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

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?

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.

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Pontiac Montana Code P1639 5-Volt Reference Low