Auto Extreme

Regardless of whether you’re a master tech or a first-year apprentice, it’s all too easy to fall into unproductive habits. These tips will help you stay on the straight and narrow.

I always buy pencils with erasers. I’ve been around long enough to know that I’m going to need those erasers from time to time. Most of us, if we’re honest, have made diagnostic mistakes and have accumulated a stash of so-called known-good replacement parts. You may, for instance, have replaced an ignition coil you thought might be weak, only to discover that the real cause of the hesitation was a faulty module ground. Or perhaps you replaced an integrated control relay and later found a bad splice in the wiring harness.

My goal with this article is to help you increase your diagnostic batting average. If together we can keep your stockpile of known-good parts from growing, that will be a good start.

1 The new part. A great many FIX posts on iATN confirm my observation over the years that one of the most common diagnostic mistakes is assuming that a new part is a good part. While parts quality certainly has risen industrywide in recent years, there still are occasional defects. Some are due to poor manufacturing methods or simple material flaws, while others are the result of mishandling or damage during shipping or (even) installation. A few defects can he traced to improper design or material selection. For the technician, the problem arises when, following a correct initial diagnosis, the new replacement component fails to correct the problem.
Most technicians who find themselves in this position understandably assume that the new part cannot be at fault. They then cast about for another explanation, often spending fruitless hours in the process. A small minority will make the opposite (correct) assumption, that their initial diagnosis was correct and the new part is faulty. But even many of these will be disappointed when a second same-brand replacement part from the same batch proves equally defective.

What, then, is the solution? For some components, an original equipment replacement part may he the sensible solution. For others, many aftermarket alternatives exist. To solve this puzzle, you must first retrace your diagnostic steps, then have faith in your conclusions. In many instances, searching the iATN FIX archives, or consulting with a diagnostic hotline, such as Identifix, will guide you toward or away from certain replacement brands for a particular component.

Lesson: When you’re certain your initial diagnosis was correct, and retesting reveals no additional problems, try a second (or third!) replacement part from a different manufacturer.

2 Pattern failures and silver bidlets. Repair hotlines make their money doing two tilings: First, they recognize, collect and catalog pattern failures. Second, they provide diagnostic guidance for those problems that are unique or at least not yet widely recognized. Similarly, the iATN FIX archives supply somewhat less organized, but often at least equally valuable, diagnostic information on a wide variety of repair problems. The pitfall for practicing technicians is jumping to a conclusion where the symptoms don’t quite match up, where there are several plausible causes for the same problem or where more accurate and complete testing would have revealed additional information or the actual underlying cause.

Thus, for example, while the conjunction of codes P1456 and P1457 in a 2001 Honda Odyssey usually indicates a faulty canister purge bypass solenoid, in areas of high road salt use, it’s quite likely the PCM will have acted as a $475 fuse, sacrificed on the shorted VSVs altar! Obviously, the time to find this out is before you replace anything. Make sure you check the PCM s ability to control the circuit. If it can’t supply an adequate ground, replace the shorted solenoid before you let the magic smoke out of a replacement PCM.

Lesson: Always allow adequate time for retesting in your initial estimate, and don’t jump to conclusions.

3 Checking the basics. Quick but thorough checks of basic fluids, belts (including tension), battery condition and connections, and sometimes even of tire pressure and tread depth, are absolute prerequisites for accurate diagnosis. Remember, too, that excessive AC ripple voltage from a weak battery or faulty alternator can cause a wide range of driveability problems.

Lesson: If you’re stumped and haven’t looked at this stuff yet, do it now. The answer to your problems might just be in that big patch of bluegreen gunk at the batteiy. There’s a reason the old big-box analyzers had you check the battery first.

4 Idiot lights and gauges. As the slang name implies, warning lights are designed to get even an idiots attention. But problems sometimes arise when they’re either ignored or taken as gospel. The first step is to verify the correct function of all dash warning lamps. Most are designed to illuminate key on, engine off (KOEO), at least for a specified interval. Most should extinguish key on, engine running (KOER), either immediately or very soon after startup. (Experienced techs know that the brake warning lamp is typically wired to illuminate whenever the battery warning lamp does. If both idiot lights remain lit KOER, don’t get sidetracked by the brake indicator; fix the charging system first.) Warning lamps that fail to illuminate as expected should be investigated early in the diagnostic sequence.
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