OBD II engine diagnostics
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OBD II Check Engine Light

OBD II Engine Diagnostics

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Are you up to speed on OBD II diagnostics? You should be because most states now use a simple OBD plug-in test to check emissions compliance rather than a tailpipe dyno test. OBD II plug-in testing is faster, easier, cheaper and more accurate than a tailpipe road simulator type of emissions test. OBD II will detect emissions problems that might not cause a vehicle to fail a tailpipe test. Consequently, emissions test failures have gone up with OBD II testing.

OBD II has been required on all new passenger cars and light trucks sold in the United States since model year 1996, including all imports. OBD II is a very powerful diagnostic system that can give technicians a greater insight into what is actually happening within the engine control system.

Early OBD II systems first began to appear on a few 1994 models, including the Lexus ES300, Toyota Camry 1MZ-FE 3.0L V6 and T100 pickup 3RZ-FE 2.7L four, plus a number of Audi, Mercedes-Benz, Volkswagen and Volvo models. In 1995, more models were added including the Nissan Maxima and 240SX. Some of these early systems are not fully OBD II compliant, meaning they may not set codes or turn on the Check Engine light for misfires, catalytic converter problems or fuel vapor leaks.

Unlike earlier OBD systems that only set a diagnostic trouble code (DTC) when a sensor circuit experienced a gross failure such as a short, open or over voltage, OBD II sets codes if a fault has the potential of causing emissions to rise. OBD II is primarily emissions-driven and will set codes anytime vehicle emissions exceed the federal limit by 1.5 times. It also will set codes if there is a gross sensor failure, but some types of sensor problems will not always trigger a code. Consequently, the Check Engine Light on an OBD II-equipped vehicle may come on when there is no apparent drivability problem, or it may not come on even though a vehicle is experiencing a noticeable drivability problem.

The determining factor as to whether or not the Check Engine light comes on is usually the effect on emissions. In many instances, emissions can be held in check, despite a faulty sensor, by adjusting fuel trim, which the engine computer does automatically. So as long as emissions remain under the legal limit, the Check Engine Light usually remains off.


onboard diagnostics check engine light The "Malfunction Indicator Lamp" (MIL), which may be labeled "Check Engine" or "Service Engine Soon" or an ISO symbol of an engine with the word "Check" in the middle, alerts the driver when an emissions problem occurs.

Depending on how the system is configured and the nature of the problem, the lamp may come on and go off, remain on continuously or flash - all of which can be very confusing because you have no way of knowing what the light means. Is it a serious problem or not? If the engine seems to be running okay, you might just ignore the light.

Remember, the Check Engine light only comes on for emissions-related failures (which includes fuel and ignition, too). A separate warning light will illuminate if there are other non-emissions problems such as low oil pressure, charging system problems, etc.

If the light is on because of a misfire or a fuel delivery problem, and the problem does not recur after three drive cycles (under the same driving conditions), the Check Engine light may go out. Don't think the car has healed itself because the intermittent problem may still be there waiting to trigger the light once again when conditions are right. Whether the light goes out or remains on, a code will be set and remain in the computers memory to help you diagnose the fault.

With some exceptions, the OBD II warning lamp will also go out if a problem does not recur after 40 drive cycles. A drive cycle means starting a cold engine and driving it long enough to reach operating temperature.

The trouble codes that are required by law on all OBD II systems are "generic" in the sense that all vehicle manufacturers use the same common code list and the same 16-pin diagnostic connector. Thus, a P0302 misfire code on a Nissan means the same thing on a Honda, Toyota, Mercedes-Benz, Chevy or Ford. But each vehicle manufacturer also has their own proprietary "enhanced" codes to provide additional diagnostic information.

Enhanced OBD II Diagnostic Trouble Codes

Enhanced codes (P1 codes), which are special OEM codes for specific vehicle applications, provide additional information and often cover non-emission related failures that occur outside the engine control system. These include ABS codes, HVAC codes, air bag codes and other body and electrical codes.

The second character in an OBD II will be a zero if it is a generic code, or a "1" if it is a dealer enhanced code (specific to that particular vehicle application).

The third character in the code identifies the system where the fault occurred. Numbers 1 and 2 are for fuel or air metering problems, 3 is for ignition problems or engine misfire, 4 is for auxiliary emission controls, 5 relates to idle speed control problems, 6 is for computer or output circuit faults, and 7 and 8 relate to transmission problems.

Generic codes that are common to all vehicle manufacturers can be accessed using any basic scan tool that is OBD II-compliant. However, older first-generation scan tool that were made pre-OBD II cannot read codes on the newer vehicles. You need an OBD II compliant scan tool to read the fault codes and system data.

Also, some scan tools do not have all the OEM codes for all makes, particularly many European applications. Accessing the OEM-specific enhanced codes may require using a dealer scan tool, which can be very expensive.


