OBD-II codes consist of a number of parts. Here is a sample OBD2 code:

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1 1 What Is OBD? OBD stands for On-Board Diagnostics. OBD-II (OBD two) is the latest industry standard, which was introduced on 1996 vehicle models. It provides monitoring of nearly all engine controls, and also some other parts of the vehicle (chassis, body, etc.). The most useful part of OBD to most drivers is the ability to read information from the vehicle. You can read trouble codes, and even monitor real-time statistics such as RPM, vehicle speed, etc. There are a variety of different devices on the market ranging from basic code scanners to advanced displays that calculate MPG as you drive. 2 OBD-II Codes Explained OBD-II codes consist of a number of parts. Here is a sample OBD2 code: P0171 Here is a breakdown of what each digit of the code means: First Character - System The first character identifies the system related to the trouble code. P = Powertrain B = Body C = Chassis U = Undefined Second Digit - Code Type The second digit identifies whether the code is a generic code (same on all OBD-II equipped vehicles), or a manufacturer specific code. 0 = Generic (this is the digit zero -- not the letter "O") 1 = Enhanced (manufacturer specific) Third Digit - Sub-System The third digit denotes the type of sub-system that pertains to the code

2 1 = Emission Management (Fuel or Air) 2 = Injector Circuit (Fuel or Air) 3 = Ignition or Misfire 4 = Emission Control 5 = Vehicle Speed & Idle Control 6 = Computer & Output Circuit 7 = Transmission 8 = Transmission 9 = SAE Reserved 0 = SAE Reserved Fourth and Fifth Digits These digits, along with the others, are variable, and relate to a particular problem. For example, a P0171 code means P System Too Lean (Bank 1). a) Do I Have An OBD-II Vehicle? Some 1994 and 1995 model year vehicles do have OBD-II. If your vehicle is 1996 or newer, U.S. federal law requires that all vehicles (cars and light trucks) sold in the United States must be OBD-II compliant. This includes all American vehicles (Ford, Dodge, GM, Chrysler, etc.) and import Asian (Toyota, Honda, Nissan, etc.) vehicles. The sure-fire way to find out if your vehicle is OBD-II compliant is to look for the standard DLC (data link connector) that all OBD II vehicles have. It will be located underneath the driver's side of the dash: Now, if your vehicle is 1996 or newer, you definitely have OBD-II. If your vehicle is older than 1996, the presence of the DLC may or may not indicate that you have OBD-II. To verify, look under the hood for a VECI label. The VECI label will indicate whether or not the vehicle has OBD II.

3 b) How Do I Reset "Check Engine" Light? After you've properly diagnosed and fixed the problem that triggered the OBD trouble code, you can reset or clear the Check Engine / Service Engine Soon light by one of two methods. Note: The code (i.e. CEL / SES light) will usually go away after a certain number of driving cycles, perhaps a few days. You can choose to wait it out or follow one of these methods. The cheap, easy, free method Simply disconnect your battery, turn on the headlight switch for about five to ten minutes, and re-connect the battery. This is pretty simple, but will likely result in you losing your radio presets and perhaps some other small things like that. Using a scan tool Most if not all scan tools have an option to clear or reset codes without the use of any tools or touching the battery. Consult your tool's manual for information on how to do it with your particular model. 3 What OBD-II Protocols Are There? There are five different OBD-II communication protocols: J1850 PWM J1850 VPW ISO9141 ISO14230 (also known as Keyword Protocol 2000) CAN (ISO15765/SAEJ2480) What do these mean to me? Generally you will need to know which protocol you have when you buy a code reader or auto scan tool. Sometimes manufacturers label the products for Ford, GM, etc. so it may be easy. However here is the quick way to tell what you have.

4 To tell which protocol you have, see the table below: Standard Pin 2 Pin 7 Pin 10 Pin 15 Notes PWM must have - must have - The connector must have pins 2,4,5,10, and 16 VPW must have The connector must have pins 2, 4, 5, and 16, but not 10. The connector must have ISO - must have - pins 4, 5, 7, and 16. Pin may have 15 may or may not be present. 4 OBD-II / Automotive Acronyms & Definitions This is the place where we tell you what various acronyms stand for, and what some other technical words mean. A through G ABS - Anti-lock Braking System A/C - Air Conditioning AIR - Secondary Air Injection System BARO - Barometric Pressure CKP - Crankshaft Position Sensor CMP - Camshaft Position Sensor CEL - Check Engine Light - same as Malfunction Indicator Lamp (MIL) or Service Engine Soon CO - Carbon Monoxide

