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How To Diagnose OBD II Emissions Problems


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The "Malfunction Indicator Lamp" (MIL) on 1996 and newer vehicles equipped with OBD II has created more anxiety and fear among motorists than any other piece of hardware in automotive history. If the MIL lamp suddenly comes on while the vehicle is being driven, most motorists realize something is wrong. But what? The lamp does not tell them. Nor does it give any indication if the problem is serious or not.

Fortunately, many OBD II problems are relatively minor and have little impact on engine performance or reliability. But some do. The only way to know for sure what is causing the MIL lamp to come on is to plug a scan tool into the system and read the diagnostic fault code(s) to find out what is going on.

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If the MIL lamp comes on while driving or remains on after starting the engine, it means the OBD II system has detected a problem and is trying to alert the driver so appropriate measures can be taken. The driver should immediately check for any other warning lamps. If none are lit and the vehicle appears to be running normally, the lamp is on because something may be affecting vehicle emissions. This may or may not have an affect on driveability depending on the cause. If any other warning lamps are on (temperature, charging, oil, etc.), the problem is probably serious and requires immediate attention.

Any time the MIL lamp is on, the cause should be investigated - the sooner the better. Ignoring it will not make it go away - unless the fault does not recur 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 lamp and erase the code.

Note: 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 running it until it 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.

Many OBD II vehicles have a memory backup for the PCM, so disconnecting the battery or PCM fuse will not turn the MIL lamp off or clear the codes. You have to use a scan tool to read and erase the codes.


OBD II is primarily designed to detect emission faults, including any kind of fault that could cause emissions to exceed federal limits by 150 percent. It also monitors the following: converter efficiency, catalyst heater (if used), 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 some A/C systems on 2002 and newer vehicles. If a fault occurs in any of these monitored systems, even if it does not cause emissions to increase 150 percent, OBD II will catch it, set a code and eventually illuminate the MIL lamp.

With most OBD II problems, the MIL lamp does not come on right away. OBD II usually waits until the problem has occurred on two separate drive cycles before it turns on the MIL lamp. This is to reduce the number of "false warnings" that might otherwise occur if the system turned on the lamp every time it saw something amiss. So if the lamp is on, it means the problem has occurred before and has occurred again. It is not just a temporary glitch but something that needs to be diagnosed and corrected.

When OBD II detects a fault, it may generate one of two types of codes. "Generic" codes, which are common to all OBD II vehicles built since 1996, have "P0" as their first two digits. "Enhanced" codes, which are special OEM codes that vary by year, make and model, have "P1" as their first two digits. All OBD II-compliant scan tools will read the generic codes, but special software is usually required to read the enhanced codes and to access other special OEM diagnostic features that may be part of the OBD II system. These include actuator tests, calibration tests and similar tests. Some of these may be available only in the factory OEM scan tool.

To access codes, you have to plug a scan tool into the 16-pin J1962 connector. You will usually find it under the dash near the steering column, but on some import vehicles the connector is located elsewhere. On many Hondas, it is behind the ashtray. On BMW and VW models, it is behind trim panels. On Volvo, it is next to the hand brake. On Audi, it is hidden behind the rear seat ashtray.


With misfire detection, the MIL lamp may come on during the first episode if the OBD II system determines the rate of misfire is really high. On most applications, the MIL lamp will blink once per second while the misfire is actually occurring. After that, the MIL lamp may go out unless the engine had been misfiring before.

OBD II will chart the rate of misfire for each cylinder along with other data such as engine speed, load and warm-up status when the first misfire was detected. It will also set a temporary fault code that will become a hard code if the same problem happens again during the next drive cycle.

If the engine runs fine during the next drive cycle and does not experience any misfire, the temporary misfire code will be erased. But if the same problem recurs on two consecutive drive cycles, the temporary misfire code will become a hard code and turn on the MIL lamp.

OBD II misfire codes will tell you which cylinder is misfiring. A code P30301, for example, would tell you cylinder number one is not hitting. But OBD II does not tell you why it is misfiring unless there are additional codes (such as a bad fuel injector or a lean fuel mixture code). If the misfire is ignition-related, OBD II can't tell the difference between a fouled spark plug or a grounded plug wire. But it can tell you if a distributorless ignition or coil-on-plug system has an open or grounded coil.

