Understanding the maze that is OBDII repair

Jan. 1, 2020
A drive cycle, the key to OBDII system repairs, is more complicated than one might think.

Since its full introduction in 1996 vehicles, on-board diagnostic II (OBDII) systems have completely changed the way technicians must approach emission system diagnostics. On today’s vehicles, there are 17 or more separate subsystems being monitored by OBDII. Also, as many as 11 monitored circuits check on the OBDII system’s running condition. What was once the “check engine” light has now become a malfunction indicator lamp (MIL).

As the OBDII system runs its checks on these individual monitored circuits, it will illuminate the MIL to advise the driver or technician of the need for service before performance deterioration results in total failure of the system or component in question, hopefully saving money on expensive repairs.

The complexity of problems detected by the OBDII system in late-model vehicles is seldom confined to just one subsystem needing repair.  OBDII runs a complex series of algorithms — programmed mathematical comparisons — continuously on some circuits, periodically on others. The system will illuminate the MIL whenever a monitored circuit indicates any of the emissions gases are likely to exceed 150 percent of the design standard.

Dealing with issues of readiness monitors, along with analyzing diagnostic trouble codes (DTCs) and running a drive cycle, all play an important part in turning off that MIL — and keeping it off.

Ensuring the MIL stays off

Even if the MIL is off, there are two problematic issues in OBDII emissions testing today. One is a vehicle’s readiness to take a test. The other is in performing a drive cycle to prepare the vehicle for its examination and to provide a quality inspection on the work performed, which includes a check to ensure the MIL is out and stays out after the repair.

In some states it is required that any vehicle submitted for an OBDII emissions test must be tested as presented — regardless of the MIL’s status. If the vehicle fails because the MIL is on and it is not ready to take the test in the first place, repairs must be made up to at least the dollar-value of the waiver limit of that state.

The system’s readiness, of course, applies to the OBDII’s 11 monitors. Three of these are continuous monitors that run around the clock. The other eight run intermittently. For OBDII-equipped vehicles, most states follow the U.S. Environmental Protection Agency’s (EPA) guidance on unset monitors. The rule of thumb is that two monitors can be unset or not ready in 1996 to 2000 model-year vehicles. For 2001 and newer vehicles, only one unset or not ready monitor is acceptable.

Today, many vehicle owners do not understand why their vehicles may not be ready for an emissions test. For the shop owner, the difficulty lies in trying to explain the problem to a motorist who feels his or her vehicle is operating fine. They have a hard time understanding why their car isn’t ready to take the test.

But failure to heed an illuminated MIL because the vehicle seems to operate normally could be costly. In cases where monitors are being unset or are not ready, the MIL will not always light up immediately to alert the owner that something needs to be checked.

Some motorists, and even some technicians, believe that disconnecting the battery for a few seconds will turn off the MIL. What this actually does is cause all the monitors to become unset or not ready. The only thing that will reset the monitors, or at least the ones that were ready, is to perform a drive cycle — a process designed to help the vehicle relearn all of the sensor values. The problem with this is that the chances of performing a successful drive cycle are slim, especially if the vehicle has not been diagnosed and repaired correctly.

The right tools are crucial

An important part of an OBD repair technician’s toolbox is the comprehensive drive cycle manual; the only source is published by Motor Manuals.  The latest 1996 to 2004 edition contains a little more than 900 individually different drive cycles for the wide range of engines now in use. Contrary to what many technicians believe, there is no generic drive cycle. A technician also needs a reference book that covers all the many DTCs. At last count, there were well over 800 of them, and that number grows each model year.

Unfortunately, there is no indicator that points to exactly which monitors may be either set to ready mode or be unset and not ready. Some late-model vehicles have a driver indication about a state of unreadiness, but there is no indication as to which monitor or monitors are affected. At this point, DTCs will have to be revealed to determine exactly what part of the emissions control system needs to be serviced.

