MACS Service Reports - 2016 - JUL2

mally the ECM would back off the injector pulse
width when doing so due to the assumption there is
a burnable vapor (gasoline fumes) being pulled into
the engine in addition to the misted fuel the injectors are spraying. That assumption really isn't an
assumption. The portion of the EVAP monitor that
monitors vapor storage deliberately skips backing
off fuel injector pulse width to factor in fuel vapors
from the evap system in order to measure the effect
of canister / tank vapors as they are pulled into the
engine, combusted and then checked for richness
by measuring leftover oxygen via the O2 sensor.
If that O2 sensor doesn't report less unburned O2
(richer mixture) when the EVAP vapor storage test
is run, it does it again on the next key up and engine
warm up cycle. Since engineers know you can't run
an EVAP test for vapor storage if the engine is not
warmed up (and the O2 sensor operating in closed
loop and operating without any problems, the test
is suspended (put off again for another day) to prevent a false OBD II code.
Conditions of engine operation called "enabling
criteria" (Fig. 1 and 2) often include some form of

tor to run. When a single monitor runs, we often
refer to that as an "OBD II Trip." When all of the
OBD II monitors have run, (Evap, Cat Efficiency,
Misfire, etc.) we call that an "OBD II Drive Cycle."
Some emissions problems are so harmful that the
ECM turns on the MIL after a single failure (during its drive cycle) of that monitor. For example,
an ignition related misfire severe enough to allow
unburned fuel to flood the catalytic converter to the
point the converter overheats and is damaged, the
MIL may come on (and flash) after a single OBD II
monitor failure. The flashing, of course, is to rattle
the nerves of the customer whom might otherwise
disregard the MIL as trivial. Other emission issues
that could flag a MIL are not as harmful to emissions. When looking in your scan tool's OBD II Status / Monitor Readiness section you'll likely see
numerous monitors that register a status either ran
/ completed vs. not ran / not completed. If those
monitors say "successfully ran" it doesn't necessarily mean the ECM has passed that monitor. It only
means the monitor / test was performed. A good
comparison would be the old pencil and paper ASE
tests. When you filled out the registration, showed
up at the test center, filled out the answer sheet with
your name on it and turn it in you took the test. Did
you pass or fail? You found out later when they emailed or mailed you the results. OBD II monitor
status (Fig. 3) is located in various locations of scan

Figure 1: shows the criteria required for the comprehensive component
monitor (TPS, ECT, IAT, etc.) to be completed.

Figure 3: shows a typical monitor status list. Completed means the test
was run - pass or fail.

tools depending on the tool. One location to find
monitor status in any aftermarket "OE Level" scan
tool (Snap On Modis / Solis / Verus, OTC Genesis
/ Pegasus, etc.) is in the OBD II generic or Global
OBD II section. The exact location's name may be
specific to the individual tool. There is a standard-

Figure 2: shows an individual non-continuous monitor's run criteria in order
for you to recreate those conditions on the vehicle.

engine warm up (typically a 40 degree rise in temperature) along with engine speeds, load condition
and a host of other items to all line up for a moni-

July 2016

MACS Service Reports

Table of Contents for the Digital Edition of MACS Service Reports - 2016