It could be a bad fuel gauge in the dash unit, but....
Usually it's "The bad".
The fuel level sending wipe arm and windings on the fuel sender wear and send erratic readings.The only cure is a replacement fuel gauge sender.
The check engine light should not be related to the erratic fuel readings.
"The good".
If you have over 200,000kms or 120,000 miles on your van you need a new fuel pump anyway,so have a complete assembly put in which will have a new fuel sender on it.
SOURCE: fuel guage issues 2003 pontiac montana
I am sorry but it sound's like your sending unit in the tank has gone bad. Expensive to replace. If your lucky, it's just the wire harness from the gas tank to the frame. "ground Wire"
SOURCE: 1999 Montana gas guage
Larry, for some reason this is a huge problem with GM vans. My parents have a 2000 Chevy Venture that does the same thing you are talking about. It is quite difficult to fix the issue just because you have to drop the tank. I have quite a bit of mechanic ability and I know I could do it myself but the sending unit costs over $200. They originally got a quote to have it fixed by a local mechanic and they told them it would be $450 to fix it. If the sending unit wasn't so expensive it wouldn't be so bad. If you know how to drop the tank it won't be too bad for you to fix yourself but won't hurt anything in the meantime if you don't fix it right away (just don't run it out of gas!!!). Let me know if you have any other questions. Thanks for using FixYa!
SOURCE: the speedometer is not working but the other
Hello Durham.
Couple of possible problems here.
Does the odometer work? If it does your speedometer is defective and will need replacement.
Or if your check engine light is on your speed sensor or wiring will likely be the issue. This will also affect the odometer.
This is caused by a bad sending unit in the fuel tank, I have
seen this many times and a new fuel sender fixes these every time.
This is very common in montanas... also for the fuel gauge (I believe its applicable for a 2000 and later), it may be an issue with the C-305 connector.
Its a pass-thru connection for wiring (from
inside cabin to outside/underneath). It connects various wires for
things in rear of vehicle(including connection to fuel pump/sensor).
While its weatherproof on bottom (underneath) it is not inside cabin,
spills/ various leaks inside can cause water intrusion and subsequent
corrosion of wires.
The connector should be under carpet to the rear of drivers seat (you
may be able to see exactly where by looking under vehicle for a bundle
of wires that come out from cabin) .
Hope helps.
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