Emissions Codes - Decoding the OBDII Diagnostic Code

When the "check engine" or "service engine soon" light comes on, it doesn't tell you what or where the trouble is, only that there's trouble. You've hooked up your scanner to the vehicle computer and retrieved the OBDII diagnostic code. What does it mean?

The "OBD" of OBDII - sometimes written OBD-II or just plain OBD2 - stands for On-Board Diagnostics. The "II" is the next generation of emissions standards and codes for all vehicles sold in the U.S. from 1996 to the present, domestic and imports.

The OBD2 system is primarily for emissions control. Its basic components are the catalytic converter and strategically-placed oxygen sensors. These as well as everything in the vehicle having to do with engine performance and emissions control are continuously monitored by the vehicle's on-board computer system.

The "check engine" or "service engine soon" light is the signal that there is a problem with the vehicle's emissions. The computer has assigned a trouble code to the problem and turned on the trouble light - technically called the Malfunction Indicator Lamp (MIL).

When you retrieve the information from the computer via an OBDII diagnostic scanner, it doesn't tell you the problem directly, such as "timing too slow" or "misfire in cylinder number four". What you get is a five-digit alpha-numeric code such as "P0304".

The first digit is a letter corresponding to the main system causing the trouble code:

B = Body
C = Chassis
P = Powertrain
U = Network

The next four digits are all numbers. There is never a letter "O" in the OBDII diagnostic code. It is the numeric digit zero ("0").

The second digit corresponds to the type of code, whether it is the generic standard applying to all OBDII-compliant vehicles, or a manufacturer-specific code.

0 = Generic codes
1 = Manufacturer-specific codes
2 = Includes both generic and manufacturer-specific codes
3 = Includes both generic and manufacturer-specific codes

The third number corresponds to the sub-system where the problem lies.

1 = Fuel and Air Metering
2 = Fuel and Air Metering (injector circuit malfunction only)
3 = Ignition System or Misfire
4 = Auxiliary Emission Control System
5 = Vehicle Speed Control and Idle Control System
6 = Computer Output Circuits
7 = Transmission
8 = Transmission

So our example trouble code P0304 indicates a problem in the powertrain. It is a generic code for trouble with the ignition system or a misfire.

The fourth and fifth numbers of the code correspond to the section of the system causing the trouble. The list of all these sections is long, but you can see how the final "04" in our example points to a misfire in cylinder number four.

Most OBDII diagnostic scanners will come with a code library of specific code meanings. A list may also be found on-line by Googling "obd2 codes list".

You will sometimes find a reference to "Bank 1" or "Bank 2" in the code explanation. These banks are generally meant for "V-type" engines. Bank 1 refers to the side of the engine with the number 1 cylinder (odd-numbered cylinders). Bank 2 refers to the side of the engine with the number 2 cylinder (even-numbered cylinders).

A misfire is a "one-trip" or "type A" problem that by itself will not turn on the MIL unless it is severe enough to damage the catalytic converter. A severe misfire will not only turn on the MIL but will signal it to flash at one-second intervals.

Other problems are usually "two-trip" or "type B" problems. When the computer first detects a two-trip problem, it stores the trouble code as "pending". If on the next driving trip the problem has passed, the pending code is erased. But if the problem is still there on consecutive trips, the computer will turn on the MIL, alerting the driver to a problem.

The MIL can be persistent. Once on, it will stay on until the problem is resolved for three driving trips. However, though the light may go out, the codes remain in the computer memory for 40-80 trips depending on the problem.

Trouble codes remaining in the computer memory will cause a failed emissions inspection whether the "check engine" light is on or not.

Of course, your diagnostic scanner can turn off the MIL and erase the codes from the computer's memory. This will not do much good, however, if the problem recurs after two driving trips and regenerates the trouble codes. The information from the scanner should be used to locate and fix the problem, not just turn off the MIL and erase the codes.

Emissions Codes - Decoding the OBDII Diagnostic Code

OBDII Vehicles

Emissions Codes - Decoding the OBDII Diagnostic Code

Emissions Codes - Decoding the OBDII Diagnostic Code
Emissions Codes - Decoding the OBDII Diagnostic Code

OBDII Vehicles

Emissions Codes - Decoding the OBDII Diagnostic Code

Emissions Codes - Decoding the OBDII Diagnostic Code

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DM-200 OBDII Wins 2 Major SEMA Awards

DM-200 OBDII Wins 2 Major SEMA Awards

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DM-200 OBDII Wins 2 Major SEMA Awards
Paul Lowchareonkul, President and CEO of PLX Devices Inc. receives 2 major SEMA 07 awards for the DM-200 OBDII for Best New Interrior Accessory and Best New ...
DM-200 OBDII Wins 2 Major SEMA Awards

DM-200 OBDII Wins 2 Major SEMA Awards

DM-200 OBDII Wins 2 Major SEMA Awards



DM-200 OBDII Wins 2 Major SEMA Awards

What Actually Happens During An OBDII Emissions Inspection

I have seen many people talk about OBD-II emissions tests on various web forums. Often with a puzzled: "I failed readiness!" or "Do I have to do an OBD-II test on my 1994 vehicle?" Since OBD-II has replaced or will replace sniffer tests for 96 and newer vehicles in most states, I thought I'd dump some introductory information about OBD-II emissions tests for people. I don't get into the details about OBD-II, the various protocols, or how to tune an OBD-II vehicle here. Only what to expect in the emissions process.

