I often compare a vehicle's power control module (PCM) to a human brain. They both require electricity to function. They both have an operating program that was installed at “birth,” in the computer's case this program is read-only memory (ROM). And, they both have a measure of learning ability, known as random access memory (RAM) in computers. Most importantly, both have means of “seeing” the outside world. The PCM has a wide array of sensors such as temperature, pressure and load, and so does the human brain.

If we humans were assembled in a factory in Detroit and we suddenly lost the ability to see, our brains would set a trouble code, light a malfunction indicator lamp (MIL) and provide a J1962 diagnostic connector.

So, if a PCM lights an MIL and sets a code, how do you go about getting to the root of the concern? What if it is an intermittent problem? If you think the PCM has failed, how do you verify that? Let me share some examples.

1991 Chevy Blazer - 4.1L Engine - Failed Emissions - MIL “ON”

Figure 1 shows the corresponding carbon dioxide (CO2) and carbon monoxide (CO) graphs for this Blazer. Notice how the CO2 red line, which is the vehicle's emissions, follows almost exactly the black line, the “normal” reading for a comparable vehicle. Now, observe the CO red line. It looks like the Grand Canyon, and yes, the peaks are flat because they're cut off. Just draw lines up the sides of the graph for your 4-inch mountains! This truck is polluting seven times the allowable standard for CO. The hydrocarbon (HC) is high, too, but this is probably a by-product of the excess CO. The oxides of nitrogen (Nox) is low; can any be formed with so much CO present?

OK, where do you start? We dove into that MIL light, found a code for the O2 sensor, and performed a visual of the sensor and its wiring. Next, I back-probed the O2 sensor with my lab scope, and went for a test drive. The O2 sensor waveform stayed below 200 millivolts, and did not respond when I introduced propane into the throttle body. This was a one-wire sensor, and the vent hole wasn't mud-packed.

So, what was my conclusion? We needed a new sensor. We installed one, verified that it was active on my scope while driving, and received a passing grade. Was this one too easy? Could I have been caught by an air injection diverter valve that was diverting downstream always? Did I verify the PCM had all its inputs, especially power and ground?

1990 Ford Bronco - 5.0L Engine - Failed Emissions - MIL “OFF”

Here is the failed emissions report for this truck in Figure 2. Notice that CO is still double the standard, but it is still cleaner than the Blazer's was. The graph in Figure 3 shows just a couple of stray red peaks that rise above the “normal” black line. Less obvious than the Blazer, but still a gross polluter.

The trouble with this truck is, there are no key on/engine off (KOEO) or keyon/engine running (KOER) codes. Where do we go from here? I began by systematically checking every system and component that could make this Bronco run too rich. I checked the AIR system for operation, the catalytic converter for efficiency, the air intake for restrictions, and the canister purge system for faults. All seemed in working order.

I found the culprits by scrutinizing the engine data. After warming up the truck, I noticed that the engine temperature only rose to 160 degrees F., so I pulled the thermostat out and found Figure 4: a wedged-open thermostat. I can see the light!

I also noticed the intake air temperature (IAT) sensor was reading on the cool side, compared with the ambient air temp, so I pulled the sensor out of the intake and found Figure 5: a dirty IAT. I cleaned it with some spray carb cleaner and the truck passed the emissions test!

How did we do on this one? With no hard sensor fault, and no corresponding trouble code, we had to scrutinize the engine data closely to see if the "brain" was getting correct information from the outside world.

1995 Pontiac Bonneville - 3.8L engine - MIL “ON” intermittently

The failure in the car occurred so sporadically that I had my customer leave it with me for several days. I caught it acting up for only a few seconds on a cold start one morning, and it set a PO101 in memory. Unfortunately, I cleared the code and it didn't act up again for a couple of days, but when it did, I was ready with my Tech 2 scanner, and my finger was on the snapshot button. Figure 6 shows the data from that snapshot with the engine running at idle. What is wrong with this picture?

If you noticed the mass air flow (MAF) value was out to lunch, you're correct. The engine ran rough when the grams-per-second (GPS) value for the MAF dropped to zero. The PCM also had the ability to recognize the problem and set the appropriate trouble code. "I must have a bad MAF," I thought, so I tapped the MAF with the engine running, but it never induced a stumble. Next, I hooked my lab scope up to the MAF signal wire where it comes right into the PCM. This way, when the MAF value dropped to zero, I could see if I was indeed losing the signal. I could also verify my power and ground to this sensor by observing the signal right at the PCM. Figure 7 shows a snapshot I recorded while watching the MAF signal. What's your diagnosis?

I decided that, barring a power or ground issue to the PCM, this Bonneville's brain was lying to me when it told me it had a MAF issue - and it had to be punished. I did find the ground in Figure 8. Cleaning it didn't help, which verified my voltage drop test. Cleaning it couldn't hurt, could it?

I voltage-dropped the grounds with all accessories on, and found no problem and the voltage on the scanner read in the 14.1 volt range, so I condemned the little silver box to the recycle bin. A new PCM fixed the problem with this car, but how certain was I that it was the root cause?

After using my snapshot mode and viewing a good MAF signal, I felt confident replacing it. Did I confirm my decision by checking technical service bulletins (TSBs)? Yes. Did I search iATN for any posts regarding this issue? Yes.

Scrutinizing Scan Data

Figure 9 shows the long- and short-term fuel trim values for a Chrysler product. What do you think about the numbers? Since anything over zero represents the addition of fuel (in percentages), what do these numbers lead you to think about what could be wrong with this engine? Since the PCM is adding fuel, we could have a vacuum leak or a lean condition. The difference would be in rpm. If the numbers remain high under load at higher rpm, then the engine may be starving for fuel. On the other hand, if the numbers are only high at idle, there is most likely a vacuum leak, which is what I found with this vehicle - an intake gasket leak. The numbers are also higher on the left bank, which is where the leak was.

A nice feature of OBD II is the addition of misfire monitors. I concede that a lot of re-flashing of PCMs has taken place for the 1995-96 model years due to misfire monitors that are too sensitive, but when they work well, they sure can help diagnose a system quickly.

Brian Manley is a vocational automotive instructor for the Cherry Creek school district in Aurora, Colo. He is an ASE master certified automobile technician and a former member of the National Automotive Technicians Education Foundation (NATEF) board of trustees. He can be reached at manley_brian@hotmail.com.

RATE THIS ARTICLE What do you think of this article? Your input will help AutoInc. develop additional articles on this subject. Share your thoughts! Your name Your e-mail address

MOST ACCESSED ARTICLES Fuel Injection Service, Not Just Cleaning The Art of Extraction EGR Systems: Operation and Diagnosis Proactive Target Marketing:_Rethinking Your Business Strategy Engine Performance: HO2S Diagnostics MOST E-MAILED ARTICLES Developing Employee Potential How Critical Thinking Can Help Your Business How to Diagnose the Ford Glow Plug What to Look for When Shopping for the Right Shop Management Software Putting a Price Tag on Complaints

AutoInc. Web Site | ASA Web Site | Have Our Work Force Opportunties Been Unfairly Limited? | An Automotive Technician's Journey: Part II | The Systems, They Are A-Changin' | Calling Around to Get the Best Price on Parts? | Hybrid Safety: Part II | Guest Editorial | Tech to Tech | Tech Tips | Shop Profile | Net Worth | Stat Corner | Chairman's Message