Check engine light comes on after refueling the vhicle. gauge doesn't read full, then it will. light goes out after a day or two.

Gas cap

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just replace my fuel pump /sending unit on my 06 Malibu, didn't matter if the pump was in or out of the tank, the gauge still reads wrong

The check engine light will set a code, to read this code you must have the system scanned with a special tool. This is not a wiring GREMLIN, trust me. As far as the gas gauge goes you will need to pull the fuel tank out and replace the gas gauge fuel level sending unit.

It sounds like you might have a poor ground connection somewhere. You can test each individual gauge for full travel by grounding the wire the goes the each gauges individual sender. In other words to check the temp gauge, ground the wire that goes to the temp sender. The temp sender is in the left cylinder head between the #1 and #2 spark plug ( looks like a brass pipe plug with a single wire coming from it) if you ground the wire with the key on (engine off) the gauge should go to the end of the scale (hot) with the wire off it should go to full cold. You can do this with the gas gauge too but not the amp gauge or volt meter. If the volt gauge is reading low (if it has one) and you know that battery voltage is good (don't guess, measure with a multi-meter) there has to be a poor connection somewhere.

Has this problem come shortly after refuelling? Do you live in a humid climate? 9 times out of 10 this is a result of the gas cap. Failing that, can you get your hands on a OBD 2 tool or go to your dealer and have them scan the truck. The Check Engine Light throws a code and the OBD2 tool reads that code. The code points to the problem.

very well sounds like the sending unit in the tank. Try running some octane booster and fuel injection cleaner in tank of the truck to loosen any deposits on the float.

This is a normal concern.
The instrument cluster uses 4 different operating modes to calculate the fuel level:
Anti-slosh (default mode)
Key OFF fueling
Key ON fueling
Recovery
After a fuel fill up, the time for the fuel gauge to move from empty (E) to full (F) ranges from 2 seconds to 55 minutes depending on which operating mode the fuel gauge is in.
The default fuel gauge mode is called the anti-slosh mode. To prevent fuel gauge changes from fuel slosh (gauge instability due to changes in fuel sender readings caused by fuel moving around in the tank), the fuel gauge takes approximately 55 minutes to go from empty (E) to full (F).
The key OFF fueling mode (2 seconds to read empty [E] to full [F]) requires 3 conditions be met:
The key must be in the OFF position throughout the entire refueling of the vehicle.
At least 15% of the vehicle's fuel capacity must be added to the fuel tank.
The instrument cluster must receive a valid key ON fuel sender reading within 1 second of the key being put into the RUN position. The key ON sample readings are considered valid if the fuel sender reading is between 15 ohms ± 2 ohms and 160 ohms ± 4 ohms.
If these conditions are not met, the fuel gauge stays in the anti-slosh mode, which results in a slow to read full (F) event.
The key ON fueling mode (approximately 90 seconds to read empty [E] to full [F]) requires 3 conditions be met:
The transmission is in PARK (P).
The key is in the RUN position.
At least 15% of the vehicle's fuel capacity must be added to the fuel tank.
In key ON fueling mode, a 30-second timer activates after the transmission is put into the PARK (P) position (automatic transmissions). When the 30-second time has elapsed and at least 15% of the vehicle's fuel capacity has been added, the fuel gauge response time is 90 seconds to read from empty (E) to full (F). When the transmission is shifted out of PARK (P), the fuel gauge strategy reverts to the anti-slosh mode. The key on refueling mode prevents slow to read full events from happening if the customer refuels the vehicle with the key in the RUN position.
Recovery mode is incorporated into the instrument cluster strategy to recover from a missing fuel level input during a refueling event. Missing fuel level inputs result from intermittent opens in the fuel sender or its circuits. Recovery mode (empty [E] to full [F] approximately 20 minutes) is initiated when the following 2 conditions are met:
The instrument cluster is in the anti-slosh (default) mode.
The actual fuel level in the tank is greater than what is being displayed by the fuel gauge

CHECK ENGINE LIGHT ON READS FUEL SENSOR
AND 5 CYLINDER

Possible bad guage, bad wiring, or bad sending unit. Check each one independently of each other, start with wiring first, then sending unit.

The nature of the gauge is that it should behave with a linear taper between full and empty. If it works when full but then malfunctions as the level drops, the variable resistor in the sending unit which is housed along with the fuel pump in the gas tank is almost certainly at fault. The sending unit will have to be replaced to fix the problem. Expect this to be an expensive endeavor.