Article > Feature Story

Check Engine Light — Opportunity or Obstacle?

By Brian Cruickshank

This month, Counterman explains how your store can leverage that little light to perk up service and sales. But beware: Pulling and interpreting OBD codes is not as simple as it seems.

When a motorist sees an illuminated Check Engine Light, she usually sees red. At least one national retailer, however, hopes to see green. This month, Counterman explains how your store can leverage that little light to perk up service and sales. But beware: Pulling and interpreting OBD codes is not as simple as it seems.

It pops up like a bad cold, usually without warning but always with a cost. When somethings wrong, the check engine light, the only visible indicator of the on-board diagnostic (OBD) systems enigmatic existence, glows with a menacing caution-light yellow on the instrument panel.

It could be something as simple as the gas cap not having been tightened enough, or something far more costly, elusive and difficult to repair.

Of course, the smart thing for a motorist to do is to take the vehicle to a qualified repair shop, have the code pulled, interpreted, diagnosed and fixed. At least thats what the industry (and the EPA) would like to happen, anyway.

But that costs money, and consequently, a popular thing to do is to simply ignore it or reach for a bit of electrical tape to cover it up out of sight, out of mind.

But theres a third alternative too. National retailer AutoZone knows this all too well with its Check Engine Light Program, which aims to leverage information gained via these codes into parts sales. The Memphis-based mega chain certainly was not the first to use OBD codes to sell auto parts, but it was the first to create a nationwide program using the idea. A little more than a year ago, AutoZone launched the program, through which stores offer free "diagnosis" of whats causing customers engine warning lights to come on. The parts retailer estimates that 20 million vehicles on any given day experience this problem. This is part of the companys program to empower consumers to take on vehicle maintenance and repair, a task made more difficult as vehicles complexity has increased.

The program has not come without its share of criticism however, and stores considering the development of their own "Check Engine Light" program should be aware of these potential pitfalls.

First is the programs possible negative effect on your wholesale customers who may be annoyed that by pulling codes, your store is stealing their business. In their view, parts professionals sell parts and technicians diagnose vehicles. Period.

The second criticism is that OBD codes can be misleading and do not constitute a final diagnosis. Merely pulling codes serves as a starting point and should not be used as an excuse to start swapping parts. Well get into that later.

The purpose of this article, then, is to educate stores on how the OBD coding system works, how to pull codes at your store (should you so choose) and to list some of the potential problems that this can bring. Your stores decision to offer this service must be weighed carefully.

Any store, equipped with the right information and equipment, can pull and interpret codes for their customers. Its really not difficult at all, but it is important for any parts store who is considering this that the OBD system is anything but straightforward. What a code says and what the problem actually is are often, but not always, two different things.

But first, here are some basics about the diagnostic system and how to "pull" codes.

The purpose of OBD II is to detect emissions problems, not driveability faults (although one could cause the other). This means that the check engine light may come on even before your customer notices any driveability problems.

When an emission fault does occur, OBD II has four levels or classifications of diagnostic codes. Some illuminate the check engine light, others dont.

  • Type A A major fault that can cause emissions to increase. This type will cause the light to come on the first time the fault occurs.
  • Type B These are less serious faults, but serious enough to illuminate the light if the fault occurs after two consecutive trips.
  • Type C These are non-emissions faults. These wont cause the check engine light to come on, but they will illuminate a "service" or other warning lamp. If the fault passes the self-test on the next trip, the light will go out, and a history code will remain in the memory.
  • Type D These are also non-emissions faults and wont make any lights come on. Like Type C faults, codes will be stored in the memory for diagnostic purposes.

When you pull the code from a vehicle, youre going to get a letter P (powertrain), followed by a four digit numeric code. If the first number after P is a 0, it is an industry standard code. But if the number is a 1, then its a "dealer" code specific to that particular make.