EVAP monitor demands

Today, one of the most difficult areas of OBD control to tackle is the evaporative fuel vapor control system (EVAP). The EVAP monitor’s state of unreadiness must be corrected, and getting this monitor to run is proving to be an involved part of OBD technology — so much so that it is creating a need for specialized training on the diagnosis and correction of EVAP problems.

Despite its complexity, the OBDII system’s MIL is most often illuminated because of simple stuff: loose connections, a broken wire or, in the case of an EVAP code, probably a loose gas cap. An EVAP monitor check also calls for the fuel tank to be no less than one-quarter and no more than three-quarters full.

As for that gas cap, some drivers simply fail to turn the cap until it clicks three times, causing the MIL to light up. In some very late-model vehicles, the new screw-on gas cap does not click itself into a secure position.

The key: Enabling criteria

Post-repair drive cycles pose their own set of problems. Several steps must be accomplished to satisfy the OEM’s specific enabling criteria and achieve a monitor’s state of readiness. All these criteria differ from one vehicle manufacturer to another.  Even within an OEM’s line of vehicles, there is little commonality. So enabling criteria for any given monitor may differ. Therefore, the drive cycle reference guide for that vehicle must be referred to.

While each OEM’s procedure is different, let’s look at a general example of what could be involved in performing a typical drive cycle. It is important to note that this is not a specific drive cycle, it is just an example of what one may involve.

First, the vehicle should be warmed up to operating temperature, and then accelerated up to a steady speed of about 30 to 35 mph. Next, the vehicle must come to a complete stop without any brake application. After that step is complete, the vehicle should be accelerated to about 50 to 55 mph for a specified number of minutes, and then coast to a stop — with no braking.

These steps, when performed correctly, complete the drive cycle, set all monitors to ready and turn off the MIL.

The difficulty with running a drive cycle like this, however, is finding a stretch of road — especially in areas with large populations — to accelerate, cruise and slow down as prescribed.

Some believe they can simulate a drive cycle on a two-wheel ASM dynamometer, but others in the industry say it just isn’t possible. A drive cycle could be performed on a four-wheel dyno if the operator follows the OEM’s prescribed routine exactly, but it is often difficult to do.

One late-model General Motors 3.6-liter V6 engine is not so easy to run through a drive cycle. This particular engine may have to go through as many as three cold-soak routines over a couple of days in order to accomplish its specified drive cycle.

No easy way out

For virtually all drive cycles, the automakers’ instructions state that two people should take the vehicle on a checkout run: one to drive the vehicle, and the other to read the drive cycle instructions. This recommendation is designed to cover any questions of subsequent liability.

Of course, the shop owner is forced to wonder how much he or she should  charge to perform a drive cycle, especially if two techs have to make the run as recommended by the OEM. Everyone knows diagnostic time is not cheap, nor is shop labor.

Some shop owners believe they can ease the complexity of running a drive cycle by  providing the vehicle owner with a copy of the vehicle’s drive cycle, with the expectation that they’ll carry it out. Bad idea. If the vehicle owner has an accident because he or she was reading the instructions a shop owner gave them instead of paying attention to traffic, that shop could be liable.

A change of thinking

Perhaps emissions testers/repair shop owners need to accept the reality that running a drive cycle, though time-consuming and complicated, is a normal part of an OBD emissions repair. And it should be listed and billed as such on the work order.

However, some shop owners rely on telling the vehicle owner to drive the vehicle normally for the next three or four days in the hope the repair was done correctly and the MIL both goes out and stays out on its own.

But if that MIL lights up and comes back in for another check, then a technician must re-check all the monitors, pull codes to determine what really is wrong and run another drive cycle in an attempt to satisfy a dissatisfied customer.

As you can see, the odds for a less-than-perfect late-model emissions repair have now been raised for everyone.

One good defense mechanism is for all techs involved in OBDII-related emissions repairs to take some technology training in the OBDII system, understand how it works and learn its designed-in strategies. And remember, the drive cycle is a final quality control check to ensure the quality of work.

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