Marinucci: ECM has final say on auto repair success

Why does a vehicle with an illuminated check engine light return to your shop, seemingly long after technicians touched it?

Completed monitors may have detected and flagged an improper repair on the vehicle's engine management system.

Technically, failing to fix the system correctly caused the comeback. The monitor, however, confirmed the mistake long after the vehicle left the bay.

Monitors are nothing more than self-tests that an engine management computer (ECM) routinely performs on components and circuits it controls.

I grossly simplified monitor basics in my last column. Here I will recap and expand that discussion to explain why some comebacks take a long time to come back.

Understandably, comebacks are as welcome as a bad check. Sometimes, the ensuing frustration blinds owners, managers and service sales personnel to several possible causes.

For example, an entirely fresh problem may have cropped up — something wholly unrelated to the repairs the techs already performed on the vehicle.

Savvy service managers coach techs to look for legitimate repair opportunities on every vehicle, but even a capable, conscientious tech may overlook an imminent failure during a vehicle inspection.

When the breakdown finally occurs, a disappointed motorist returns to your tire dealership or service shop seeking answers.

Please keep a reasonable perspective on automotive breakdowns. For example, I noted a June column that the average age of light vehicles has reached 12.2 years.

In my view, an average age greater than 12 years old is a significant statistic.

The older a vehicle is, the more realistic it is to find multiple problems on it instead of a single one. Furthermore, the potential for numerous problems per car means techs should check each vehicle as thoroughly as practically possible.

In addition to these issues, a botched repair on an engine management system may cause a belated comeback. Managers and service writers have told me that this exasperated them for two reasons.

For one thing, the problem brewed in the vehicle for an unusually long time — perhaps months — until the dreaded check engine light turned on again.

For another, the vehicle performed fine when a tech road tested it after the repair.

In fact, it still ran well when the customer brought it back to you.

First of all, remember that monitors are extensive, highly detailed self-tests that the ECM performs on the engine management system. Tests take time.

Many consumers — including anxious car owners — demand instant gratification. However, an ECM tests according to a strict, preprogrammed regimen.

It could take months for an ECM to complete some of its self-tests. Therefore, this potential consequence is an automotive fact of life rather than a mysterious aberration of some sort.

Second, an ECM has an extraordinary ability to compensate for various failures within the engine-management system.

A system breakdown may illuminate the check-engine light, increase tailpipe emissions and reduce fuel economy, but overall engine performance may feel normal in spite of the problem(s).

Third, and most critical, an ECM has the final say on the quality of your technicians' work. Eventually, the computer evaluates repairs on the engine management system by running monitors.

The results of these self-tests determine the success of the repair job — period.

Let's look just a bit closer at these extensive, highly detailed self-tests called monitors. First of all, the ECM runs some monitors constantly rather than periodically.

These "continuous" monitors usually flag problems within the engine-management system quickly, but the ECM runs other monitors periodically instead of constantly. This category of self-tests aptly are called non-continuous monitors.

Conceivably, the ECM could run non-continuous monitors within minutes or hours of when the customer drives away. However, it may take days, weeks or months to complete some non-continuous monitors.

Variables such as customer habits, driving conditions and the local weather may delay some non-continuous monitors from running for an unforeseen length of time.

The reason is that an ECM strictly observes critical operating conditions — often called enabling criteria — when trying to run monitors.

For example, a certain amount of time may have to elapse before an ECM tries to run a monitor; any condition that interrupts this "countdown" scuttles the test.

In other situations, the ECM expects to see engine coolant temperature or ambient air temperature increase to a certain value between the time that the driver starts the engine and shuts it off.

Realistically, it could take days for a driver to complete a valid sequence of ignition switch-on and switch-off events that satisfies the ECM.

Sometimes, a monitor requires driving a vehicle at a specified road speed for a certain length of time. In some areas, stop signs, stop lights and routine traffic make it very difficult to meet those test criteria easily and quickly.

The evaporative emission system stores gasoline vapor that older vehicles simply released into the atmosphere. At the proper time, the engine purges and burns this stored fuel vapor.

The ECM runs a monitor on the evaporative emission system. A frequently overlooked requirement is that the fuel level must be within one-quarter to three-quarters of a tank of fuel.

All the other criteria for the monitor may be present, but the ECM will not attempt to test the evaporative system if the fuel level is outside this specified range.

The key takeaway here is that performing a self-test may be relatively easy; meeting the prerequisite test criteria may not.

Therefore, completing some monitors takes time.

OK, suppose that all the factors fall into place, enabling an ECM to complete a self-test. If that occurs, we may say that the monitor has run.

A tech may call this a monitor that has run to completion or is "complete."

By consulting a scan tool, service personnel easily can see which monitors are or are not complete.

Sometimes, the display indicates a completed monitor with a check mark or "OK."

Next, suppose the ECM finishes its self-test on a particular component or subsystem. Completing a monitor is no guarantee that the component or subsystem passed the self-test.

A completed monitor — without any trouble codes present — means a passing grade for that component or subsystem. Your techs' repairs on those areas succeeded.

However, a completed monitor accompanied by a check-engine light and trouble code(s) tells you that the component or subsystem failed the ECM's self-tests.

This means the repairs were unsuccessful — time to troubleshoot those items a second time.

Savvy managers recognize that road testing repaired vehicles is a prudent precaution.

Checking repaired vehicles for "pending" troubles codes with a scan tool is equally important. A pending code flags a problem that has not illuminated the check engine light yet — but will turn it on in the future.

A pending trouble is just another way an ECM flunks the recently performed repairs.

Readers may have a lot to ponder after this grossly simplified discussion of diagnostic monitors, but when all is said and done, remember that the ECM's self-tests determine the effectiveness of your techs' repairs on engine management systems.

Another key takeaway is that an accurate diagnosis and a thorough repair job is your business' best insurance against comebacks — regardless of the specific monitors and test criteria involved.