MIL Trouble Code 12 Meaning

Having long known that trouble code 12 (as flashed by the
Malfunction Indicator Light, or MIL, AKA the "CHECK ENGINE" light
after one does the start with key off, switch key on, key off,
key on, key off, key on, leave key on and count groups of flashes
until done, then turn key off) does not mean what my '96 Jeep Cherokee
service manual says it means, I've been curious about it. I've just
completed some homework and reached my own conclusion about what code
12 really means.

The widely-reported meaning is "battery recently disconnected"
(supposedly within the last 50 key-on cycles), but I've known for a
long time that this isn't quite right.

I have now concluded that code 12 means "emissions systems not
yet proven good". It seems to be reported when:
a.) an emissions-related "monitor" has not yet run its testing,
b.) an emissions-related trouble code is stored, or
c.) ??? maybe some other circumstance I haven't learned yet,
perhaps some numbers of consecutive passing runs of
various emissions-related "monitors" after codes
are cleared.

This neatly explains why:
1.) a battery disconnect causes code 12 (all "monitors" would then
be flagged as not yet having run their tests),
2.) code 12 can appear without a battery disconnect or loss of
connection having occurred (code 12 shows up when certain
other trouble codes get recorded), and
3.) it can take MANY more than 50 key-on cycles for code 12 to
go away, even if no other trouble code is reported
(probably depends on how one's driving patterns compare to
the trigger conditions for running of the various "monitors").

The word "monitor" seems to be jargon for any of various sets
of firmware routines responsible for monitoring and|or testing various
sets of subsystems within the vehicle.

I hope this helps lessen the confusion over code 12.

Happy JEEPing!

 

Your assumptions are interesting. The 12 code shows up when battery
has been disconnected , for sure. It also shows up if/when your
battery drops below a certain voltage at anytime, especially when
cranking. This can happen at any time, even to a perfectly good
battery. That is why you see a code 12 alot of times.
Thanks for your time.

 

This code occurs any time the engine controller microprocessor has a
low voltage reset. This occurs when the voltage AT THE ENGINE
COMPUTER goes below 6 volts.
This can occur during the first millisecond of engine cranking if the
battery or any of the connections that carry the starter current get
rusty and build up a higher than normal resistance. A few extra
milliohms will do this. During a start cycle the lowest battery
voltage occurs when the starter contactor closes but the starter and
engine has not yet begun to spin. This does not last very long, less
than a millisecond. You need to use a digital storage oscilloscope to
see this.

Get your self a good DVM, and a one amp or more load resistor. Then
learn what a 4 wire KELVIN resistance connection means. Then find
the high resistance that is causing the problem. ( E=IR ) Once you
understand what you are doing you can find the problem in less than a
hour most of the time.
John.

 

The main thing is, don't let the 12 code bother you too much. Unless
you know you have a battery/electrial problem,the code is a common
one.

 

Richard Benner Jr did pass the time by typing:

Or if it pops up with lots of other codes. I had one throw about ten
other codes, checked all sorts of thing and one of the last things I did
was pull my test meters power leads off the battery... wouldn't ya know
it, the batteries ground connection popped right off. DOH!

 

Never heard of a 1 AMP load resistor before. 1 OHM, yes.

 

No disagreement there.

No such problem, in my case. Code 12 does not just
spontaneously appear on my vehicle. Its recent appearance
was due to spray-cleaning the air horn, which caused storage
of a couple misfire codes. I've cleared those, and Code 12
will, as it always has, go away and stay away ('til next
provocation) when it's darn good and ready. Whether that
comes sooner or later it definitely won't be related to the
"50 key-on cycles" claimed in the service manual.