Gas gage says empty,when it is half full.Why wont car start

it will help if you tell us what kind of car it is and dose this problem happen after you fill it while the car is running or is it off ?

Map sensor is bad

probably not all that difficult to fix it you have patience. The needle going way past full is an indication of a short before the sender unit or in the sender unit. Pull the power wire from the sender unit and see what the gauge does. IT should stay on the empty mark. IT may be just a fuel sender unit .

what car is it?

Sell it as qoick as possible before it pennys you to death, Like all the other over price toys out there in the last 20 years,

Not computer. You have air in the cooling system. Bubble is moving around. Park on an upward incline, take the radiator cap off, run the engine and keep the radiator full. Let it warm up and let the thermostat open (until you see flow in the radiator). Then fill to the brim and keep full until it stops going down. Also open any bleed ports you can find in the cooling system (there may not be any) and let any air out of them. Let the engine run and check for any bubbles that continue to come out. If bubbles continue to come up, you have a blown head gasket that is forming the bubble and causing the heater and temp gage issues. If bubbles do not come out, maybe you have fixed the problem by getting all the air out. Please let me know if you have any questions.

fuel pump went out

What causes the gas gage to work is the sending unit works in resistance. As the resistance goes up, the float goes to empty, as resistance goes down, it goes to full. Resistance in this case equals ground. So it sounds like the ground has been lost. check for a bad wire and under the frame back to the tank is a good place to start since that is where a lot of corrosion is posible.

Since the Uplanders have the long (front to back) skinny gas tank, with the outlet to the rear; if you run out of gas with the front/nose inclined lower than the rear, then it can take upwards of 5 to 8 gallons to get it started again. It was written up in a GM TSB. Two gallons won't start it. When the gage reads less than 1/2, it's not a good idea to park with the front lower than the rear...unless you carry a full 5 gal. refill. Thank you GM engineering.

the gas gauge is on the fuel pump assembly with in the fuel tank, you must have bumped it during reassembly. the only solution is removal. pull it out check for alignment and binding, reassemble being very careful. hope this helps.