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OBD2 plug-in diagnosis with scan tool

OBD II Emissions Testing and Diagnostic Codes

Copyright AA1Car Adapted from an article written by Larry Carley for Underhood Service magazine

Are tailpipe emissions tests going away? That seems to be the case for 1996 and newer vehicles. Many states have changed or are in the process of changing their vehicle inspection programs to Onboard Diagnostic II (OBD II) computer plug-in checks for late-model cars and trucks. In some states, such as Illinois, the dyno tailpipe tests have been eliminated altogether and replaced with OBD II plug-in tests (1995 and older cars no longer have to be tested).

In most instances, an OBD II check replaces the need for an I/M 240 or other loaded-mode tailpipe test. If a vehicle fails the OBD II test, some states give the vehicle owner the option of taking a "second-chance" tailpipe test. Older vehicles that do not have OBD II still have to take a tailpipe test in most states.

So far, 25 states and the District of Columbia have made the switch to OBD II testing. These include Alaska, Arizona, California (in areas that require "enhanced" emissions testing), Colorado, Connecticut, Georgia, Illinois, Indiana, Kentucky, Maine, Massachusetts, Missouri (St. Louis), Nevada, New Hampshire, North Carolina, Oregon, Texas (Houston and Dallas/Ft. Worth), Utah (Salt Lake City), Vermont, Washington and Wisconsin. In 2004, New Jersey, New York, Pennsylvania (Pittsburgh and Philadelphia) and Virginia added OBD II testing.

In some states, owners of 1996 and newer vehicles that fail the OBD II test can take a second-chance tailpipe test. If they pass the tailpipe test, they are considered to be in compliance with the law - even if the Malfunction Indicator Lamp (MIL) is on.

For the latest information on current state emissions testing programs, see OBD Program Status.


An OBD II test is a simple plug-in computer check that verifies four things:

1. The Vehicle Identification Number (VIN).

2. That the vehicle's OBD II system is ready (all required readiness monitors have been set).

3. The status of the MIL lamp. The lamp must be functioning correctly and come on when commanded to do so. Otherwise, it must be off, indicating no codes.

4. That the vehicle has no diagnostic trouble codes that would cause the MIL lamp to come on.


OBD II is primarily emissions-driven and will set codes anytime a vehicle has a fault that may cause emissions to exceed federal limits by 1.5 times. It does not necessarily mean the vehicle has a real emissions problem. Many times it does not. But if the nature of the failure is such that it might cause emissions to rise, OBD II will set a code anyway. That is why some states allow motorists to opt for a second-chance tailpipe test if their vehicle fails an OBD II test. In many instances, the vehicle will pass the tailpipe test.

OBD II monitors misfires, converter efficiency, catalyst heater (if used), the evaporative system, air injection system (if used), fuel trim, oxygen sensors, exhaust gas recirculation (if used), secondary air system (if used), the coolant thermostat (starting in 2000), positive crankcase ventilation system (starting in 2002) and even the A/C systems on some 2002 and newer vehicles.

If a situation develops in any of these monitored systems that could cause a real or potential emissions problem, OBD II will watch it, set a code and eventually illuminate the MIL.

Some codes take time to mature and will not turn on the MIL lamp immediately. OBD II may wait until it detects the same problem on two separate drive cycles before it converts a pending code into a mature code and turns on the MIL.

OBD II Plug-In Diagnostics

All OBD II-equipped vehicles have a common J1962 16-pin diagnostic connector and use the same "generic" fault codes. This means all you need is an OBD II-compliant code reader or scan tool to check readiness status, and to read and clear codes. The state emissions programs require vehicle inspection facilities to use a more sophisticated plug-in tool that also records vehicle data for record-keeping purposes, but otherwise they are using the same basic scan tool technology as everybody else.

To access the OBD II system all you have to do is plug a code reader or scan tool into the 16-pin connector (Note: there are no "manual flash codes" on OBD II systems). The connector is usually located under the dash near the steering column. But on some vehicles, it can be hard to find. On many Honda models, the plug is located behind the ashtray. On BMW and VW vehicles, it is behind trim panels. On Volvo, the plug is next to the hand brake. On Audi, you may find it hidden behind the rear seat ashtray.


