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OBD2 Diagnostic Help

Copyright AA1Car Adapted from an article written by Larry Carley for Import Car magazine
If your Malfunction Indicator Lamp (MIL) is on, it means the Onboard Diagnostics II system (OBD II) has detected an emissions-related problem. OBD II is designed to turn on the MIL if a problem occurs that may cause emissions to exceed federal limits by 150 percent. The problem has to occur more than once, and it must be significant enough to create a potential emissions problem (one serious enough to prevent a vehicle from passing an emissions test).

In the real world, the MIL often comes on for what seems like trivial reasons (like a loose gas cap). But there's no way to know what's triggering the light until the vehicle is diagnosed. The problem may be something minor that has little or no affect on driveability, or it may be something more serious that is affecting engine performance.

The mysterious nature of the MIL, which most people call the "Check Engine" light, terrifies and confuses a lot of motorists. Except for a few luxury vehicles that actually display a fault message when the MIL comes on, most provide no information whatsoever other than indicating that something is wrong. You have no way of knowing if the problem is major or minor, or what it will ultimately cost to have the problem diagnosed and repaired.

Some motorists, on the other hand, seem unfazed by warning lights. As long as their vehicle continues to run, they see no urgency to have their engine checked, to slow down or to do anything out of the ordinary. Others are optimists and hope that if they keep on driving, the light will magically go out. Sometimes it does, much to their relief. But when the light refuses to go out, or it comes and goes like the ups and downs of the stock market, they panic and don't know what to do.

Anyone who repairs late-model vehicles today for a living knows that diagnosing complex emissions and driveability problems is not as simple as reading a code and replacing a part. OBD II is a great system that has a tremendous amount of self-diagnostic capability, but it only identifies faults in particular circuits or systems. It does not tell you which component to replace. That can only be determined after doing additional diagnostic work to isolate the fault.

Some problems such as misfires and evaporative emissions leaks can be very challenging to nail down. Misfires can be caused by ignition problems, fuel problems or compression problems. The underlying cause might be fouled spark plugs, bad plug wires, a weak ignition coil, dirty injectors, a shorted or open injector, low fuel pressure, a vacuum leak, a leaky head gasket, burned exhaust valve or a camshaft with a bad lobe. No simple plug-in diagnosis will give you the answer until you do a lot of other checks.


Tailpipe emissions testing may soon be a thing of the past, if the growing list of states that are instead using OBD II testing is any indication. The OBD II test is quick and easy, does not require an expensive dyno or emissions analyzer and gives a pass/fail indication in a minute or less. There is no risk of damage to the motorist's vehicle (as may be the case when running a vehicle on a dyno), and the reliability of the OBD II test is actually better than a tailpipe emissions test. Why? Because the OBD II system monitors emissions 24/7, 365 days a year. There are no arbitrary cutpoints that can be fudged one way or the other to pass or fail more or fewer vehicles. In other words, everybody dances to the same tune and must meet the same standards.

OBD II is also much better at detecting evaporative emissions leaks, and a drop off in converter efficiency. If the MIL is on and there's a code for an EVAP or converter problem, you can usually bet the problem is real. The problem may not have any noticeable affect on driveability or performance but, technically, it is in violation of the standards - and must be fixed before the MIL will go out and stay out.

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 air flow 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 inches in diameter (0.020 inches for 2000 and newer 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-inch hole is the size of the head of a pin.


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 BMWs and VWs, it is behind trim panels. On Volvos, the plug is next to the hand brake. On Audis, you will find it hidden behind the rear seat ashtray.


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. 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 to come on.

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.

Most OBD II codes take time to mature and will not turn on the MIL 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.

The bottom line here is if the light is on, the vehicle will not pass the OBD II test. The problem must be fixed and the MIL must stay out before the vehicle passes.

obd monitors


One of the EPA's requirements for using a plug-in OBD II check in lieu of a tailpipe test is to make sure the OBD II system has run all of its monitors and that the monitors have all passed. But there's a catch. Some import vehicles 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, and one readiness monitor for 2001 to 2003 vehicles.