If an emissions problem is being caused by engine misfire, the OBD II light will flash as the misfire is occurring. But the light will not come on the first time a misfire problem is detected. It will come on only if the misfire continues during a second drive cycle and will 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. Causes here would be anything that might affect only a single cylinder such as a fouled spark plug, a bad coil in a coil-on-plug ignition system or distributorless ignition system with individual coils, a clogged or dead fuel injector, a leaky valve or head gasket.

The OBD II system detects a misfire on most vehicles by monitoring variations in the speed of the crankshaft through the crankshaft position sensor. A single misfire will cause a subtle change in the speed of the crank. OBD II tracks each and every misfire, counting them up and averaging them over time to determine if the rate of misfire is abnormal and high enough to cause the vehicle to exceed the federal emissions limit. If this happens on two consecutive trips, the Check Engine light will come on and flash to alert the driver when the misfire problem is occurring.

Misfire detection is a continuous monitor, meaning it is active any time the engine is running. So too is the fuel system monitor that detects problems in fuel delivery and the air/fuel mixture, and something called the "comprehensive monitor" that looks for gross faults in the sensors and engine control

Other OBD II Monitors

Other OBD II monitors are only active during certain times. These are the "non-continuous" monitors and include the catalytic converter efficiency monitor, the evaporative system monitor that detects fuel vapor leaks in the fuel system, the EGR system monitors, the secondary air system monitor (if the vehicle has such a system), and the oxygen sensor monitors.

On many 2000 and newer vehicles, OBD II also has a thermostat monitor to keep an eye on the operation of this key component. The thermostat monitor is required on all 2002 and newer vehicles. On some 2002 model-year vehicles, there also is a PCV system monitor, which became standard on all vehicles in 2004.

The catalytic converter monitor keeps an eye on converter efficiency by comparing the outputs from the upstream and downstream oxygen sensors. If the converter is doing its job, there should be little unburned oxygen left in the exhaust as it exits the converter. This should cause the downstream O2 sensor to flatline at a relatively fixed voltage level near maximum output.

If the downstream O2 sensor reading is fluctuating from high to low like the front sensor, it means the converter is not functioning.

The Check Engine light will come on if the difference in O2 sensor readings indicates hydrocarbon (HC) readings have increased to a level that is 1.5 times the federal limit. For 1996 and newer vehicles that meet federal Low Emission Vehicles (LEV) requirements, the limit allows only 0.225 grams per mile (gpm) of HC - which is almost nothing. Converter efficiency drops from 99 percent when it is new to around 96 percent after a few thousand miles. After that, any further drop in efficiency may be enough to turn on the Check Engine light. We are talking about a very sensitive diagnostic monitor.

The EVAP system monitor checks for fuel vapor leaks by performing either a pressure or vacuum test on the fuel system. For 1996 through 1999 vehicles, the federal standard allows leaks up to the equivalent of a hole .040 inches in diameter in a fuel vapor hose or filler cap. For 2000 and newer vehicles, the leakage rate has been reduced to the equivalent of a .020 in. diameter hole, which is almost invisible to the naked eye but can be detected by the OBD II system. Finding these kinds of leaks can be very challenging. According to one expert, you can find a .040 in. leak with an ultrasonic leak detector but not a .020 in. leak. For such a small leak, you need a smoke or dye-type detector.

scan tool obd monitors incomplete
Three OBD monitors on this vehicle are NOT READY:
the Catalyst, EVAP and HEGO oxygen sensor monitors.


An essential part of the OBD II system are the "readiness monitors". These are self-tests the OBD II system runs to make sure everything is functioning normally. When a test runs and passes without any faults, the OBD II system runs the next monitor and the next until all have completed. This may take some time because some monitors require specific driving conditions before they will run. Also, if a fault is found during any test, it may prevent subsequent monitors from completing.

The misfire detection, fuel system and continuous system monitors are active and ready all the time, but the non-continuous monitors require a certain series of operating conditions before they will set - and you cannot do a plug-in OBD II test unless all of the monitors are ready. For more information on this subject see OBD II Emission Monitors Not Ready.

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.

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 55 mph plus highway speed driving will usually set most or all of the monitors (the converter and EVAP system readiness monitors are the hardest ones to set). So if you are checking the OBD II system and find a particular monitor is not ready, it may be necessary to test drive the vehicle to set all the monitors.

OBD II Readiness Issues

Some older import vehicles have known readiness issues. Many 1996-98 Mitsubishi vehicles will have monitors that read "not ready" because setting the monitors requires very specific drive cycles (which can be found in their service information). Even so, these vehicles can be scanned for codes and the MIL light without regard to readiness status.

On 1996 Subarus, turning the key off will clear all the readiness flags. The same thing happens on 1996 Volvo 850 Turbos. This means the vehicle has to be driven to reset all the readiness flags.