One of the EPA requirements for using a plug-in OBD II check in lieu of a tailpipe test is to make sure the OBD II system is ready and has run its system monitors. Some vehicles (mostly some import models) have readiness issues when it comes to setting all the OBD II monitors. Consequently, the EPA currently allows up to two readiness monitors not to be set prior to testing 1996 to 2000 model year vehicles. For 2001 and newer vehicles, only one readiness monitor not set is allowed. After that, ALL readiness monitors have to be completed before the vehicle can take a plug-in OBD II emissions test.

When OBD II runs a self-check on a particular component or system, it lets you know by setting readiness prompts or "flags," which are displayed on your code reader or scan tool. If OBD II has run all the available monitors and finds no faults, the MIL remains out and the vehicle passes the emissions test. But if all the required monitors have not run, the vehicle cannot be given an OBD II test. The motorist must drive the vehicle and come back again, or take a tailpipe test if that is an option.

If OBD II detects a fault when running a monitor, the setting of a code may prevent the remaining monitors from running. A bad oxygen sensor, for example, will prevent the catalyst monitor from running.

emissions testing, oxygen sensor

Known OBD Readiness Issues

Getting all the monitors to run can be tricky on some vehicles. Each monitor has certain operating requirements that must take place before the self-check will run.

To set the converter monitor, for example, the vehicle may have to be driven a certain distance at a variety of different speeds. The requirements for the various monitors can vary considerably from one vehicle manufacturer to another, so there is no "universal" drive cycle that will guarantee all the monitors will be set and ready. To be sure, you may have to follow the vehicle manufacturer's specific drive cycle for the year, make and model of vehicle.

As a general rule, doing some stop-and-go driving around town at speeds up to about 30 mph followed by five to seven minutes of steady 55 mph highway speed driving will usually set most or all of the monitors. Consequently, if you are checking an OBD II system and discover that one or more of the monitors have not run, it may be necessary to test drive the vehicle to set the remaining monitors.

Some vehicles with known readiness issues include 1996-1998 Mitsubishi cars (which require a very specific drive cycle), and 1996 Subarus and Volvo 850 Turbos (turning the key off clears all the readiness flags, so don't turn the vehicle off after driving). On 1997 Toyota Tercel and Paseo, the readiness flag for the EVAP monitor never will set, and no dealer fix is yet available. Other vehicles that often have a "not ready" condition for the EVAP and catalytic converter monitors include 1996-1998 Volvos, 1996-1998 Saab and 1996-�97 Nissan 2.0L 200SX.

OBD Drive Cycles

If the MIL comes on while driving, or remains on after starting the engine, it means OBD II has detected a problem. The lamp will usually remain on - unless the fault does not reoccur in three consecutive drive cycles that encounter the same operating conditions, or the fault is not detected for another 40 drive cycles. If OBD II sees no further evidence of the problem, it will turn off the MIL and erase the code.

An OBD II drive cycle is not just turning the ignition key on and off or starting the engine. A drive cycle requires starting a cold engine and driving the vehicle until the engine reaches normal operating temperature. The next drive cycle does not begin until the engine has been shut off, allowed to cool back down and is restarted again.

On some vehicles, the drive cycle also includes the cold soak time between trips. On some vehicles, the EVAP monitor won't run unless the vehicle has not been run for eight hours. There is no way to bypass or get around such requirements, so you have to do whatever the system requires. And if that means waiting, you have to wait.

Reading OBD Trouble Codes

If OBD II has detected a fault, you should find one or more "generic" codes (which start with the prefix "P0"), and maybe one or more "enhanced" codes (OEM-specific codes that start with a "P1"). All OBD II compliant code readers and scan tools should be able to display generic codes, but some do not display all the OEM enhanced codes. As a result, you may not get the full picture of what is going on if you are using a tool with limited capabilities.

The same goes for accessing many OBD II diagnostic features such as history codes, snapshot data and special diagnostic test modes that require two-way communication and special scan tool software. For example, some of the OBD II diagnostic features that are currently accessible with an OEM factory scan tool are not yet available on aftermarket scan tools. This may limit your ability to diagnose and repair certain types of problems.

An inexpensive Palm Pilot or other Personal Digital Assistant (PDA) with scanner software and cable, or even a DIY type of code reader can be used to read and clear most OBD II codes on 1996 and newer vehicles. This type of tool often can be used to make a quick diagnosis, and in many cases you do not need anything else. But for advanced diagnostics, you need a professional grade scan tool or software package with advanced capabilities.

