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Use this procedure to verify that the logic module in the instrument cluster can drive the fuel gauge. Depress and hold the trip odometer button. Turn ignition key switch to the RUN position (do not start engine). After a few seconds release the trip odometer button. That places the cluster in self diagnostic mode. All the warning lamps should be illuminated. The gauges (speedometer, tachometer, fuel, temperature) will move to full clockwise position and then move incrementally counter clockwise to resting position.

If the fuel gauge moves as described then you know the logic module in the cluster is functioning properly and it can drive and ,make the fuel gauge respond appropriately. If the fuel gauge does NOT move then the issue is in the instrument cluster and the solid state driver for the fuel gauge.

If the instrument cluster self test indicates all is well, then there is a problem with an improper signal or no signal from the fuel sending unit in the tank and / or a problem with the wiring to the instrument cluster and the fuel level sending unit.

This is a situation where having access to a sophisticated scan tool that a dealership service department has will make quick work of this diagnosis. The appropriate scan tool can check for open or shorts in the fuel level sending circuit wiring and inform accordingly.
 

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1966 Crown Coupe, 2016 200 S AWD, 1962 Lark Daytona V8.
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If your cluster tests out ok, try some Techron in the gas tank. If the problem is a coating on the fuel level sensor, the Techron is known to help.
 

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Oh, techron ? of chevron is a real thing ? I've ocassionally had a engine that would run on fewer than 4 cyl at startup. but usually clears up in a few mins or once you get 1/2 throttle or so or get some temperature (one of those - I just start driving it and it gets better). Maybe I'll put techron of Chevron in it. I've got a bunch of carb cleaners from Walmart that I keep putting in at fillup, ir may or may not be causing/curing the problem.
 

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Techron is the only fuel additive Chrysler (FCA) actually approves. I run it in my vehicles every 6 months or so.
 
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