When diagnosing misfires, it is important to use tools that allow you to actually see what is going on. A basic scan tool that reads serial data spit out by the PCM cannot tell you what the firing voltage is or what the ignition pattern looks like. Nor can it tell you if the serial data is accurate or correct. For that kind of information you need a scan tool, DVOM, graphing multimeter or oscilloscope that can look at sensor voltages directly, and/or a scan tool or scope that can also display primary and secondary ignition patterns. If the vehicle has a distributorless or coil-on-plug ignition system, you'll also need the appropriate inductive pickups to get a good ignition pattern signal from the coils.

A random misfire problem (code P0300) means the misfire is jumping around from cylinder to cylinder and that multiple cylinders are experiencing a misfire problem. This is usually due to a lean fuel condition which, in turn, is being caused by a vacuum leak, an air leak in the intake manifold, dirty injectors, low fuel pressure or an EGR valve that is stuck open and leaking exhaust into the intake manifold. If a random misfire code is also accompanied by a P0171 code (cylinder bank 1) or P0174 (cylinder bank 2), it will help isolate the lean fuel condition to one side of the engine or the other. If you find any codes in the P0400 to P0408 range, it indicates an EGR-related problem.


OBD II monitors the operation of the fuel delivery system anytime the vehicle is driven. This includes the fuel injectors, fuel pressure, the operation of the fuel pump and pump relay, oxygen sensors, feedback fuel control loop and fuel trim adjustments. If OBD II detects any problem here, it will log a code and turn on the lamp if the same problem occurs on two consecutive drive cycles.

Most vehicles use some type of short-term and long-term fuel trim adjustments to maintain the proper air/fuel ratio. OBD II keeps an eye on fuel trim and will turn on the MIL lamp if the system reaches either the minimum or maximum limit for fuel trim adjustment. Underlying problems here might include vacuum leaks, one or more dirty or leaky fuel injectors, a weak fuel pump, a bad or biased O2 sensor, etc.

Any time an O2 sensor problem is suspected, the sensors response should be checked to make sure (1) it is oscillating from rich to lean, (2) that it goes to maximum voltage output (0.9 v) when the fuel mixture is rich, (3) drops to minimum voltage output (0.1 v) when the mixture goes lean, and (4) responds quickly to changes in the fuel mixture. A sluggish O2 sensor can lag too far behind changes in the fuel mixture to allow the PCM to maintain the right air/fuel ratio. The first three items can be checked with a DVOM or graphing multimeter, but measuring response time requires a scope that can measure milliseconds. Some specs say the O2 sensor should respond to changes in the air/fuel mixture in 300 milliseconds or less, while others specify a response time of 100 to 125 milliseconds.

OBD II also monitors the evaporative emissions (EVAP) system, but only once during a drive cycle. The purpose here is to detect leaks that allow fuel vapors to escape into the atmosphere. OBD II does this by applying vacuum or pressure to the fuel tank, vapor lines and charcoal canister. If OBD II 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 up model year vehicles, which is the size of a pin prick), the EVAP system is malfunctioning and OBD II sets a code.

If you have a P0440 code indicating a fault in the EVAP system, 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 vapor leak, you may need a leak detector that uses smoke and/or dye.

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OBD II also keeps constant watch over the operation of all the sensors (the "comprehensive component" monitor) every time the vehicle is driven. A fault here such as an open, short or loss of signal will almost always set a code.

If you find a code for a particular sensor circuit, the next step is to figure out where the fault lies. Is it the sensor, a bad connector, the wiring or the PCM?

A quick way to see if a sensor is providing good input is to use your scan tool to see what the PCM is seeing. If the sensor data looks good and changes normally in response to changing rpm, throttle position, load or whatever, chances are the sensor is good but the system is being affected by something else.

To understand why a particular sensor code has been set if the data looks good, you have to know something about the diagnostic strategy the OBD II system uses to determine a good reading from a bad one. This is where things can get real complicated if a sensor or other component apparently checks out good, but continues to set a code.

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.

We heard one story about a 1997 Kia Sportage that kept turning on the MIL lamp and setting a code for the mass air flow (MAF) sensor. The MAF sensor was replaced but the code kept coming back. The MAP sensor was again replaced and the wiring checked, but the code continued to reappear. The real problem, it turned out, was actually a bad throttle position (TPS) sensor. On this particular vehicle, the OBD II system uses the TPS voltage setting to check the calibration of the MAF sensor. Because the TPS sensor

wasn't reading the proper voltage at idle, the OBD II system thought the throttle was open, but the MAF wasn't reading enough air flow. So it set a code for the MAF sensor when, in fact, a bad TPS reading was causing the glitch.