On 1997 Toyota Tercel and Paseo models, the readiness flag for the EVAP monitor will never 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 1996-98 Volvos, 1996-98 Saabs, and 1996-97 Nissan 2.0L 200SX models.


An official OBD II emissions test consists of three parts:

1. An inspector checks to see if the MIL light comes on when the key is turned on. If the light does not come on, the vehicle fails the bulb check.

2. A scanner is plugged into the diagnostic link connector (DLC), and the system is checked for monitor readiness. If more than the allowed number of monitors are not ready, the vehicle is rejected and asked to come back later after it has been driven sufficiently to set the readiness flags. The scanner also checks the status of the MIL light (is it on or off?), and downloads any fault codes that may be present.

If the MIL light is on and there are any OBD II codes present, the vehicle fails the test and must be repaired. The vehicle also fails if the DLC is missing, has been tampered with or fails to provide any data.

3. As a final system check, the scanner is used to command the MIL lamp on to verify it is taking commands from the onboard computer.

If the OBD II light is on, or a vehicle has failed an OBD II emissions test, your first job is to verify the problem. That means plugging into the OBD II system, pulling out any stored codes and looking at any system data that might help you nail down what is causing the problem.

Long-term fuel trim data can provide some useful insight into what is going on with the fuel mixture. If long-term fuel trim is at maximum, or you see a big difference in the numbers for the right and left banks of a V6 or V8 engine, it would tell you the engine control system is trying to compensate for a fuel mixture problem (possibly an air leak, dirty injectors, leaky EGR valve, etc.).

OBD II also provides "snap shot" or "freeze frame" data, which can help you identify and diagnose intermittent problems. When a fault occurs, OBD II logs a code and records all related sensor values at that moment for later analysis.

Once you have pinpointed the problem and hopefully replaced the faulty component, the final step is to verify that the repair solved the problem and that the

OBD II light remains off. This will usually require a short test drive to reset all the readiness monitors and run the OBD II diagnostic checks.

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.


You cannot work on OBD II systems without some type of OBD II-compliant scan tool or scanner software. Choices here include:

Aftermarket scan tools with the appropriate software and J-1962 16-pin universal OBD II diagnostic connector. Before you buy a scan tool, scanner software or a scanner app for a smart phone, check with the manufacturer to make sure it will work on your year, make and model of vehicle. Some inexpensive DIY scan tools, scanner software and apps only display generic OBD II codes. Other products only include a limited list of enhanced manufacturer codes. Some of these products on work on domestic vehicles or have very limited import coverage (like most Asian but no European coverage). Many BMWs require special scan tools or scanner software that is BMW specific.

Dealer scan tool. The main advantage with a dealer scan tool is access to ALL of the codes and data the vehicle can provide, including powertrain, body, ABS, airbag and other codes. OEM scan tools do it all, including PCM reprogramming, and bi-directional two-way communication and system testing. But the drawback is price. OEM scan tools can cost thousands of dollars, and they only work on one make of vehicle, not a broad range of vehicles as is the case with professional grade aftermarket scan tools and scanner software.

A basic These inexpensive tools, software and apps can display fault codes but little else. They can tell you why your Check Engine light is on, but are little or no help beyond that in diagnosing a fault because they can't read sensor data and other system information.

Scanner software for a laptop, tablet or personal computer (PC) or a smart phone app. The software provides the capability to read and display fault codes and system data. Some require a cable to connect your device to your vehicle while others can use a bluetooth or WiFi enabled plug in the OBD II connector to allow a wireless link. Capabilities range from very limited to very good. Again, make sure the product will work on your vehicle before you buy it, and that it is capable of providing the kind of diagnostic information you can use to fix your problem.

If a vehicle has an OBD II EVAP code (P0400 series codes), you may need some type of leak detection equipment to find the cause. Choices here include ultrasonic leak detectors that listen for sound waves produced by air or vapors escaping through an opening, smoke detectors that generate smoke which allows leaks to be spotted visually, and dye detectors that use a visible or ultraviolet dye to reveal leaks.


For advanced diagnostic work, a digital oscilloscope is a great tool for displaying sensor waveforms. Momentary faults that happen too quickly for a conventional voltmeter or even the OBD II system to detect can often be revealed by observing and comparing waveforms. Do not expect to hook up a scope and start making a diagnosis right away because learning how to use a scope takes some time and experience. You need to learn how to distinguish good waveforms from bad ones - which means looking at vehicles that do not have problems as well as ones that do. The International Automotive Technicians Network (www.iatn.com) maintains an online library of waveforms for members that is a great resource.

Related Products:

To OBD II Guide
Get OBD II Quick Reference Guide (click here)

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Larry's Guide to Check Engine Light Diagnostics
(PDF ebook)

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