For some jobs, you also may need a tool that can graph or display waveforms. That means buying a digital storage oscilloscope if you do not buy a high-end scanner that can do both. Most scan tools display data stream values, which is what the PCM tells it to display. If the PCM is misreading a sensor input or is substituting bogus information, you have no way of knowing without actually testing the circuit or component in question. That is where a scope comes in handy.

When a scope is hooked up to a sensor or circuit, it shows what is actually going on inside that device or circuit. Voltage is displayed as a time-based waveform. Once you know how to read waveforms, you can tell good ones from bad ones. You can also compare waveforms against scan tool data to see if the numbers agree (which is a great way to identify internal PCM faults).

A scope also allows you to perform and verify "action-reaction" tests. You can use one channel to monitor the action or input, and a second, third or fourth channel to watch the results. For example, you might want to watch the throttle position sensor, fuel injector waveform, crank sensor signal and ignition pattern when blipping the throttle to catch an intermittent misfire condition.

OBD II Pattern Failures

On some vehicles, OBD II may set a DTC when in fact there is no real emissions problem. These are called "pattern failures" and occur on a variety of different applications. Many of these can be found in factory technical service bulletins. AllataDIY is an excellent resource for this kind of information.

One such pattern failure is a code P1406 on GM cars. This code indicates a fault with the EGR valve. Replacing the EGR valve may not solve the problem because the MIL comes back on and sets the same code. The real problem here is that the OBD II system is not allowing enough time for the EGR valve to respond when it is commanded to open. The fix is to reprogram the computer so OBD II will allow more time for the appropriate response from the EGR valve.

Engine Misfires
If an emissions problem is being caused by engine misfire, the OBD II lamp may flash as the misfire is occurring. But the lamp will not come on the first time a misfire problem is detected. It will only come on if the misfire continues during a second drive cycle and set a P0300 series code.

A P0300 code would indicate a random misfire (probably due to a vacuum leak, open EGR valve, etc.). If the last digit is a number other than zero, it corresponds to the cylinder number that is misfiring. A P0302 code, for example, would tell you cylinder number two is misfiring.

Unfortunately, OBD II will not tell you why the cylinder is misfiring. That you have to determine by doing more diagnostic tests once you have isolated the misfire to a particular cylinder. The cause could be a fouled spark plug, bad spark plug wire, weak ignition coil, dirty or dead fuel injector, or a burned exhaust valve.

Random misfires that jump around from cylinder to cylinder also will set a misfire code (P0300). The underlying cause is often a lean fuel condition, which may be due to a vacuum leak in the intake manifold or unmetered air getting past the airflow sensor, or an EGR valve that is stuck open.

Converter Trouble Codes

OBD II monitors the operation of the catalytic converter with a second "downstream" oxygen sensor in the tailpipe behind the converter. It compares the upstream and downstream O2 sensor readings to estimate how efficiently the converter is working. A good converter should show little downstream O2 sensor activity with few cross-counts.

If you look at the two O2 sensor readings on an oscilloscope, the upstream O2 sensor should be flipping back and forth from rich to lean (high voltage to low voltage) while the downstream O2 sensor should be flat-lined. If converter efficiency has dropped off due to old age or contamination, the downstream O2 sensor reading will look like the upstream reading.

EVAP Codes

OBD II monitors evaporative emissions by checking for fuel vapor leaks once a drive cycle. OBD II does this by applying vacuum or pressure to the fuel tank, vapor lines and charcoal canister. If it detects no airflow when the EVAP canister purge valve is opened, or it detects a leakage rate that is greater than that which would pass through a hole 0.040" in diameter (0.020" for 2000 and up model year vehicles), it indicates a fault.

If you find a P0440 code that indicates a fuel vapor leak, finding the leak can be a challenge. The first place to start is the gas cap. A loose-fitting or damaged cap can allow enough air leakage to set a code. To find a leak in a vapor hose, you may need a leak detector that uses smoke and/or dye. A 0.020" hole is the size of a pin.

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OBD II Guide

Understanding OBD II Driveability & Emissions Problems

Fixing Emission Failures

All About Onboard Diagnostics II (OBD II)

Advanced Diagnostics: Mode 06

Basic Emission Control Systems Overview

Exhaust Emissions Diagnosis

Troubleshooting a P0420 Catalyst Code

Catalytic Converters

Exhaust Gas Recirculation (EGR)

Positive Crankcase Ventilation (PCV)

Driveability Diagnosis: Misfires

Sensing Emission Problems (O2 Sensors)

Emissions testing update

Evolution of I/M 240

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