The Check Engine Light is a light you do not want to see. It pops up like a bad cold, usually without warning but always with a cost. When something goes wrong with your vehicle's emission control system, the Check Engine Light is often the only visible indication that the vehicle is polluting. Trouble is, the light could mean almost anything, from something as simple as a loose gas cap to something far more costly, elusive and difficult to repair.
Of course, the smart thing to do when the light comes on is to take the vehicle to a qualified repair shop, have the code pulled, interpreted, diagnosed and fixed. Or, buy a scan tool and read the code yourself. But many motorists simply ignore the Check Engine Light or reach for some electrical tape to cover it up — out of sight, out of mind.
But there is 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. AutoZone currently offers free "diagnosis" of what is causing the Check Engine light to come on. The parts retailer estimates that 20 million vehicles on any given day experience this problem. This is part of their program to empower consumers to take on vehicle maintenance and repair, a task made more difficult as vehicle 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. One criticism is that trouble 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 a means to sell parts.
Anyone can read trouble code with a scan tool or code reader. The tool plugs into the vehicle diagnostic connector and displays a number or message. It is really quite easy. The hard part is figuring out what the code actually means and how to fix the problem that is setting the code.
CHECK ENGINE LIGHT DIAGNOSTIC TROUBLE 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 the driver 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 do not.
Click Here for a list of generic OBD II Trouble Codes.
When you read the code, you will see 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 generic code. But if the number is a 1 then it is a "dealer or OEM" code specific to that particular make.
PULLING CODES WHEN THE CHECK ENGINE LIGHT IS ON
When the light comes on, the engine management system has recognized a problem that is causing an increase in emissions. But by itself, the Powertrain Control Module (PCM) cannot 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 Check Engine Light, the computers memory needs to be probed. On 1996 and newer vehicles equipped with Onboard Diagnostics II (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 computer, a scan tool or code reader must be plugged into the diagnostic connector. 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 trouble codes on older vehicles. Some tools may read trouble codes in domestic vehicles but not imports (unless equipped with an extra import cartridge). Some tools that read OBD II trouble 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 it is 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.
INTERPRETING TROUBLE CODES
It makes no difference who pulls the codes, the numbers are the same. However, what you do with the code after you have 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, the Check Engine Light is on and you find a code P0301. This is a misfire code (03) for cylinder number one (01). Does this code tell you what is wrong with the car? Other than that it is misfiring, there’s nothing else to go on. Often it is the why part of the equation that reveals the true path to proper diagnostics. You might be tempted to replace the spark plugs and wires. Do not 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 alone, it is very difficult to make an accurate diagnosis. Additional tests are usually needed to nail down the cause of the fault.
For a detailed look at all the operating parameters that can set various fault codes, Click Here to view a PDF file on GM 4.6L diagnostic parameters.
The bottom line here is don't waste your money replacing parts if you don't know the parts are bad or not. Diagnosing sensor, driveability and emission problems on late-model vehicles requires know-how, experience, additional diagnostic checks and reference specifications for the vehicle. What's 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.
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