The temperature gauge always read hot or maximum even if the engine is just started. And the fuel gauge always read full even if the fuel tank is not full.

If the gas gauge of your 2007 Dodge Nitro R/T always shows a full reading, there could be a few possible causes for this issue:

1. Faulty fuel level sensor: The fuel level sensor is responsible for measuring the amount of fuel in the tank and sending that information to the gauge on the dashboard. If the sensor is malfunctioning or stuck in the "full" position, it will consistently display a full reading on the gas gauge.
2. Wiring or electrical issue: There might be a problem with the wiring or electrical connections between the fuel level sensor and the gauge. If there is a loose or damaged wire, it could prevent the accurate transmission of fuel level data to the gauge.
3. Instrument cluster malfunction: The problem could be with the instrument cluster itself, where the gas gauge is located. If there is a fault in the cluster's circuitry or display, it may cause the gauge to always show a full reading.
4. Gauge calibration issue: In some cases, the gas gauge might be improperly calibrated, resulting in it always displaying a full tank. Recalibrating the gauge might resolve the issue.

To address this problem, it is recommended to take the vehicle to a qualified mechanic or an authorized Dodge service center. They will have the necessary diagnostic tools and expertise to identify the exact cause of the issue and perform the required repairs. They may need to inspect the fuel level sensor, check the wiring connections, or evaluate the instrument cluster to resolve the problem.

A wire or connection is getting shorted during installation and reinstallation

Remove the gauge cluster and check for a bad ground. If it is bad you can resolder the trace on the back of the cluster back to the groung pin for the connector.

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if you have 2 fuel gauges working of a tank switch there may be some resistance in the circuit of the second circuit that increases the reading of the gauge. Personally I have the tanks balanced out with a levelling hose between them the the gauge reads the capacity left in the tanks IE full-- both tanks-- full, 1/2 both tanks 1/2 With the way you have it set up then if one gauge is faulty and shows full when it is empty you can find yourself in a spot of bother when you need the other tank

How do you know it gets hot? Are you going by the temperature gauge on the dash? Or have you checked under the hood and found out the engine really is running hot? ...Reason I ask is, something just doesn't sound right in your description. After just 5 minutes from starting and driving away, the engine would not yet be at normal operating temperature. That would explain the heater not yet warm. But the temp. gauge says hot?
If so, I would replace the temperature sending unit on the engine. the one for the dash gauge, and see if that was the problem-not reading the temp. accurately.
Couple of other things: Are you sure you got all the air out of the coolant system? Start the car with the radiator cap off and let it run until the engine is warmed up fully, then shut it off and install the cap. Does your reservoir tank stay about half full where it should be, between the full-hot and full-cold marks?
If your problem persists. I would have the coolant system pressure tested, and see if you could get a true temperature reading of the coolant when it seems to be running hot.
Good luck.

they could be seperate problems. the fuel level sender in the tank may be bad. the 2 wire coolant temp sensor may be faulty. there may be a bad ground some where or a bad connection in the harness. although it is common for the circuit boards in the cluster to go bad and you get funny readings on one half or the other. i would start diagnosing the cooling problem first. those engines have an air bleed valve above the t-stat. make sure there isnt a pocket of super heatad steam in the system (first indication of a small leak). the cts also controls the fan relays through the pcm. are the fans comming on as the temp guage passes half way? what does the guage do when the key is on, engine off, cts unpluged, and the connector jumpered or grounded? the guage should have a full sweep if it doesnt sweep the cluster or harness are suspect

Usually if the gas gage does not work yes it is true it may be the sensor if it is electronicly controlled. the sncor will more than likley be in the tank. here is how to remove your gas tank.

1. Before servicing the vehicle, refer to the precautions section.
   
2. Relieve fuel system pressure.
   
3. Drain the fuel tank.
   
4. Remove or disconnect the following:
   
   • Negative battery cable
     
   • Fuel filler and vent hoses
     
   • Fuel tank skid plate
     
   • Fuel tank wiring connectors
     
   • Fuel supply and return lines
     
   • Fuel tank
     
   • Fuel pump assembly
     Fuel pump assembly mounting
     

To install:

1. Install or connect the following:
   
   • Fuel pump assembly
     
   • Fuel tank. Tighten the bolts to 27 ft. lbs. (36 Nm).
     
   • Fuel supply and return lines
     
   • Fuel tank wiring connectors
     
   • Fuel tank skid plate
     
   • Fuel filler and vent hoses
     
   • Negative battery cable
     
2. Start the engine and check for leaks.
   

your ground wire from your fuel tank to the chassis has come disconnected

You don;t say whether the fuel gauge is reading accurately, or whether it's always on empty. If the gauge is always showing empty, it's most likely a problem with the sending unit in the fuel tank. Check the resistance across the two terminals of the sending unit.

On a C900, the fuel sender level unit is separate from the fuel pump and located on the right side top of the tank, accessible by removing the trunk floor lift up panel (the pump is located on the left side top of the tank.
Possible problems are: electrical line off or broken, float not working (it's fairly delicate); or gauge not working.

Test the resistance of the sending unit with the lines disconnected, the resistance should be 2.6 to 3.8 ohms full tank, 63.3 to 67.5 ohms empty tank; and test for voltage across the top of the sender unit, if no voltage, that's likely the problem!

If the gauge is otherwise reading accurately (showing 'full' for a full tank, etc) then the problem is likely a gauge malfunction and can be simply replaced.

Good luck!

Saabreur