First, some really quick background: California required emissions control systems (Catalytic converters) on cars sold in California starting the 1966 model year. This was adopted across the US in 1968, and eventually became the Clean Air Act of 1970 which required emissions standards that were hard for manufacturers to meet. Ultimately, car makers found that switching to electronically controlled engine management allowed them to meet these demands. These electronics became more sophisticated and more standardized as time went on, and now we have a standard protocol (OBD-II) that all these computers adhere to.

Today: Many States, California, Massachusetts, New York, Illinois, Washington, New Hampshire, and on and on have mandatory emissions inspections in some or all counties. Typically this is used as part of the car registration and renewal process. Up until 2000, this just meant popping the car on a dyno, sticking a sniffer in the tailpipe and measuring what percentage of the air coming out of the vehicle is clean. However, in 2000 the EPA started pushing an "OBD-II emissions test" and many states adopted it, or are in the process of adopting it.

An OBD-II test consists of the emissions computer plugging into the OBD-ii port on the car (usually under the dash or hidden behind the cigarette lighter) and asking the car's computer whether the emissions equipment on the vehicle is working within the required efficiency limits. This is advantageous over the sniffer because its much faster, more consistent (in theory) and harder for those of us that like fast cars to just jury rig a huge catalytic converter the morning before inspection in order to pass with flying colors in our fire-spitting vicious mobiles

The way OBD-II tests actually work, is pretty simple. The car's ECU waits for a set of conditions that are representative of normal driving and then checks the values of a few sensors to make sure that under that condition the equipment is working exactly as it should. Typically it examines systems such as the catalytic converter, the Exhaust Gas Recirculation (EGR), and the evaporative emissions. It stores these results internally and the computer just queries for these results at inspection time. The inspection machine also checks the computer to see if any error codes are set, and if the check engine light (also know as a Malfunction Indicator Light (MIL)) is set. If the tester determines that the emissions equipment is in order, there are no internal errors which the car didn't see fit to tell you about and the check engine light is not on, the car is given a pass without needing any further emissions related tests.

For the car to definitively say that the vehicle is in working order it needs to examine its systems for a set number of "driving cycles". Driving cycles are defined differently for different car companies. For some a driving cycle is the time between when the key is first turned on, until it is turned off. For others, it is any 10 minute period of non-idle driving, etc. This is where the concept of readiness is introduced. When the vehicle's ECU is reset for any reason (replacement, low battery, faulty sensors, etc) it resets the 'readiness monitors' inside the ECU. These are a number of flags that determine whether or not the car is prepared to validate the condition of the emissions systems. The vehicle then must be driven for a preset number of drive cycles to give the ECU all the information it needs to get out of the unready state and actually test the emissions systems.

If a car is inspected while it is not ready, the car will return 'not ready' to the inspection computer. On cars sold between 1996 and 2000 an emissions test is considered a fail if any 2 of the emissions systems return not ready. On later vehicles, you are only allowed one not ready. The typical remedy for failing readiness is to drive the car around for up to 500 miles and trying the test again. After that, if you still fail, a dealer or somebody with factory diagnosis equipment is required to force the vehicle to run the emissions tests regardless of readiness.

Modified Cars: OBD-II represents a problem for cars with modified emissions systems. In the past, you could just keep the factory exhaust in a corner of your garage and slap it on for inspection, but now the electric tattle-tails will still catch you. You can use a datalogger, such as the car-code logger: http://www.obd-2.com/ to check whether your car has passed readiness and thinks it is in an emissions happy state. This is of course for use with off-road only vehicles only.

A common problem for modified cars is secondary o2 sensors. Part of the OBD-II tests involves determining the efficiency of catalytic converter (or pre-cat catalytic converters) by checking the value of the oxygen sensors which are located after the cat. If your off-road vehicle does not have this equipment the vehicle will likely fail. A common solution for this problem is to install an oxygen sensor simulator which gives the car's ECU a false signal that mimics what it is expecting to see so it thinks everything is in order. A quick internet search for "o2 simulator" will return further information on this topic.

Finally, remember that you will need to drive the car a lot of miles to get readiness to pass. In some extreme cases over 1000 miles is required. So if you are planning on taking the car down for a few months in order to do a long project, and your time for inspection is coming up you are better off getting the vehicle inspected before taking it off the road and resetting the ECU.

What Actually Happens During An OBDII Emissions Inspection

OBDII Vehicles

What Actually Happens During An OBDII Emissions Inspection

What Actually Happens During An OBDII Emissions Inspection
What Actually Happens During An OBDII Emissions Inspection

OBDII Vehicles

What Actually Happens During An OBDII Emissions Inspection

What Actually Happens During An OBDII Emissions Inspection

BestCustomerReview

What Actually Happens During An OBDII Emissions Inspection

ItemFeatures

What Actually Happens During An OBDII Emissions Inspection

What Actually Happens During An OBDII Emissions Inspection
What Actually Happens During An OBDII Emissions Inspection



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What Actually Happens During An OBDII Emissions Inspection

What Actually Happens During An OBDII Emissions Inspection Specifications

ItemSpecifications
What Actually Happens During An OBDII Emissions Inspection

What Actually Happens During An OBDII Emissions Inspection

*** Product Information and Prices Stored: OBDII Vehicles
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What Actually Happens During An OBDII Emissions Inspection