Tech Stuff

Check engine light diagnostics


Figure 1: Here’s a screenshot of a DTC scanned using a quick code reader.
<p>Figure 1: Here&rsquo;s a screenshot of a DTC scanned using a quick code reader.</p>

Craig Truglia is an ASE A6 and A8 certified technician who presently works as a service writer for Patterson Autobody, a repair facility in Patterson, N.Y. A former shop owner and editor of several automotive repair magazines, Truglia combines his Columbia University education with the real-world experience he sees daily in the automotive repair field. Technicians Truglia and Fred Byron took part in diagnosing the different vehicles in this article.

For most auto repair shops, diagnostics are not where the money is. A good diagnosis is needed to figure out what parts (and these days, sometimes software files) are needed to repair a drivability issue.

It is safe to say that the most profitable field of diagnostics involves the skills and knowledge necessary to figure out what is required to allow the monitors to pass a state emissions inspection. As states such as New York adopt CARB emissions standards, emissions diagnostics may become increasingly important (yes, that means no more 49-state legal catalytic converters as of 2014).

Many experienced technicians get needlessly flustered diagnosing check engine lights. With the right diagnostic strategy that takes advantage of service information and proper procedure, many routine mistakes and time wasters can be avoided.

Step 1. Scan codes using generic OBD II.

Don’t waste timing booting up a big and expensive scan tool, unless it is the only one in the shop (or all the repair information resources are already on the tool.) Let’s be honest — sometimes we can almost complete a diagnostic by seeing a DTC alone (see Figure 1). Misfire codes on Fords, due to bad ignition coils, come to mind.

Many technicians make the mistake of using OEM enhanced functions on a scan tool as opposed to generic OBD II to diagnose check engine lights. There are two problems with this. First, it takes too long. Second, some aftermarket scan tools have a tendency to say there are current DTCs that really are not current. No one wants to chase down a code that is not going to turn off the light.

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