Onboard Diagnostics


Why can't the computer tell exactly what's wrong?


Let's say your vehicle just isn’t running properly. The "check engine" light is glowing, your gas mileage has dropped and you've got a rough idle when you come to a stop. A no-brainer, right? It must be time for your technician to do an analysis to find the problem.

In this age of high-tech, low-polluting automobiles, modern diagnostic procedures involve things like digital code scanners, computer data streams and electronic engine analyzers. As a result, motorists sometimes assume the "computer" will have the answer to every problem. It’s not that simple; this electronic device is just another tool to help technicians diagnose and repair your vehicle as quickly and accurately as possible.

An accurate diagnosis requires a combination of the proper knowledge, skill, training and equipment and an up-to-date information base that provides quick access to thousands of pages of technical reference data. Let's look at how the diagnostic process works, and why it's not always as simple as it might seem.

Any vehicle built since the early 1980s has an onboard diagnostic (OBD) system that monitors various sensors located throughout the vehicle (the newer the vehicle, the more sophisticated the system). These sensors report data such as vehicle and engine speed, coolant temperature, manifold pressure, throttle position, etc., to the central computer. By continuously monitoring this data stream, your car’s computer can make adjustments to optimize the vehicle's performance, fuel economy and emission levels. That is, until something goes wrong.

Suppose one of those sensors sends a signal that's out of the normal range, or perhaps no signal at all. Recognizing this as a sign of possible malfunction, the computer may do one or more of the following, depending on which of the vehicle's systems appears to have the problem:

1. It may store a fault code or diagnostic trouble code (DTC) in its memory.
2. It also may illuminate the malfunction indicator lamp (MIL) on the instrument panel (sometimes known as the "Check Engine" or "Service Engine Soon" light). This alerts the driver that a possible malfunction has been detected.

If the missing or corrupted signal is one that's essential if the engine is to continue to operate, the power train control module (PCM) may decide to switch to a fail-safe or "limp" mode. This allows the computer to ignore the faulty signal and use preset generic data instead. On some vehicles, this may be called the limp-in mode. Once a vehicle has switched to this mode, the engine may perform poorly, but at least it will run. (This may explain your rough idle and reduced fuel economy.)

In rare cases, loss of a sensor signal could disable the engine completely. If the PCM has no way to monitor engine speed, for example, it may have no other choice except to shut down the engine.

Okay, so now your OBD system has caused the MIL to light up, your PCM has stored a DTC and your engine is running in limp mode. Are you thoroughly confused yet? It's time to "limp" to Quality Care Auto, where a technician will identify and solve your problem.

We usually begin the troubleshooting process by connecting your vehicle's computer to an electronic code scanner or diagnostic computer. Putting the OBD system in diagnostic mode allows retrieval of any store fault codes.

The method for retrieving the codes may vary, but the important thing to realize is that a fault code seldom reveals exactly what's wrong with your vehicle. Instead, it gives the technician a starting point from which to begin an analysis. A fault code only reveals that something abnormal has occurred in a particular circuit. It does not reveal exactly what caused the abnormality. Was it a bad sensor? A failed part? A short in the wiring? A loose connection? A false code that shouldn't be there at all, or perhaps a malfunction in the PCM itself? These are some questions the technician must answer.

Armed with the DTC codes, your technician can then refer to a diagnostic chart that lists step-by-step tests and procedures to follow in sequence so the faulty component can be identified and either replaced or repaired.

Depending on the nature of the problem, testing procedures may be time-consuming, often involving an analysis of the data stream from the sensors while the engine is running. This may require a "flight test," in which the car is driven on the road while connected to portable hand-held diagnostic equipment.

The process can be complicated when it works exactly like it's supposed to. And when it doesn't? That's when our trained technicians, skills and information resources are taxed to the fullest.

When troubleshooting a specific problem, a variety of mysterious "glitches" that might hamper the diagnostic process may occur. For example, a fault code may not be present, either because it was never stored or has already been erased by the PCM. The technician may have to get it to reappear before the problem can be isolated.

If a "false" code is found, it may be caused by a voltage spike, an improperly grounded circuit or a failure to erase and old code. If this is suspected, the solution may be to erase the code from memory and then test drive the vehicle to see if it reappears.

If a problem occurs intermittently, there may not be any codes or clues to follow until the malfunction occurs again. ("Gee, I'm sure it was doing it last week!")

Sometimes a problem cannot be detected by the OBD system. A timing belt that has "jumped" slightly out of position may be unrecognizable to the PCM, for example.

Some vehicles have problems that may not appear until they've been driven for a few years. If these problems generate false codes in a particular model of car or truck, the manufacturer may issue a technical service bulletin (TSB) to alert technicians about the glitch and save them from hours of fruitless testing. Your Quality Care Auto technician may need to check a TSB information source to check for bulletins pertaining to your car’s problem.

Computers are certainly an essential part of the diagnostic process, but they don't have all of the answers--at least not yet. Isn't that why you bring your car or light or medium truck to us?




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