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Get OBD II Strategies on your Radar

Theory and practice combined show the power and limits of on-board diagnostics.
Friday, September 1, 2006 - 00:00

Theory and practice combined show the power and limits of on-board diagnostics.

Diagnostic trouble codes (DTCs) can give you trouble, but they become easy to diagnose and repair with a good understanding of on-board diagnostics (OBD) II and scan tools. First, what exactly is OBD II? Well, if you remember what OBD I is, you can think of OBD II as OBD I on steroids. All you really need to know is that OBD II goes beyond OBD I by adding test strategies and hardware designed to identify vehicle problems that may result in increased vehicle emissions, either tailpipe or evaporative emissions (EVAP).

How does OBD II know when to store a DTC? Results of a battery of lab tests called Federal Test Procedures (FTP) established the standards used by OBD II. All new vehicles sold in the United States are designed to pass the FTP tests of tailpipe emissions under different operating conditions. In addition, the vehicle EVAP system also is tested to ensure it won't leak hydrocarbon (HC) vapors into the atmosphere. The results of the FTP testing establish standards for comparison to actual vehicle operating conditions. If a vehicle is performing outside of these standards, OBD II will notice, and issue a DTC as a result. For example, when emissions exceed 1.5 times the FTP standard, the malfunction indicator lamp (MIL) will illuminate and a DTC will be stored.

During normal vehicle operation, OBD II runs a test we refer to as monitors, a diagnostic strategy program stored in the Powertrain Control Module (PCM). Continuous Monitors are always running when the vehicle is in operation. In contrast, Non-Continuous Monitors run only once per trip. In other words, Non-Continuous Monitors perform a specific diagnostic test and then stop when the test is complete.

You may recall that in Ford vehicles equipped with OBD I, the key on/ engine off (KOEO) and key on/engine running (KOER) options allowed you to perform a test that is very similar to how today's OBD II monitors run. By the way, the KOEO and KOER test options still exist on Ford vehicles equipped with OBD II.

The first time a non-continuous monitor runs to completion, its readiness status changes from Not Ready to Ready and stays that way until either the DTCs are erased or the PCM loses battery power, erasing keep-alive memory (KAM). Monitors can be difficult to set to Ready if you do not follow some basic rules and drive-cycles.


  • Engine thermostat is working properly.
  • Engine coolant/ antifreeze mixture is correct.
  • Crankshaft relearn is completed.
  • No DTCs or pending DTCs are stored.
  • Vehicle fuel level is above 15 percent and below 85 percent.

Another good tip for hard-to-run monitors on some vehicles is to force a DTC, then erase the DTC and perfrom the generic drive-cycle.


The following steps demonstrate a generic drive-cycle, which will run the non-continuous monitors to completion (Ready) within 30 minutes on most vehicles.

STEP 1: Ensure the vehicle has been "sitting" without even a single start-up, for at least eight hours before the test. In certain makes of vehicle, this is necessary to ensure that the EVAP monitor functions properly. This is especially important in certain Ford vehicles that include a six- to eight-hour cold-soak as part of the EVAP monitor enabling criteria.

STEP 2: Prior to driving the car, warm the engine to normal operating temperature.

STEP 3: Drive the vehicle for 10 minutes at highway speeds.

STEP 4: Drive the vehicle for 20 minutes in stop-and-go traffic, including at least four idle periods. Do not turn the ignition off at any time during the cycle. You can connect a scan tool and take a passenger along to watch and inform you when the monitor test status changes from Not Ready to Ready.

If these steps don't work, you may need to erase computer memory and start over. Often a good first step is to disconnect the battery cables with the key out of the ignition. This will erase learned computer values and place the computer in a "fast-learn" mode that accelerates monitor completion.

A word of caution: Disconnecting the battery in some vehicles can cause significant problems. Some "drive-by-wire" throttles and radios with security codes may refuse to work after the battery is reconnected. This is especially problematic on some European makes. At a minimum, be prepared to reprogram KAM for radio presets and clock settings following a battery disconnect.


A good foundation in any subject is essential in order to obtain an efficient and conclusive diagnosis, and the Diagnostic Link Connector (DLC) is a good place to start. The DLC is the location at which you connect your scan tool and obtain information from the vehicle. The DLC is a 16-cavity connector that always has power at Pin 16 and always has chassis ground at Pin 4 and signal ground at Pin 5. If your scan tool won't display communication, check the connections at the DLC. On many vehicles, Pin 16 is on the same fused circuit as the cigarette lighter. Many times, "stuff" just happens to find its way into the socket, or the occupants of the vehicle overload the circuit, causing the fuse to blow. The first step to checking the DLC for good power and ground is to make sure your digital multimeter (DMM) is baselined by testing at the battery, and then checking Pin 16 for B+ voltage and Pins 4 and 5 for 100mV or less voltage drop with the circuit loaded. To perform a proper voltage drop on the ground pins, load the circuit by plugging your scan tool into the DLC and back-probing pins 4 and 5.


1. Never insert large test lead probes into the DLC pins from the front. Back probe the connections at the DLC to avoid expanding the terminals. Damage to the DLC terminals may result in poor scan tool connections and lost data.

2. Look for pins that have been damaged, pushed out through the back or are missing from the DLC 16-cavity connector. Most of the DLC connectors have approximately six pins.

3. Just because your scan tool powers up does not mean you have good voltage and grounds. Many of today's newer-generation scan tools have their own internal battery that allows the scan tool to power up. Always check to see if you have power, ground and serial data if you experience no communication or missing data.

The OBD II system knows there's a problem if inputs/sensors and outputs/actuators fail. It does this by monitoring electrical circuits and sensor signals. For instance, misfire is detected using the crankshaft speed/position sensor.

EVAP leaks are detected using control valves and fuel tank pressure sensors. Exhaust gas recirculation (EGR) operation is checked by comparing manifold pressure or oxygen sensor readings both before and after the commanded EGR opening, checking for changes in manifold pressure or exhaust oxygen content.

The PCM runs three types of test:

Passive: Simply looks for problems.

Active: Activates a component to see if it works.

Intrusive: This is a special Active Test that may affect the way the vehicle runs. Intrusive tests may cause some brief "weird" symptoms, such as a slightly rough idle. These minor symptoms are not to be confused with a true vehicle failure.

Now that you've reviewed a brief history of OBD II and the DLC connector, you're ready to move on to a good diagnostic strategy for repairing vehicles. First, you need some good information sources. You should also have a good support hotline as a backup. And of course, don't forget your diagnostic equipment. Diagnostic equipment for OBD II includes a DMM; scan tools with the latest generic/global Controller Area Network (CAN)-compliant software and OE software if possible; a Power Probe; low-current amp clamp; a labscope; a Fuel System Analyzer (FSA); and a smoke machine to search for EVAP leaks. And last but not least, make sure you are fully trained on electricity, meters, scopes, scan tools and OBD II.


These case studies illustrate the need for more than just a brief overview to effectively diagnose and repair OBD II vehicles. Technicians need to bridge the gap between a general overview and real-world diagnosis and repair. The best way to master these techniques is by attending hands-on classes and practicing on your own. No one knows a good waveform from a bad without looking at the good, the bad, and the ugly from many vehicles. If you spend some time and money learning OBD II now, you'll save yourself a lot of time and money later.

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