One time-saver here is to hook up a DVOM, graphing multimeter or oscilloscope to the sensor itself and compare the "real" sensor readings to what your scan tool displays using serial data from the PCM. If the values agree and are within normal ranges, you can assume the sensor, connectors, wiring and PCM are all working properly. But if the readings do not agree, there is a problem in the connector or wiring, or the PCM may be substituting bogus data for the real data.


Other OBD II monitors include the catalyst heater, catalytic converter efficiency, secondary AIR, O2 sensor heaters, EGR system, PCV system, thermostat and A/C system (where used). These are all "non-continuous" monitors and are not set until certain driving conditions have been met. The converter efficiency monitor, in particular, is a hard one to set and may require driving the vehicle at various speeds and loads so the OBD II system can get a good look at what is going on. for more information about OBD monitors and their ready status, Click Here.

The converter monitor compares the reading of the upstream and downstream O2 sensors to see if the converter is working efficiently. If you hook up a scope to both O2 sensors and compare the waveforms, the upstream O2 sensor should be fluctuating up and down from rich to lean with voltage readings going from 0.6 or more volts down to 0.3 volts or less. The downstream O2 sensor, on the other hand, should remain relatively flat. If the downstream O2 sensor is fluctuating in sync with the upstream O2 sensor, it means the converter is not doing much.

Most converters start out at about 99 percent efficiency when new, and quickly taper off to about 95 percent efficiency after 4,000 miles or so of driving. As long as efficiency does not drop off more than a few percentage points, the converter will do a good job of cleaning up the exhaust. But if efficiency drops much below 92 percent, it will usually turn on the MIL lamp.

With vehicles that meet the tougher LEV (Low Emission Vehicle) requirements, there is even less room for leeway. A drop in converter efficiency of only three percent can cause emissions to exceed federal limits by 150 percent. The LEV standard allows only 0.225 grams per mile of hydrocarbons, which is almost nothing.

If you have a vehicle with a converter efficiency code, don't assume the converter is bad and replace it until you have checked for air leaks at the exhaust manifold, head pipe and converter. If possible, you should also hook up a scope and compare the upstream and downstream O2 sensor readings to verify both are working properly.

One thing to keep in mind about non-continuous OBD II monitors is that they may not catch a problem until the vehicle has been driven several times and conditions are right to detect the fault. Consequently, any time you are troubleshooting an OBD II problem it is very important to use a scan tool that can tell you if all the monitor readiness flags have been set. If one or more monitors are not ready, the vehicle will have to be driven under varying speeds and loads until all the monitors are set. Then, and only then, will you get an accurate diagnosis from OBD II.

Some import vehicles have readiness issues when it comes to setting all the OBD II monitors. Turn the key off on a 1996 Subaru and it 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 Tercels and Paseos, the readiness flag for the EVAP monitor never will set, and no dealer fix is yet available. Other vehicles that may show 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 models.

Once all the monitors have been set, OBD II does an excellent job of detecting faults that affect emissions. In fact, OBD II has proven itself to be so effective that some states are now using a simple

OBD II plug-in emissions testing has replaced I/M 240 and ASM loaded mode dyno testing in most states, so why not put OBD II to work for you when it comes to diagnosing emission problems.

* Basic scan tool for reading and clearing OBD II generic fault codes.

* Enhanced scan tool for reading OEM "P1" codes and accessing other special OEM test functions.

* DVOM for measuring live sensor voltages, checking wiring continuity and grounds.

* Graphing Multimeter or Oscilloscope for displaying and analyzing sensor waveforms.

* Oscilloscope or enhanced scan tool for displaying ignition patterns.

OBD emission problems More Articles About Onboard Diagnostics:

All About Onboard Diagnostics II (OBD II)

OBD Monitor Not Ready

OBD II Plug-In Emissions Testing

Fixing Emission Failures

EVAP system

EGR system

PCV system

Advanced Diagnostics: Mode 06

Controller Area Network (CAN) Diagnostics

Exhaust Emissions Diagnosis

Emissions testing update

Driveability Diagnosis: Misfires

Troubleshooting Intermittent Engine Problems

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