Marinucci: Clearing codes also cancels crucial data

Think twice before erasing trouble codes because it has negative, irreversible consequences for vehicle owners and service professionals alike.

Clearing one or more fault codes pleases a motorist because it turns out the dreaded check engine light on the instrument panel.

However, the step also permanently deletes useful diagnostic data such as monitors and freeze frame. This information may help technicians diagnose complex engine control systems more quickly and accurately than they could otherwise.

Precise troubleshooting benefits consumers because it ultimately saves them time and money — an outcome that many motorists and service sales personnel overlook.

Monitors represent crucial self-tests that the engine control computer performs on its circuits and components.

A freeze frame literally is a snap shot of key operating conditions at the moment an engine control computer sets a trouble code and illuminates the check engine light.

Techs may troubleshoot and repair the cause of a trouble code without ever referring to the monitors or freeze frame. But overall, this data has proven to be helpful, rather than hurtful, for hard-working techs.

Furthermore, monitor and freeze frame information may be invaluable during difficult diagnoses.

I have watched car owners, techs and service writers fiddle with self-diagnostic formats for more than 40 years. Among some motorists, the ability to turn out a check engine light has reached hero status.

Clearing trouble codes and turning out the light has become as American as apple pie — especially for cash-strapped car owners.

Remember that a vehicle continues running after a common fault code appears, but the check engine light turns on and remains on. Traditionally, some folks couldn't care less if this scenario increases fuel consumption as well as tailpipe emissions.

Actually, all that matters is that the car keeps running.

Motorists who do not prioritize proper maintenance and repair may think the check engine light is a needless nuisance.

Predictably, someone who shuts off the light for them — particularly free of charge — is a hero.

Some auto parts retailers have recognized the appeal of fast, free services — especially those that may increase parts sales to do-it-yourselfers. Therefore, a counter person may carry a scan tool or code reader into the store's parking lot in order to erase fault codes from a customer's vehicle.

Beware of another factor here. Relatively recently, inexpensive trouble code readers have appeared on the market. These compact tools retrieve as well as erase codes.

For one thing, code readers are popular among backyard mechanics. These amateurs' vehicles may end up in your service bays when they could not diagnose problems themselves and/or needlessly replaced parts.

Furthermore, some of these shade-tree mechanics happen to be service writers at your dealership or service shop. Undoubtedly, they have good intentions.

These employees may play hero, clearing trouble codes out in the parking lot for hard-luck car owners. For one thing, they are overlooking or ignoring the consequences of erasing codes.

For another, they risk creating a counterproductive image of a quick-fix artist.

Some owners, managers and service sales personnel brush off technical discussions, asserting that the subjects are too technical for them to understand.

But I have found that topics such as freeze frame and monitors only are as difficult as they choose to make them.

Following a grossly simplified explanation, these people usually grasp the topics — especially the value of free diagnostic features such as freeze frame and monitors. Let me try to grossly simplify them here.

First, the typical vehicle rolling into your bays has an industry-standardized diagnostic format called OBD-II. Typically, this format appeared on cars and light trucks in 1996.

OBD-II systems feature a common diagnostic connector, usually located somewhere under the left side of the dashboard. By connecting the proper scan tool to this connector, techs can examine a common list of OBD-II test values such as engine speed, road speed, coolant temperature, throttle opening and so forth.

Second, the OBD-II format provides a standardized set of diagnostic trouble codes — often called DTCs. Making DTCs common across vehicle manufacturers simplifies diagnosis for techs who work on a variety of makes and models.

Third, the format includes a standard troubleshooting feature called freeze frame. The engine control computer saves this snap shot of vital measurements the moment is sets a trouble code.

Whenever the computer stores multiple fault codes, it takes the data snap shot when the first code appears.

A tech can review freeze-frame data easily with a common, OBD-II-compliant scan tool. Usually, the tech has the option of printing the freeze frame results and/or downloading the information into a PC.

Among the freeze-frame data are operating conditions such as engine load, RPM, coolant temperature, vehicle speed, etc.

You need not be a technical ace to appreciate how much these readings may ease a tech's troubleshooting procedure. What's more, this bank of information is free.

Fourth, monitors are extensive self-tests that the engine control computer performs on its circuits and components. Typically, newer OBD-II systems do more self-tests than earlier versions did.

Techs review monitors, like freeze frame, with a scan tool. If the engine control computer has finished a particular self-test, the scan tool usually labels that monitor "complete" or "completed" for some performance-related monitors.

I could write chapter upon chapter on OBD-II monitors. Suffice it to say that each monitor usually represents a set of extremely detailed, highly accurate tests — trust the results.

Suppose the scan tool shows a completed oxygen sensor monitor and the computer has not set any oxygen sensor-related trouble codes. If that is the case, rest assured that the oxygen sensor is operating to the computer's satisfaction.

This means that oxygen sensor performance is not relevant to the diagnosis at hand.

On the other hand, suppose that the scan tool shows an incomplete oxygen sensor monitor. The tech does not know the results of the self-test until the oxygen sensor monitor is complete.

To put it another way, an incomplete monitor means more diagnostic questions than answers.

Now, suppose a tech sees a completed oxygen sensor monitor — accompanied by an oxygen sensor trouble code. Typically, this scenario flags a legitimate problem; the tech should test the sensor itself and look for possible causes of sensor failure.

Overall, OBD-II monitors constitute an extraordinary record of self-tests the engine control computer already has performed for you — take advantage of it.

I will conclude by recalling classic mystery movies: Heroes try to preserve proof of dastardly deeds. But villains try to conceal or destroy evidence of their guilt.

Erasing a trouble code and turning out a check engine light may appear heroic. But ultimately, erasing vital data such as freeze frame and monitors seems villainous.