I have a 2000 Dodge Neon and don't have a real OBD reader yet. The other day I started to turn my car on and, before I got to the ignition the speedometer came on with a "p" and a "1684" (the check engine light has been on for a while). I tried to get it to happen again later. I'm not sure what I did, but I eventually got it to work once but still can't reproduce it.

Does anyone know what happened and how it can be done consistently?

Edit: I would like to know how I can consistently read codes on my speedometer. I am not specifically asking about P 1684 (although tips are definitely appreciated anyway).

2 Answers 2


This can be reproduced by turning the ignition on and off 3 times (somewhat quickly). ON, OFF, ON, OFF, ON. On the last ON switch, the speedometer should cycle through any error codes.

I also found a site that documents this. Different vehicles seem to have different variables: http://www.allpar.com/fix/codes.html

According to the site, this should be the case on all (at least somewhat modern) Dodge/Chrysler/Plymouth vehicles. It might be good if somebody with a different brand of vehicle could test it out.


Apparently, code 1684 indicates a loss of power to the powertrain control module.

One of the risks seems to be corrosion on the battery terminals or cables. As the easiest way to clear this code is to disconnect the battery (resetting the code and the check engine light at the same time), that would be a good time to check the battery posts for corrosion.

This sounds like a non-critical code but, as always, it's possible that a non-critical code was caused by something more important going progressively more awry. On that note, you should probably visit a shop with a code reader before disconnecting the battery so that you can find out what in the world caused the check engine light.

  • Thanks. I think I phrased my question somewhat vaguely (see edit); that's not quite what I'm asking. Still, I will try the battery thing. When I got it to work the second time, I saw two other error codes. One was 0171, which I sort of worked on but haven't been worrying about. The other was my TCC Solenoid circuit, which I replaced a month ago. So I'll look into that and try to see what's going on.
    – jtpereyda
    Apr 8, 2011 at 0:28

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