My 2002 nissan frontier truck keeps getting hot after i have new head gasket ,hoses ,thermostat ,water pump ,clutch fan,radiator all new. any ideas? also has new heater core also.

check thermostat in boiling water make sure it opens. make sure it is not fitted up side down, check the radiator pressure with a gauge there may be a leaking head gasket. also check the relief valve on the radiator cap.

Do the fans even start when the engine gets hot

BLOWN HEAD GASKET

Well you covered the major points of the system.
It is possible one of the hoses is collapsing internally, or a blockage inside the radiator in spite of it being flushed.
I would try to see if the radiator and the block will flow with the hoses off.
Another possible cause is something blocking the air flow thru the radiator.
Was it running hot before the pump was replaced ? Any chance the pump is defective ?

Good idea on the fans. Keep it that way till you fix the problem. Won't hurt anything, just may shorten fan life-but a new fan is much less expensive than a new engine.
Watch the temp gauge on your dash. The thermostat should open as it warms up, before the gauge gets up to half-way. You can tell when the thermostat starts to open-the upper radiator hose will start feeling hot, or at least start warming up. You will get a rough idea if the thermostat is working properly. By the time engine is at normal operating temperature, the thermostat should be fully open and the hose will feel quite warm. If the hose is still cool when engine is at operating temp., the thermostat may be opening too late, as in wrong thermostat. Not very likely, but always possible.
How did the shop check the head gaskets? Did they do a compression test? With new pump, new radiator, new hoses...should be good, but still runs hot? Any bubbling in your overflow tank? You may want to consider having a block test done, where they check for exhaust gases in the coolant. That would be a head gasket leak causing hot running condition. Maybe the shop checked the gaskets this way, or maybe they missed something? About the only other thing I could think of would be a coolant circulation problem inside the engine, a restriction or something, that an engine flush might clear up. A shop could do a reverse flush of the engine if there is a circulation problem.
Good luck, and keep us posted on your progress. Does your mechanic have any other suggestions?

Check that the fan is working properly, whether it is electric or a belt driven fan. A car will overheat if the fan doesn't function as originally designed.

yes, you will have to change the fan, get a bigger one and go flush the radiator

How do you know the water pump works?
Is the radiator clean outside and inside?
Remove the hoses and flush it out.
Is the thermostat in upside down?
Have you pressure tested for a bad head gasket? Leave the rad cap off and look for bubbles.

Ok well there are a number of possibilities. One to keep in mind is that some water pumps are really cheap and the fins will corrode and rust off leaving nothing behind to actually pump the water. If the pump really is working fine then there are two more possibilities. 1 the radiator is clogged, 2. there may be a head gasket leak. 3. you are sure your gauge is working right? 4. you didnt say what year or kind of car it is.. Is the electric fan working? When the engine is overheating, are both upper and lower radiator hoses hot? If the water pump or the thermostat are failing the upper radiator hose can be hot while the lower can still be relativly cool. If the radiator is plugged or the fan isnt working then both hoses can be Hot. There is also a remote possibility that water passages in the head gasket could be plugged.

2 Inspect Belt Incorrectly routed, adjusted, tensioned, missing, or worn water pump belt(s). 3 Inspect Oil Pan Gasket - Performance Ruptured, cracked or leaking radiator hose. 4 Inspect Radiator Cap Worn or damaged radiator cap 5 Inspect Thermostat Thermostat stuck closed 6 Inspect Fan Blade Broken, missing, or defective fan blade(s). 8 Inspect Water Pump Damaged, worn or leaking water pump. 9 Inspect Intake Manifold Plenum - Perform Leaking water pump gasket. 10 Inspect Cooling Fan Control Faulty cooling fan control or circuit. 11 Inspect Cooling Fan Switch - Radiator Faulty radiator cooling fan switch or circuit. 12 Inspect Engine Temperature Sensor Faulty engine temperature sensor or circuit. 13 Inspect Temperature Switch Damaged or faulty temperature switch or temperature switch circuit. 14 Inspect Fan Clutch Worn, loose or faulty fan clutch. 15 Inspect Ported Vacuum Switch Damaged, leaking, or faulty ported vacuum switch. 16 Inspect Radiator Obstructed radiator core or radiator cooling fins. 17 Inspect Head Gasket - Performance Head gasket leaking coolant into cylinders here is a list of things to check Shaun