Engine light

Ok here goes.
Just bought a 2002 Rodeo. Runs fine.
My check engine light comes on but goes half when gas tank is at or less than 1/4 tank.
Gas gauge takes a while to give an accurate reading.

When idling the gas symbol comes on , the fan starts, and the gas gauge goes to emtpy. When I put in drive the gas gauge goes back to semi-accurate reading. I know what twenty bucks looks like from inside the vehicle.

Please help don't want to get ripped off. Mechanic said $500.
Could it be a faulty gas guage inside the gas tank ?
Gas injection OEM ?

Heading to florida and don't want to look at check engine light, fan turning on, gas gauge dancing all over the place. lolololol

If you have a CEL I would get the codes read (autozone does it for free).

Your gas guage issue sounds like a bad sending unit. Search the FAQ's for more info.

before getting too deep into tracking down each of those weirdo symptoms, i'd look at the charging system, specifically the alternator, but first check all the wires, grounds and battery terminal wires and connectors.

//bc

p0463 fuel level sensor circuit high input
open in fuel sending unit
mechaniic will tell you he has to replace the entire fuel pump
go to the nearest isuzu dealer purchase fuel sending unit for a 2000 model and a fuel pump seal for the 2002
have your mechanic install the sending unit the part is around $50.00
labor should not be more than $150.00 thats him charging $75.00 an hr.

My cousin had the same constraints with his GM alternators and here is something he gave me for you to read and to know where start with regarding your problem, it is basically about alternator and little about battery:

Specifically, car alternator works like generators but innovations made its function far more complex. The car needs loads of power to make its components work. The vehicle's electrical system is one of the sources of energy that supplies power to the lighting system, ignition coils, climate control parts and many other auto components. However the power it generates is not enough to support the whole vehicle. The battery also provides power but still it is not enough to make the whole system work. The car has several power sources but it roots down to the simple yet complicated energy maximizer, the alternator.
Alternator is the bridge that connects the battery to the gas tank. It transports the mechanical energy from the crankshaft through a fan belt. Its main role is to transform the unused power from the car's engine into electrical energy which empowers the different systems of the vehicle. As mentioned, GM high output alternator works like generators. Older cars use generators instead of alternators. Old fashioned generators has a wound stater which creates a magnetic field that causes the armature, the coil of wire that can be seen on the component, to move.

On the other end of the armature are brass segments that produce electrical current by using carbon brushes. Each segment is attached to the armature shaft. However it is considered as the weak spot of the system because the centrifugal force produced, as the armature rotate at high RPM, can cause commutator explosion. Explosion can be prevented by driving the vehicle at speeds less than the standard acceleration level of the engine. Decreased speed causes the generator to produce little or no current at all posing another negative effect.

p0463 fuel level sensor circuit high input
open in fuel sending unit
mechaniic will tell you he has to replace the entire fuel pump
go to the nearest isuzu dealer purchase fuel sending unit for a 2000 model and a fuel pump seal for the 2002
have your mechanic install the sending unit the part is around $50.00
labor should not be more than $150.00 thats him charging $75.00 an hr.