Motor Age Garage: Acing the Exam - Diagnosing misfires and cat codes on today's cars requires information specific to the vehicle you are testing. - Motor Age - Automotive training, certification & pa
When I first started in this business, you could diagnose most running problems based on a generalized set of rules. Of course,
this was in the day of carbureted engines and mechanical ignition systems. Today, the Engine Control Module (ECM) does a pretty
good job of keeping an eye on things. While there are general methods used by the ECM for testing, the specific testing procedures
programmed by an individual manufacturer can have an impact on how you proceed with your diagnosis and repair.
Maligned Misfires
After all, if the ECM considers a test as "failed" and sets a code, your repair must meet the requirements for that particular
system. Otherwise, the MIL light will come back on.
My 'Check Engine' Light Is 'On'
Worn gaps require more energy from the coil, leading to heat and ultimate failure of the coil.
Today's subject is a 2002 Mitsubishi Lancer that came in with a "Check Engine" light complaint. There were two codes stored
in the ECM. The first listed was a P0421, a catalyst efficiency code, and the second was a P0304, a cylinder No. 4 misfire.
Considering the mileage on the car, it certainly was possible that the catalytic converter was bad. And we all know that misfires
can quickly overheat the cat and damage the substrate. But is that what the ECM is testing for? Time for a little research.
Oil makes a good conductor of electricity, but then the plug doesn't get it.
The ECM on an on-board diagnostic (OBDII)-compliant car is charged with monitoring the various engine systems for any weaknesses
that will have an impact on emissions. It does this by conducting tests on the systems grouped into monitors. All powertrain
related codes fall into one of these monitors. These monitors can be continuous or non-continuous, meaning that they are either
run over and over during a given drive cycle or run only once per drive cycle. There's a little more to it than that, but
that's enough for now.
Considering what I'd seen so far, this dusting on the coil made me suspicious. Not a "for sure" sign of failure, but it did
bear closer inspection.
Each monitor is made up of one or more individual tests that fall into three categories: circuit tests (for open circuits
or shorts to power or ground), rationality tests (comparing inputs to see if they make "sense") and functional tests (does
the component or system being operated function as expected). Functional tests can be intrusive, meaning the ECM takes control
and actively tests a component, or passive, meaning the ECM tests the component during its normal operation. Knowing which
of these three test categories a code falls into can help you design your testing methods to mimic the ECM's testing technique
and cut down your diagnostic time.
The Cat Efficiency Monitor
Generally, the ECM tests the efficiency of the catalytic converter by gauging its ability to store oxygen, which the cat needs
to burn off the emissions left over after the combustion process. This test typically relies on input from the pre-cat oxygen
sensor and the post-cat sensor. Right off the bat, we know that these two sensors have to be in good shape. After all, the
ECM's decision is only as good as the information it receives. I've seen several "false" cat codes caused by the recent replacement
of a post-cat oxygen sensor combined with an aged, lazy pre-cat sensor.
On this vehicle, all of the methods used are an example of a functional test. The test(s) can be intrusive or passive and
can occur at idle or at cruise. I would like to know how this ECM runs its tests so I can try and duplicate it in the bay
or on the road. Looking up the information on the Diagnostic Trouble Code (DTC) Set conditions, I learn that this monitor
has to have the following conditions met before it will run its test:
engine rpm less than 3,000
MAF frequency between 63-169Hz
intake air temperature over 14 degrees Fahrenheit
barometric pressure over 11 psi
throttle open
ECM in closed loop
vehicle speed over 0.93 mph.
No doubt the coil energy was going everywhere but across the gap.
I also learn that the monitor checks the two sensor signals in 10-second increments, and does this seven times before flagging
a problem. The code itself is triggered when the ratio of switches between the pre-cat sensor and the post-cat sensor is over
0.75:1. In other words, if the two sensor signals start to mirror each other, the ECM thinks there is a problem with the cat.
Peter Meier is an ASE-certified Master Technician, member of iATN and full-time tech in Tampa, Fla. His experience reaches back over 30 years, and his contributions to Motor Age reflect a wide variety of experience with almost every make and model.
Articles by Peter F. Meier