When the light comes on, the engine management system has recognized a problem that is causing an increase in emissions. But by itself, the PCM cant tell you what is causing the problem or what parts need to be replaced. It can only alert the driver that a problem has occurred and record a code.

To find out what triggered the light, the computers memory needs to be probed. On 1996 and newer vehicles equipped with OBD II, there is a common 16-pin diagnostic connector usually located under the dash near the steering column. On older vehicles, the shape and location of the diagnostic connector varies and may be located almost anywhere (under the dash or seat, behind the glove box or kick panel or anywhere in the engine compartment.) The diagnostic connector is the access portal to the engine management system. It is through this portal that you can pull or record the trouble codes.

To read trouble codes in the computers memory, a scan tool or code reader must be plugged into the diagnostic connecto . This requires not only the right diagnostic connector adapter for the vehicle application but also the right software in the scan tool or code reader so it can read and display the information.

But be careful: Some scan tools will only read OBD II-compliant vehicles. Others may only read Ford, GM or Chrysler codes on older vehicles. Some tools may read codes in domestic vehicles but not imports (unless equipped with an extra import cartridge). Some tools that read OBD II codes may only read "generic" codes that are common to all makes and models, but not the "enhanced codes" that are vehicle specific. If your store wants to be able to read all makes and models, make sure the tool you purchase is able to interface with all vehicles.

On pre-OBD II Ford, GM and Chrysler vehicles, the check engine light can also be made to flash out codes by jumping or grounding certain terminals on the diagnostic connector.

On older Japanese cars, some computers have small LED lights that flash out codes. Code retrieval procedures vary greatly on older vehicles, so its important to look up the details in a service manual.

The more expensive tools will give you the code number and a short description of what the code means. The less expensive tools will only display a number, which then has to be looked up in a reference manual.

There is also software available that allows a desktop PC, laptop PC or even a Palm Pilot to function as a code reader or basic scan tool (this requires a special adapter cable to connect the PC or Palm Pilot to the diagnostic connector on the vehicle.) Some of the better software also displays sensor values and other data.


It makes no difference who pulls the codes, the numbers are the same. However, what you do with the code after youve pulled it is often the dividing line between technician and everyone else. The numbers are often like literary red herrings: They can be false friends, pointing you in wrong diagnostic directions.

For example, lets say you pull code P0301 out of a late-model vehicle. This is a misfire code (03) for cylinder number one (01). Does this code tell you whats wrong with the car? Other than that its misfiring, theres nothing else to go on. Often its the why part of the equation that reveals the true path to proper diagnostics. You might be tempted to sell the customer a set up plugs and wires. Dont do it. In this case, the underlying cause might be low compression (a burned exhaust valve or leaky head gasket), an ignition problem (worn or fouled spark plug, bad plug wire or weak ignition coil), or a dirty or dead injector.

Based on the code, it is nearly impossible to make a recommendation to a customer, other than to send them to a qualified technician for further diagnosis. But at least you can point them in the right direction.

The bottom line here is dont pretend youre a technician if youre not. And dont sell a part unless youre absolutely sure that a new part will fix the fault. Diagnosing sensor, driveability and emission problems on late-model vehicles requires know-how, experience, additional diagnostic checks and reference specifications for the vehicle. Whats more, some problems can only be fixed by referring to Technical Service Bulletins (TSBs) from the vehicle manufacturer. "False codes" and other glitches that may be in the engine management system may require reprogramming or replacing certain original parts with revised parts. This kind of information can only be found by researching a TSB database for the vehicle in question.

In the end, trouble codes are not a final diagnosis but a starting point for further diagnosis by one of your good technician customers. Considering all the intricacies of the OBD and engine management systems, pulling and interpreting OBD codes should be used more as a service to your DIY customers and less a way to sell parts.

Reading codes will reveal the nature of the problem (sort of), but it can also mislead some customers into thinking they now have a diagnosis and can start replacing parts. Use this information wisely.

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