Being Proficient: The Art Of Diagnostics

diagnosticDiagnostics is one of the few challenges a technician will face that there is little (if any) preparation for.

I’m not slighting the Apprenticeship program… it’s a good starting point, But it doesn’t come close to preparing technicians for the road ahead. They’re taught system operation and how to use a trouble code/symptom to find the appropriate trouble tree in the service manual to solve problems. This approach will only solve a small percentage of problems accurately and with minimal wasted time. The reality: job requirements to become a master diagnostician go well beyond what’s currently being taught.

So what requirements are needed? This isn’t an official designation but refers to an individual who is proficient in the Art of Diagnostics. This is someone who can successfully tackle any diagnostic problem.

Equipment will play a part in performing accurate diagnostics. Currently, we’re able to purchase both generic and OE scan tools. Check the offerings very carefully before purchasing any scan tool or scope. Many technicians purchase tools based on reputation and past performance—not always a good idea. For generic scan tools, I recommend Autel and Auto Enginuity; for a lab scope I’d recommend the PICO 4 channel automotive kit. No Aftermarket scan tool can compete with the OE dealer tool and thanks to the CASIS agreement, we in Canada are able to purchase the factory scan tools. I’d suggest you purchase the factory tools for your top two car lines and use a generic tool for everything else. As an example of how important the correct tools are, I recently had a call to work on a 2003 Chev Avalanche with an intermittent stalling and no start complaint. When the truck acted up, the gauges in the instrument cluster also acted up.

The vehicle was running fine when I saw it… the gauges worked properly. I suspected a data bus problem of some sort, so I wanted to monitor the Class 2 bus. The Factory Tech 2 scan tool has a bus monitoring function and I checked out the screen shown in Figure 1. You can see that the PCM is not synced with the other modules, indicating a problem with the PCM. Without the bus test this would have been very difficult and time consuming to diagnose. Generic scan tools don’t offer this vital test.

Focus time

Once you have equipment out of the way, you can focus on diagnostics. The key to diagnostics is identifying, then troubleshooting the problem you’re facing. This sounds simple enough…but the reality is that most of us diagnose symptoms and not the problem causing the symptom(s). Let me explain what I mean.

I recently performed diagnosis on a 2007 Ford Ranger for a shop working on the vehicle with a misfire complaint on cylinder #6. This shop checked the cylinder compression, replaced the spark plug, checked for proper spark to the cylinder and swapped the injector from another cylinder into the #6 cylinder. It made no difference! When I started on the vehicle, I verified that the problem was on cylinder 6—clearly misfiring. (See Figure 2.) The engine was then run at various RPM rates and observations revealed that at times the cylinder would start functioning as shown in Figure 3. From the heavy traces you can see that cylinder 6 misfires sometimes and not on other occasions. To pinpoint the problem, you must take into account the various possible faults that could cause a cylinder to misfire, including: fuel to the cylinder, ignition spark to the cylinder, the ability of the cylinder to breathe properly.

Since checking the breathing ability of the engine is relatively easy, I started there. I used a vacuum transducer on the intake and a sync probe around the #6 spark plug wire. Since this is a waste spark system, there should be a signature pulse from the #6 wire for every three breathing events of the cylinder. Figure 4 shows the engine running normally. The blue trace is the engine vacuum signature, the red trace is the sync probe from cylinder 6. You can see from the trace that the engine has 6 even vacuum events. This indicates that each intake stroke is producing the same amount of vacuum… as it should be. Figure 5 shows the vacuum trace when the engine is acting up. You can see from the trace, the engine no longer has even vacuum pulls. This indicates a problem with the engine valve action of some sort. Now I’ve identified the cause of the misfire.

From here it’s a simple matter of examining the valve train to determine the root cause of the problem. Replacing the spark plug and injector was an example of chasing a symptom—while identifying the valve problem identified the problem area so we could focus on solving the problem, not the symptom.

Chasing a symptom

Here’s another example. The vehicle is a 2003 Pontiac Sunfire with a rough idle complaint and a P0300 (random misfire) code stored in the PCM. The engine runs fine off idle and on the road. The following parts were installed to try and fix the problem: spark plugs, spark plug boots, coil pack and ignition module. None made any difference to the way the vehicle ran.

Our diagnosis started with a scan for codes and we confirmed the P0300 code. The engine certainly runs rough at idle but doesn’t appear to be misfiring. Next we looked at the vehicle data stream, in this case fuel trim data was observed to try to determine the problem area. Figure 6 shows the vehicle at idle and Figure 7 shows the vehicle under load. From the data we can see the engine is running rich at idle but normal under load. At idle, Long Term Fuel trim is at -11 and the front oxygen sensor voltage is very high. This doesn’t indicate a misfire, but an overly rich engine. Likely the misfire code and the rough running were the reasons ignition parts were replaced. The problem hadn’t been identified. In our attempt to identify the problem, we have an engine that’s rich at idle but normal under load. This is indicative of a fuel system problem or engine control problem.

Now we must determine the correct problem area to proceed. A fuel pressure test revealed the engine had the correct fuel pressure, so this ruled out the fuel supply system as a problem and left the engine control system. This engine uses a Speed Density type of fuel calculating system to calculate airflow into the engine. One of the major sensors is the MAP sensor.

The MAP sensor relays engine load information to the PCM so the PCM can add the correct amount of fuel. We checked the MAP sensor data with the scan tool and found the MAP reading was low. We then checked engine vacuum and measured about 15 inches of mercury vacuum. It should have been above 18 inches. Low vacuum at idle, but normal off idle, indicates the engine is not pulling air properly at idle. Since the problem went away when the car was driven, we ruled out a plugged exhaust. Now we have the problem area—low engine vacuum at idle. The problem turned out to be carbon on the intake valves as the root cause. Again, a clear case of chasing a symptom without identifying the problem area!


As a diagnostic technician, resources like Identafix and D-Tips will be of limited use and could lead you in the wrong direction. These resources are good for the average technician with limited diagnostic abilities, but rarely used by diagnosticians. is an exception to this—you can post and get in contact with some really smart people who provide knowledge above and beyond what can be found almost anywhere else.

How does one become a diagnostician—take the required training in order to understand all major automotive systems. This will be an ongoing process and continue as long as you’re diagnosing problems. You’ll also be required to do a lot of research and testing until you have an arsenal of tests you’re satisfied with and can rely on to give you predictable results that you can then deploy quickly. You’ll be required to learn the various pieces of test equipment you’ll use and spend the required time with them until you can use them almost without thinking about it. You must be able to think outside the box, recognize how to troubleshoot a system based on nothing more than your knowledge and the system’s wiring diagram. I’d also suggest you develop a network of likeminded individuals to brainstorm with—you’ll be dealing with problems most technicians aren’t equipped to handle. This process is well worth the effort.

The Art of Diagnostics is about identifying the problem, not chasing the symptom. To do this will require a lot of effort on your part. As vehicle complexity continues to increase, diagnostic technicians will be in great demand and the need will only grow as time goes on. Make the effort to become a diagnostic technician.

Photos: Mark Lemay

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