In recent years, many states that formerly did tailpipe emissions testing have changed to a faster, easier and less expensive method of checking emissions compliance. It is an OBD II plug-in emissions test.
An OBD II test is a simple plug-in computer check that verifies four things:
1. The Vehicle Identification Number (VIN).
2. That the OBD II system is "ready" (all system readiness monitors have completed).
3. The status of the Check Engine lamp. The lamp must be functioning correctly and come on when commanded to do so. Otherwise, it must be off and remain off, indicating there are no faults and no trouble codes.
4. There are no emission trouble codes in memory 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 doesn�t 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 are allowing 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.
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 Hondas, the plug is located behind the ashtray. On BMW and VW vehicles, it is behind trim panels. On Volvos, the plug is next to the hand brake. On some older Audis, you will find it hidden behind the rear seat ashtray.
When OBD II runs a self-check on a particular component or system, it lets you know by setting readiness 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 can't 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.
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.
Some vehicles require very specific drive cycles (called "drive traces" if you perform them on a road simulator or dyno) to activate certain self-checks like the catalyst and EVAP monitors.
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.
Most of the known readiness issues are on older cars, including 1996-1998 Mitsubishi (which require a very specific drive cycle), and 1996 Subaru and Volvo 850 Turbo (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! Other vehicles that often have a "not ready" condition for the EVAP and catalytic converter monitors include 1996-1998 Volvo, 1996-1998 Saab and 1996-1997 Nissan 2.0L 200SX.
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.
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 scan 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. These may require an expensive professional grade scan tool. For more information on this subject, see ScantoolHelp.com.
On some vehicles, OBD II may flag a fault 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.
One such pattern failure is a code P1406 on older 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 isn't 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.
If an engine is experiencing a 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 won't 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.
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.
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. A loose gas cap will usually set a P0440 codes.