Auto Service World
Feature   May 1, 2006   by Dennis Mellersh

Countertalk: Knowledge Building: Your Role in OBD II

As the traditional repair and services garage increasingly becomes more of a computerized and electronics-driven solutions provider, the counterperson’s role as advisor to trade customers becomes more complex.

The question of emissions-oriented technology is a case in point. Emissions test technology, and OBD II capabilities in particular, were mandated and designed to help effectively manage and maintain the emissions levels of automotive vehicles at ideal levels that are not as harmful to the environment.

Yet, in many instances, these technologies can be used at a diagnostic level for much more than simple emissions compliance. They can be highly effective tools in helping diagnose drivability problems.

Paradoxically, as electronics become more prevalent in aftermarket tools and equipment, the knowledge, experience, common sense, and intuition of both the jobber and the service provider are needed more than ever in the area of diagnostics.

As one service writer at a major Toronto car dealership says, “Emissions testing will tell you what’s coming out of the tailpipe, but not why. OBD II on the other hand will tell you what’s happening with the vehicle in terms of what’s wrong, but it won’t tell you what the emissions levels are.”

In other words, in the case of emissions testing, such as with exhaust gas analyzers, you can determine what gases and hydrocarbons are coming out the tailpipe and at what levels, but you still need knowledge and experience to determine what these levels signify.

In the case of OBD II, with the specific diagnostic trouble code (DTC) PO303, for example, a technician can find out that the #3 cylinder is misfiring, but he will need knowledge and experience to be able to find out why, and then he’ll probably need other diagnostics techniques and tools to solve the misfire problem.

Emissions readings of tailpipe exhaust gases using gas analyzers can be an effective diagnostic troubleshooting tool. As noted by Toyota Motor Sales USA, Inc., “Use of a four- or five-gas exhaust analyzer can be helpful in troubleshooting both emissions and drivability concerns…Use of an exhaust gas analyzer will allow you to narrow down the potential cause of drivability and emissions concerns, focus your troubleshooting tests in the areas most likely to be causing the concern, and save diagnostic time. In addition to helping you focus your troubleshooting, an exhaust gas analyzer also gives you the ability to measure the effectiveness of repairs by comparing before and after exhaust readings.”

You can actually learn a lot from tailpipe emissions. Following are some examples Toyota gives for causes of problems identified by exhaust gas analysis.

If there are excessive hydrocarbons, common causes could include:

* Ignition system failures

* Excessively lean air/fuel mixture

* Excessive exhaust gas recirculation (EGR) dilution

* Restricted or plugged fuel injectors

* Incorrect spark timing

* Insufficient cylinder compression

If there are excessive oxides of nitrogen, causes could include:

* Cooling system problems

* Excessively lean air/fuel mixture

* Improper oxygen sensor operation

* Improper or inefficient operation of EGR system

* Improper spark advance system operation

* Carbon deposits on intake valves

If there are excessive levels of carbon monoxide, causes could include:

* Excessive fuel pressure at the injectors

* Leaking fuel injectors

* Crankcase fuel contamination

* Plugged PVC valve or hose

The above is by no means an exhaustive description, but a thorough knowledge of exhaust gas testing and analysis tools can be an excellent means of identifying the causes of drivability problems as well as the identification of emissions problems. As Jim Fox of Maremont puts it, “There are various times or situations other than mandated inspection cycles when people will want to know what is coming out the exhaust–such as for engine tuning diagnostics purposes.”

OBD II was mandated on vehicles beginning with 1996 models primarily to identify problems that are resulting in less than ideal emissions performance by the vehicle. As with exhaust gas analysis however, OBD II can be an excellent diagnostic troubleshooting resource for drivability problems.

The reason OBD II is so effective in this area is that OBD II, through its DTCs, can pinpoint faulty operational situations. The codes are read or identified through electronic scanners, or can be indicated using the car’s readouts and special procedures to access them.

Again, as with exhaust gas analysis however, interpretation, knowledge, and experience are the keys to turning operational specifics into successful diagnosis and solutions of drivability problems. There are literally thousands of DTCs, and without the knowledge of how to interpret them, simply seeing the code displayed may be of little value. An excellent source for detail information on DTCs and other OBD II background is The standard codes are also listed in the Automotive Service Data Book, copies of which can be obtained in print or CD form through

Here are some key DTC areas the OBD-codes site identifies and their corresponding engine operating areas:

PO200 to PO299 = fuel and air metering (injector circuit)

PO300 to PO399 = ignition system or misfire

PO400 to PO499 = auxiliary emissions controls

PO500 to PO599 = vehicle speed controls and idle control system

The beauty of OBD II is the specificity of its DTCs, which will identify, for example, exactly which cylinder is misfiring. For example, PO301 = cylinder 1 misfire detected, PO302 = cylinder 2 misfire detected, and so on sequentially down to PO312 = cylinder 12 misfire detected. Here are a few other examples which demonstrate just how specific the DTCs are: PO420 = catalyst efficiency below threshold; PO101 = MAF circuit malfunction; PO217 = engine overtemp condition; PO230 = fuel pump primary circuit malfunction.

For each code there is a meaning, a symptom, a cause, and a possible solution. So again, just having the right electronics reading, whether it is through scanning tools or other hardware and software, does not substitute for knowledge built over years that will enable the service provider to correctly interpret emissions and OBD II readings.

