On-Target Troubleshooting

I've met and worked with a lot of technicians over the years, the majority of whom were honest, intelligent individuals. A few could be considered "Top Guns," able to handle nearly any diagnostic situation handed to them. What separated these few from the majority? I believe there are several factors, all of which you can easily develop for yourself.

Step 1: Verify the Complaint

The object, of course, is to duplicate the problem the customer is having. This can be as simple as seeing the Malfunction Indicator Lamp (MIL) on for yourself, or taking a test drive with the customer and feeling what they feel. Doing so provides some initial idea of what the problem may be, and also allows you to form a baseline for verifying your repair after it's completed.

Step 2: Gathering Information

The primary goal in this step is to learn as much as you can about the problem you are troubleshooting. This starts in the verification phase when you saw the problem yourself and/or learned as much as you could from the customer about when and how the problem occurs.

Since you already have the customer's view of the problem, next ask the onboard computer (ECM or PCM) what it thinks. Connecting in Global OBDII mode with your scan tool is the fastest way to access some basic information. See what the monitor statuses are, if any codes have been stored and if any are pending. If a code has turned on the MIL, and it is related to a continuous monitor, be sure to check and record the freeze frame information. Depending on the problem, take a look at current data from the Parameter Identifications (PIDs) that may be related to the fault. Using Global OBDII here ensures you are reading accurate information and not a substituted value the ECM may be using. One of the last bits of information you want from the computer is the current calibration identification, just in case there has been a reprogramming Technical Service Bulletin (TSB) issued by the manufacturer.

Since you may want to ask the ECM a few more questions later, DO NOT clear the codes. Doing so will clear freeze frame and Mode $06 data as well, and you may still need that. Clear the codes only when you are ready to complete the final step of this process.

When dealing with misfire or fuel trim issues, be sure to note which cylinders and/or banks are affected. If only one cylinder is misfiring, what is unique to that cylinder? If one bank, then what is special only to that one? Is the issue occurring at idle with no load or at higher rpm under load? All of these observations will help you later when you move to the next step.

Now it's time for you to do some research. Use your service information system to study up on the theory and operation of the systems you are troubleshooting. No generalities allowed here...each manufacturer has its own little way of doing things. The same applies to looking up the code information. Even though the code may be a standard SAE designation, a P0301 (cylinder No. 1 misfire) for example, each manufacturer is still free to determine how and why the code is recorded by its ECM. Knowing exactly what the enabling conditions are and what the specific ECM testing method is will help you later in testing the system yourself.

Look for applicable TSBs while you are doing your research. Depending on the source, more than 40 percent of today's drivability issues are a result of programming. Many others are related to redesigned components. You can troubleshoot all day long and not figure this out, so don't spin your wheels unnecessarily. If you do find a related TSB, test to verify that this is indeed the cause before following it blindly.

If this is the first time you are dealing with this particular problem, there are online resources also available. Sites like iATN (International Automotive Technicians' Network) and Identifix are two such resources that have databases of known problems and their possible solutions. While there are fees associated with some of these sites, the cost can be recuperated in time saved. It may be that a tech on the other side of the country has seen this same problem several times and posted his findings. As with TSBs, test to make sure his fix applies to your problem before throwing the parts at it.

Don't rush yourself in this phase. The more time you invest here, the less time you'll spend in actually locating and repairing the problem. Once you are comfortable that you fully understand the nature of the problem you are facing, the operation of the system(s) involved and the definitions of any codes you found, you are ready for the next step.

Step 3: Decide On a "Most Likely" Cause

Grab a cup of coffee or a cold drink and take a few minutes to think about what you've learned so far. By now, you probably have a few ideas of what the root cause of the complaint may be. Decide on what the most likely cause may be.

For example, if you are diagnosing an "insufficient EGR flow detected" code on an early Ford, you would have learned that the Differential Pressure Feedback sensor (DPFE) is responsible for informing the ECM of actual EGR flow, and that it is a common failure item. This code could also be caused by a restriction in the EGR flow path from the valve to the intake, or a failure of the EGR itself. The most likely cause would be the DPFE.

Misfire and fuel trim codes can be caused by several variables. The input devices the ECM uses to track the results of these monitors may be questionable, and any factor that affects the combustion of the mixture in the combustion chamber comes into question. Part of the gathering information phase may include some baseline testing of the engine's mechanical health, the ability of the fuel system to deliver the proper charge and the ignition system's ability to provide the correct spark. With this information in hand, you may decide that one system or the other is the most likely culprit.

Now that you've decided on the most likely suspect, it's time to see if you're right.

Step 4: Test Your Suspect

If you're following a diagnostic "flow chart," shame on you. While helpful, it has been my experience that technicians will follow the flow chart only as long as the testing described is convenient or easy to do. In addition, if you don't know what you are testing and why, the results may be misleading. While flow charts can be helpful in understanding the problem and how it can be tested, it is best to follow your own flow chart, based on what you've learned.

So design your own tests appropriate to the component or system you are testing. The objective here is to either verify your suspicion or prove yourself wrong. If your test(s) verify you were correct, the problem is identified and you can proceed with the repair. If not, then incorporate what you've learned into the gathering information phase, and rethink your most likely cause.

Back to the Ford EGR code as an example. You now know that the DPFE is a common failure, so you decide that it is the most likely cause. Using your scan tool, you record the KOEO voltage reading of the sensor. It typically is less than 1.0 volt. You then start the car and manually open the EGR valve while watching this voltage. It should increase with EGR opening, and the engine should try to stall if EGR is being fed through the intake at idle. If the engine idle is not changing as you open the valve, then you know no EGR gasses are passing through. It may be time to rethink your most likely suspect. If it does, but the DPFE voltage isn't changing, then the odds are your first choice is your best one. After you verify the sensor has power and ground, you've proven the sensor is at fault.

You may decide that a mechanical issue is causing the single cylinder misfire code you are troubleshooting. Testing engine compression, either with a gauge or a scope, may be your first step, and if it reads low you may follow up with a cylinder leakdown test to further isolate the cause of the loss.

Continually educating yourself on testing techniques will provide you with a storehouse of testing methods to choose from, and speed up your testing time. There are a lot of new techniques using a lab scope and pressure transducer, for example, that can make short work of testing for mechanically related issues like low compression, sticking valves and cam timing variances. In some cases, though, "old school" still works the best...like using a manifold vacuum gauge as a quick measure of engine health.

Remember your TSB search? There are also TSBs issued by the manufacturers on how their systems work and testing methods you can use to speed up your diagnosis. Keep an eye out for these as well, read them, and add that knowledge to your arsenal.

Continue your process of testing and analyzing the results of your testing until you have nailed down the cause. Now you can make the repair, and complete the process.

Step 5: Verify the Repair

Now that you have the fault cornered, it's time to actually make the repair. But you're not done yet...not until you've verified that repair.

You can do that by retesting the same way you tested to find the problem, to see if the test now passes. In the case of a related Diagnostic Trouble Code (DTC), now is the time to clear the code and rerun the monitor to see if the code stays gone. You can also mimic the monitor and test the system the same way the ECM does. You did read up on that, didn't you?

This step is important for two reasons. First, there may be additional faults combining to cause the main complaint your customer told you about. Second, you want to make sure that the car is ready for delivery to your customer to avoid a comeback. Customers expect their car to be repaired correctly the first time they pay you, and they aren't happy if they have to come back again for the same issue. Often times, they won't come back...ever. So don't neglect this important step. Always verify the repair before pronouncing the job done.

Take the time to follow a logical approach to your troubleshooting and you'll fix more problems than ever before. Who knows? You may even become your shop's "Top Gun!"