connection somewhere on the battery earth

maybe a haynes repair manual would help.check your auto parts store or on ebay

fault on a live connection,but check all your earths aswell

battery is dead, check the battery and charging system.

Do you get any dash lights when you turn the key on? If not, you could just have a dead battery.

Checked Cables how?

Looking & Touching isn't doing anything

You have to do a voltage drop test or remove
& clean the ends. I would do both

Remove the starter & have it tested at parts store,
assuming you don't know how to do a voltage or
current test while cranking.

battery,alternator,fuelpump,startergoodluck

there is a shift lock solenoid that cnnects to the shift cable. You will need to access that and replace it

i assume you are saying need to turn light out/ service engine soon light if so there are two ways you can hook to a scanner that has the ability to turn off light or you can unhook both battery cables from battery touch the two cables together for about 10 secs. reconnect to battery if the problem that caused the light to come on has been corrected the light will go out upon completion of drive cycle by touching the cables together you have drained the kam keep alive memory and light should go out