2001 Pontiac Montana.
What is the code p1639 5-volt reference circuit
mean and what can i do to fix it?

Answer:
Hi Kevin, thanks for visiting. Code p1639 is 5 volt
reference circuit for the a/c pressure sensor and
the fuel tank pressure sensor. The pcm sends 5
volts to theses sensors to operate, and the pcm
did not see the correct voltage back.

You Need to
check voltage. Check for 5 volts in
the disconnected connector to each of these
sensors, while the other is still plugged in. If less
than 5 volts at one of them, unplug the other and
see if it goes up to 5 volts, if it does, then that
sensor is pulling down voltage, and needs to be
replaced. If less than 5 volts at either, then you
have a wiring problem, or a bad pcm.
Visitor reply:
i did notice that my fuel gauge does not work now goes up and
down for no reason. Does this sound like it is related?

Answer:
Well, the level sensors in the tank are pretty common for acting
up like that, it is on a different circuit, but the level sensor and
the fuel tank pressure sensor do share the same harness, and
the same ground (low reference).

Now that a problem with the level sensor circuit shows up, do
this...
Under the van, right behind the drivers seat is a couple of
connectors that come through the floor board and go toward the
rear of the van. Unplug those connectors from the under body,
look in connectors and body connector to see if there is any sign
of green or corroded terminals.

Have had some wired electrical problems because of that.
Sometimes even get
ABS codes. This would be the next thing to
check, since that is in common with the fuel level circuit/harness.
Let me know, am kind of optimistic about you will find a problem
in one or all them.
Pontiac Montana Code P1639 5-Volt Reference Low

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