Figure 1 The PCMs monitor strategy
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In every shop I've ever worked in, I've seen techs stumped by what should have been relatively simple repairs. It's not because
they didn't have the factory scan tools or because they were less than intelligent. But when the malfunction indicator lamp
(MIL) is on, it's just easier to "send it to the dealer" if a simple part swap or "silver bullet" doesn't cure the problem.
It seems that too many of us don't understand how a particular system works and think it's too hard to figure out. So let's
see what we can do to change that mindset.
Let me challenge you first with this statement: The powertrain control module (PCM) will only command the MIL to to illuminate
if a test it has run has failed. Sometimes it will even rerun the test several times before it alerts the driver that there
is a problem. Now who is going to test these systems more thoroughly, you or the PCM?
By understanding the PCM's test strategy and how the systems it tests are supposed to work, you will make your diagnostic
routine more efficient and reduce, if not eliminate, your comebacks.
BEFORE YOU GRAB A SCAN TOOL
PCM Monitors
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The first step in thinking like the PCM is to understand some basics. Every powertrain diagnostic trouble code (DTC) is associated
with a "monitor." A monitor is a test or tests of an emissions-related system or component. Some monitors, specifically the
misfire, fuel and comprehensive component monitors run all the time; these are referred to as "continuous" monitors.
Others, such as the evaporative emission system (EVAP), exhaust gas recirculation (EGR) and catalyst efficiency monitors,
are run once per drive-cycle and are referred to as "non-continuous" monitors. A drive-cycle is completed when all the conditions
needed for a given monitor have been met and the monitor has run to completion. Some monitors need very little time to be
completed, others require very specific operating conditions. Each monitor has its own criteria, and its own defined drive
cycle.
When a monitor is running, the PCM will look at the results and determine if a fault has occurred. For certain faults, it's
programmed to turn on the MIL and record the fault after the first failure. These are the "one-trip" DTCs.
Most DTCs, however, require the PCM to see the same fault occur over two consecutive trips. "Trips" means a completed drive-cycle
for the monitor in question. These are the "two-trip" codes and will be recorded as "pending" on the first noted failure.
If the same fault occurs the next time the monitor runs to completion, it will "mature" and the PCM will command the MIL to
turn on.
Figure 2 Incomplete monitors can be hiding problems that havent been discovered yet.
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If not, the pending code will be cleared. Early OBD II systems will report pending codes in Mode $07 only for failures in
the continuous monitors. Later OBD II and controller area network (CAN) systems will report all pending codes in this mode.