In terms of which main tools the service provider should use, Patrick Dubois, national sales manager of Vetronix–Bosch Group, rates them in order of priority.

“First, you need an OBD II scan tool; second, a gas analyzer; and third, an engine analyzer. The point is that you need the OBD II scanning tool to identify the failure and then once the failure is identified, you can do analysis with the other two tools. Although you need all three to be truly effective, the first one, the OBD II scan tool, is the most important. Gas analyzers are really like a blood test for the vehicle. They operate generically and are not vehicle-sensitive or -specific. The big benefit of all of these tools for the shop’s productivity and profitability is that they enable the shop to pinpoint problems, which saves a lot of time.”

A technical spokesperson for Delphi Product & Service Solutions stresses the importance of real-world knowledge in OBD II diagnostic troubleshooting.

Delphi indicates that there could sometimes be problems in the interpretation of OBD II readings. To help with diagnostics, Delphi offers a lot of technical assistance and support in its TechSource program. This includes detailed Tech-Tips and newsletters, service bulletins, as well as product and service information seminars. Training includes CDs, online programs, and a technical call centre. “The Tech-Tips component consists of original material written from real-world situations based on technicians’ calls to and discussions with the Delphi technical call centre,” Delphi says.

Emissions readings and OBD II readings can be highly useful tools in diagnosing both situations resulting in drivability problems as well as emissions-compliance issues. Proper use of these tools can reduce comebacks and job revisiting, and generally can save a lot of trial and error and manual searching time and thus improve productivity and profitability for the service provider.

Jobbers, as the traditional virtual “meeting place” for service providers and their concerns, are in an excellent position to help these trade customers understand the issues involved. In this respect the counterperson can serve as productive technical information source, and the jobber operation as a technical training source and reference centre.

Today it is a challenge for traditional aftermarket service providers to build their core businesses, with OEM dealership networks exerting such a strong pull on the consumer through extended warranties and other devices.

Jobbers are in a position to help traditional service providers to compete by helping them to offer and promote increased value-added diagnostic capabilities such as thorough emissions testing and OBD II.


At the first contact point of dealing with the customer at the service provider shop level, the service writer is very important.

One of the key functions the service writer should perform is to ask the customer questions about what specific problems the customer is experiencing with their vehicle, so that drivability problems can be described accurately.

A diagnostic checklist of questions can be useful in this regard. From a diagnostics perspective the questioning must be very specific, such as whether the problems are experienced on start-up or after the vehicle has been running for some time, or occur when restarting a warm engine.


True or False? OBD II and its associated electronic tools reduce the need for intelligent technicians.

False: While OBD II is good for pinpointing malfunctions, the intelligence and experience of the technician are required more than ever before to pinpoint causes and figure out solutions.

True or False? When the MIL light comes on, the customer should stop the car as soon as possible or turn into a repair facility immediately.

False: While the MIL or SES or “check engine” light means there is a problem, it means to seek service soon. It is not necessary to stop the engine.

True or False? Emissions testing is only useful for emissions regulations compliance such as clean air programs.

False: Effective emissions testing can identify many drivability problems.

True or False? If the MIL comes on, it will never go off unless the car is serviced.

False: If the problem, such as a cylinder misfire, corrects itself within a certain number of drive cycles, the MIL will usually go off by itself.

True or False? You have to be a computer geek to be able to interpret emissions readings and OBD II DTC readings.

False: Readings are not complicated. In fact, the traditional knowledge and experience of the service technician is much more important than computer literacy.

True or False? With vehicles being designed to go longer between maintenance cycles you don’t need to replace O2 sensors until they die and stop working.

False: Periodic replacement of the O2 sensors should be done as a matter of good maintenance. A defective but still “live” O2 sensor can cause emissions and drivability problems.

True or False? DTCs can be cleared by disconnecting the battery for a specific interval.

False: You may be able to get the MIL to go off by doing this, but the codes are still there and will remain there until the malfunctions the codes are identifying are fixed.


KOEC: Key On, Engine Cranking

KOEO: Key On, Engine Off

KOER: Key On, Engine Running

KSM: Knock Sensor Module

LT: Long Term Fuel Trim

MAF: Mass Airflow Sensor

MAP: Manifold Absolute Pressure Sensor

MC: Mixture Control

MDP: Manifold Differential Pressure

MIL: Malfunction Indicator Lamp

MST: Manifold Surface Temperature

MVZ: Manifold Vacuum Zone

NOX: Oxides of Nitrogen

O2S: Oxygen Sensor

OBD II: Onboard Diagnostics,

Second Generation

ODM: Output Device Monitor

OL: Open Loop

PCM: Power Train Control Module

PSA: Pressure Switch Assembly

RM: Relay Module

SDM: Sensing Diagnostic Mode

SFI: Sequential Fuel Injection

SRI: Service Reminder Indicator

SRT: System Readiness Test

ST: Short Term Fuel Trim

TB: Throttle Body

TBI: Throttle Body Injection

TCC: Torque Converter Clutch

TFP: Throttle Fluid Pressure

TP: Throttle Position

TPS: Throttle Position Sensor

TVV: Thermal Vacuum Valve

VAF: Volume Airflow

VCM: Vehicle Control Module

VR: Voltage Regulator

VS: Vehicle Sensor

WOT: Wide Open Throttle

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