When OBD II runs a self-check on a particular component or system, it lets you know by setting a readiness "flag" or indicator, which can be displayed on your code reader or scan tool. If OBD II has run all the available monitors and all the monitors have passed (and no faults have been found) the vehicle should pass the OBD II plug-in test. But if all the required monitors have not run, the vehicle canno 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.

Mike Cole of the National Center for Vehicle Emissions Control and Safety (NCVECS) at Colorado State University says 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. NCVECS has compiled all the known drive traces for various vehicles on a CD and offers the package to technicians for about $40. For more information, you can visit www.ncvecs.colostate.edu.

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 drive the vehicle more to set the remaining monitors.

With the EVAP monitor, the vehicle may require a certain period of inactivity (such as sitting overnight) and certain ambient temperature conditions (such as above freezing) before the EVAP monitor will run.

Some vehicles with known readiness issues include 1996 to 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 Tercels and Paseos, 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 Saabs and the 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 doesn't 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 others, the EVAP monitor won't run unless the vehicle has sat 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.

scan tool companion information


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's going on if you're 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 can often be used to make a quick diagnosis, and in many cases you don't need anything else. But for advanced diagnostics, you need a professional-grade scan tool or software package with advanced capabilities.

For some jobs, you may also need a tool that can graph or display waveforms. That means buying a digital storage oscilloscope, if you don't buy a high-end scanner that can do both. Most scan tools display datastream 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's where a scope comes in handy.

When a scope is hooked up to a sensor or circuit, it shows what's 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.

Diagnostic Parameters

For a detailed look at the operating parameters that can set various fault codes, Click Here to view a PDF file on GM 4.6L diagnostic parameters.



  • To improve the gas mileage and to constantly maintain the operational efficiency of the engine, it is very important to replace your customers' oxygen sensor every 50,000 miles under normal driving conditions.
  • There are several factors, such as a fouled spark plug, vacuum leak, an EGR leak, excessive fuel pressure, MAP (manifold absolute pressure) sensor, MAF (mass air flow) sensor and incorrect ignition timing, that can affect the normal functioning of the oxygen sensor.
  • The sensor can be contaminated from engine coolant, excessive oil consumption, additives used in sealants, and the wrong additives in gasoline. A contaminated oxygen sensor will not produce the proper voltage and will not switch properly.
  • It is very important that the oxygen sensor's heater electrical circuits be in excellent condition. Inspect the vehicle's wiring harness for damage or corrosion or loose fit.
  • Since the oxygen sensor is a part of system feedback operation and the sensor output signal can affect the operation of the mixture solenoid on carburetor vehicles and the pulse width on fuel-injected vehicles, a malfunctioning oxygen sensor will cause the engine to run rich or lean.

    ~ A rich condition means the engine produces high carbon monoxide. If this condition persists for a long period of time, it will cause too much carbon buildup on the spark plugs and will decrease engine performance.

    ~ A lean condition means the engine produces high hydrocarbons and can produce nitrogen oxide, and this will cause the combustion chamber heat to rise as a result. This will also lead to damage to the spark plugs.
  • The PCM uses an oxygen sensor to ensure the proper air/fuel ratio. Based on the oxygen sensor signal, the PCM will adjust the amount of fuel injected into the intake air stream.
  • OBD II vehicles require two oxygen sensors: one before the catalytic converter (used by PCM to adjust the air/fuel mixture), and one after to determine the efficiency.
  • Many factors can cause engines to misfire such as a defective spark plugs, distributor cap and rotor, a malfunctioning injector(s), bad CTS, etc.

- courtesy of Denso Sales North America

Misfire Diagnosis

If an emissions problem is being caused by an 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 sets 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've 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 will also 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 air flow sensor, or an EGR valve that is stuck open.

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