T
thoman
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!
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!