A simple 4-step diagnostic strategy

Diagnostics is one of the greatest and most important challenges facing technicians today. There is an ever increasing amount of diagnostic tools and codes to get up to speed on, and pressures to use those tools and codes effectively to diagnose and repair a vehicle correctly the first time that vehicle comes into the bay.

It’s not easy to accomplish, but a diagnostic strategy will help a technician get to that ‘first-time-right’ goal.

Let’s begin by highlighting a customer’s problem and show just how this diagnostic strategy can be used to source the “cause” of the problem.

Ms. Jackie Smith is the owner of a 2000 Chevy Blazer, (4×4) with a 4.3ltr V-6 engine. She reports to you that, on occasion, the MIL light comes on for brief periods of time, and then turns off. While the MIL light is on, the engine seems to run OK, although one time, it seemed to run a bit rough.

She has already taken her vehicle to a couple of other repair shops, and has had the following repairs/services completed:

1 Scanner Diagnostics: A P-0300 = Random misfire code was identified once. When you search for codes, none are found.

2 Tune-up: One of the previous shops completed this necessary service. Ms. Smith said it had been two years since the last tune-up.

3 Injection System Flush: Because the problem resurfaced within a few days after the tune-up, Ms. Smith returned to the same shop, and agreed to have the injectors cleaned.

After each of these services were done, the vehicle’s performance had greatly improved, and for the next few days, all seemed to be fine — that is until today, when she drove her Chevy Blazer into your bay.

Bearing the look of frustration, Ms. Smith repeats her story, and the work done to you. During the conversation, she mentions that she had the battery replaced about three weeks ago. After a few questions from you, she admits that the problem first appeared after the battery was replaced.

Now that you have learned all that you can from this customer, how should you proceed with your diagnostics? Is there a diagnostic procedure (a flow chart if you prefer to call it), that can be used to identify the cause of Ms. Smith’s vehicle’s problem?

Simply, there is a diagnostic procedure you can follow and it is referred to as: Symptom-to-System-to-Component-to-Cause. This diagnostic strategy allows a technician to isolate, locate, and repair a customer concern in a logical sequence.

To see how this strategy works, let’s apply it to Ms. Smith’s vehicle:

SYMPTOM: Intermittent MIL Light On

SYSTEM: PCM

COMPONENT: Engine Mechanical? Engine Sensor(s) — Specifically the CAM sensor, since it is used for “misfire” detection, and may create this problem, if the distributor is not timed correctly.

CAUSE: __?

Before identifying what the actual cause is, let’s take a moment to review the findings: The engine is mechanically sound. The distributor cam retard setting is “0” degrees (specs are “0” +/- “2” degrees). Again, you review the vehicle’s repair history, and note that the problem surfaced shortly after the battery was replaced. You now refer to the TSB’s for this vehicle’s engine, and find one published by GM, that says: “Anytime the crankshaft, reluctor, sensor, PCM, or the battery, is/are replaced, a crankshaft relearn procedure should be performed. If this is not done, incorrect misfire code(s) could be set.”

The reason for this is that the PCM “learns” the crankshaft sensor’s reluctor imperfections, and if the battery is disconnected, the PCM must be “re-taught” those relucator settings.

What you need to consider and keep in mind at all times is that most scanners contain a program, entitled “Special Tests.” Listed under this heading are features (sub-programs), entitled:

Fuel Trim Relearn

Cam Retard/Sync

CRANKSHAFT RELEARN *

So the actual cause of Ms. Smith’s vehicle problems was failure to perform a crankshaft relearn procedure. Once you’ve performed this procedure, and completed a “verification — road-test” (which could take up to 30 minutes), the vehicle can be released to Ms. Smith, and the problem will “permanently” disappear. And you will have a satisfied customer.

Mike Duguay has been a licensed technician since 1974, opening Duguay’s Auto Repair in 1992. He has also been a substitute instructor for the Vancouver Community College Automotive Department. In 2003, Mike began working as an instructor with Injectronics Training www.injectronicstraining.com.