2003 Altima engine cold and off, a/c off, collant level full,outside temp 30 and cooling fans start running byself, also 5-10 min after car is parked running light come on by self

temp switch fail most likely. fail safe keeps fan running so engine will not overheat. temp switch will be screwed into engine block. may be cover by cover, hoses or something else but pretty easy to replace. DO COLD for the sensor is in the collant port of the engine and will leak a little when changing. Good luck hope this helps.

So you had it at several repair shop that can't find anything
or don't believ your loosing coolant

Sorry I can't guess I would need the car to check & test

Some major issue like a Head Gasket ??

To start with check the coolant level in radiator...Your vehicle is equipped with a head temp sensor that is designed to shut motor down before it reaches temp. that will result in engine damage..If level is ok then i would look at the water pump at being your next culprit.In either case i would definetly check your temp. sending unit to make sure its operational due to the fact you state that vehicle doesnt get hot...Another thing you can check is to make sure that your cooling fan turns when car gets hot...If fan doesnt come (after you have made sure coolant temp sensor is operational) check fan relay located in power distribution box in engine compartment...

Sounds like the same old intake manifold gasket leak to me. (If it is equipped with a 3.1L engine) Check your engine oil to see if it looks like a chocolate milkshake. For the 3.1L or the 3.8L engine, make sure the coolant level is full. Check the level in the RADIATOR, not just the coolant recovery reservoir. (take the radiator cap off only when the engine is cold)

check that radiator is not blocked by debris leaves etc...

u should be able to see coolant easily, remove the overflow cap and check the level. the proper level is maker on the tank (cold andhot levels marks) clean it off to see it.

Hi it sounds like you have a temp sender playing up I think there are 3 on your engine,

first useing a scanner or temp gun check the actual engine temp. if it is hot check for fan opperation, air lock in the cooling system water pump operation I think youll find the prblem to be the fan

It may be the thermostat, but you should also check:-
1) Coolant level is at correct level (full) when checked cold- prior to running,
2) Cooling fan/s are running normally as they should when coolant temp rises,
3) A coolant hose on the suction side of the water pump is not collapsing (due softening) causing restricted coolant flow,
4) Radiator coolant core is not blocked, or radiator heat dispersion air fins blocked with insects/grass,
5) Engine does not have other running problems that cause it to overheat.

it is not engine thermostat.Your engine temp is normal right? doesnt sound like expansion valve. which is basically thermostat for a/c system. make sure your cooling fans are coming on and no trash on condensor this is in front of radiator.