Fuel gauge doesn't move despite milage driven and digital readout for miles left to fill fluctuates up and down. Dealers' computer says all normal with car but I've driven over 175 miles on one tank of gas and needle shows just below FULL.

Fiats have a reputation for electrical/electronic problems as they age and even when they are not very old they can manage strange things that even sometimes defeat the dealers, though the same can be said of many modern vehicles. I used to look after a Ducato motorhome and fortunately didn't need to get involved with much electrical work - the only faults were caused by the coachbuilders rushing around and being careless and not by Fiat.

I am not sure if the temp gauge is driven by the engine management or is a traditional circuit but either way the temperature gauge is in effect a voltmeter of sorts. Assuming the gauge is ok, the reading will only fluctuate if the voltage in the circuit fluctuates and that could mean the supply voltage is fluctuating or the resistance of the ground path is fluctuating.

With the traditional circuit the gauge is supplied by an electronically stabilised voltage and the ground path resistance is varied by the engine coolant temperature sensor and fluctuations could be anything from a faulty instrument voltage regulator, through faulty wiring or connections to the temperature actually fluctuating.

If the gauge is driven by the engine management the same approximately applies but checking the circuit becomes much more complex due to the need for circuit diagrams and specialised test equipment, though these would not be needed just to check the security of the gauge in the printed circuit of the instrument panel.

Try disconnecting the battery for a few minutes, then maybe the computer will relearn the correct fuel level.

This has got blown head gasket written all over it. How many miles are on the vehicle. Whens the last time the thermostat was changed?

Its very simple. If the gauge is showing half full then fill the tank and see how much fuel it takes. If the amount you add is approximately 50 percent of the fuel tank capacity then you know the fuel gauge is accurate . In that case the computer is giving an inaccurate reading and should be ignored. All you need is an accurate fuel gauge - it does not matter a row of beans what the computer says if the fuel gauge is reading correctly.

The sending unit in gas tank is broken - hence the wrong readings of how much gas is in the tank. Replace unit.

Oh my, there is no reset. You would need to remove the pump module and check the gauge sender part, maybe the float arm got jostled or bent when installing.
Sorry to tell you this, it will have to be redone. This time when you pull the unit out of the tank, take a moment to connect the battery, connect the electrical connector to the pump module, turn the key to on, then check that raising and lowering the float arm makes the gauge read right.
It's either this way, or let it be and figure your gas left in tank by number of miles driven.

Since the guage moved, your wiring to your pick up is still good. Bad news is that if it doesn't move or moves all at once from full to 1/4, your float sensor is probably sticking or just bad. To clean or replace it would require you to drop the tank and pull the sending unit out. Sorry.

There's a float inside the tank that the fuel gauge works off of. If you step on the brakes or accelerate hard it causes the gas inside the tank to slosh around and you can see the fuel gauge move when you do this. If you know the vehicle has gas in it, the best way to check if the gauge is bad is to drive around enough to where you should have burnt a noticeable amount of gas. If the gauge doesn't move, then there's something wrong with it.


Since the gauge moved when you braked hard, I would assume it's working properly because it picked up on the level change inside the tank where the float's at.

What I do is go by miles per gallon and fill the tank when I've driven enough for it to drop. As long as the odometer works it's all good. You can fix it if you want to go to the trouble. It's either a bad card in the tank module or a resistor fell off in the instrument panel.
Panels can be refluxed just like motherboards by tossing it in the oven for a few seconds. Tank modules have to be replaced.

Could be the float or it could also be electronic card problem that this float inside the tank that is sending the signal to the marker, or problems contact Latarjet electronica.Thanks