5 CO2 - Carbon Dioxide DEPS - Digital Engine Position Sensor DLC - Data Link Connector DIS - Distributor-less Ignition System DTC - Diagnostic Trouble Code ECT - Engine Coolant Temperature EEPROM - Electrically Eraseable Programmable Read Only Memory EGR - EGR stands for Exhaust Gas Recirculation. It is part of the vehicle emissions system, and is used to reduce combustion temperature and pressure to control Oxides of Nitrogen. There are generally three parts to the EGR system: the EGR valve, an actuator solenoid, and a differential pressure sensor (DPF). These things work together to deliver the correct amount of recirculation based on engine temperature, load, etc. EVAP - Evaporative Emissions System FTP - Federal Test Procedure H through M H2O - Water HO2S - Heated Oxygen Sensor HC - Hydrocarbon HVS - High Voltage Switch IAC - Idle Air Controller IAT - Intake Air Temperature IC - Ignition Control I/M - Inspection/Maintenance KS - Knock Sensor MAF - Mass Air Flow (Sensor) MAP - Manifold Absolute Pressure MIL - Malfunction Indicator Lamp - same as Check Engine Light or Service Engine Soon MR - Magneto-Resistive MSC - Mass Storage Cartridge N through T NOx - Oxides of Nitrogen OBD - OnBoard Diagnostics OBD-I - On-Board Diagnostics Generation One OBD-II - On-Board Diagnostics Generation Two OBPA - Off-Board Programming Adapter O2S - Oxygen Sensor

6 OSC - Oxygen Storage Capacity PCM - Powertrain Control Module SES - Service Engine Soon - same as Malfunction Indicator Lamp (MIL) or Check Engine Light (CEL) SPS - Service Programming System TCC - Torque Converter Clutch TDC - Top Dead Center TP - Throttle Position TPS - Throttle Position Sensor TWC - Three Way Catalyst U through Z UART - Universal Asychronious Receive & Transmit VCM - Vehicle Control Module VIM - Vehicle Interface Module VPW - Variable Pulse Width VSS - Vehicle Speed Sensor WOT - Wide Open Throttle 5 What Is Bank 1 and Bank 2? There are a number of OBD-II trouble codes that refer to either Bank 1 or Bank 2. There are conflicting messages on Bank 1 versus Bank 2, so we're trying to set the record straight. Quite simply, Bank 1 refers to the side of the engine that has the #1 cylinder. Bank 2 is the opposite side of the engine. It's not correct to simply state that driver's side is always cylinder #1 or vice versa. Even on cars with a tranverse mounted engine, bank #1 is referring to downstream from the side of the engine with cylinder #1. 6 What Are Fuel Trims? There are often questions asked about fuel trims. Some may have noticed the fuel trim PIDS (parameter I.D.'s) on their scan tool datastream and wonder what they're for. So, what are fuel trims and what do they do? A proper understanding of fuel trims can result in a quicker diagnosis and alert you to future problems with your vehicle.

7 Basically, fuel trims are the percentage of change in fuel over time. For the engine to operate properly, the air/fuel ratio needs to stay within a small window of 14.7:1. It has to remain in this zone under all the various conditions an engine encounters every day: cold start-up, idling in heavy traffic, cruising down the highway, etc. So, the engine computer is trying to maintain this proper air/fuel ratio by fine-tuning the amount of fuel going into the engine. As it adds or takes away fuel, the oxygen sensors monitor how much oxygen is in the exhaust and respond by telling the computer. The oxygen sensors could be likened to the computer's "eyes" that watch the mixture of oxygen in the exhaust. The computer monitors this input from the heated oxygen sensor(s) continuously when in closed-loop. If the O2 sensors inform the engine computer that the exhaust mixture is lean, the computer adds fuel by lengthening injector pulse, or "on-time", to compensate. Conversely, if the O2 sensors inform the computer that the exhaust is rich, the computer shortens injector pulse, adding less fuel to compensate in order to bring the rich condition down. This change in fuel being added or taken away is called Fuel Trim. Really, the oxygen sensors are what drive the fuel trim readings. Changes in o2 sensor voltages cause a direct change in fuel. The short term fuel trim (STFT) refers to immediate changes in fuel occurring several times per second. The long term fuel trims (LTFT) are driven by the short term fuel trims. LTFT refers to changes in STFT but averaged over a longer period of time. A negative fuel trim percentage indicates a taking away of fuel while a positive percentage indicates an adding of fuel. Think of it like this: You're driving from the beach at sea level into the mountains. On a short-term basis, you may go up and down several hundred feet at a time as you ascend the mountainous terrain. But, over the long-term, you are actually ascending from low to high altitude several thousand feet, gradually. It's similar with short and long-term fuel trims. STFT are immediate ups and downs in fuel, while LTFT are what is occurring over a longer period. A normal STFT reading will generally fluctuate between negative and positive single digits 2-3 times per second. Usually they'll stay around positive or negative 5%, but they may occasionally go up towards 8 or 9% depending on the efficiency of the engine, age of the components, and other factors. A normal long term fuel trim reading will appear to stay the same, giving a long term average of fuel added. It, too, should be close to zero, positive or negative single digits under normal circumstances. It will fluctuate much slower, possibly appearing static.

8 Normal STFT Reading If you experience ST or LT fuel trims that are in the double digits positive or negative this would indicate a abnormal adding or lessening of fuel. This could be due to leaking fuel injectors, an unmetered air leak or something similar. For example, if the O2 sensors are reading lean due to, say, a vacuum leak, the engine computer will compensate by adding fuel. The STFT will start to climb immediately to reflect the computer adding fuel. While the computer is adding fuel it still watches the O2 sensors until the O2 sensors are indicating that the lean condition no longer exists and proper Air:Fuel ratio is met. The computer will maintain this heightened addition of fuel until the leak is corrected. The scan tool will show STFT readings that are in the positive double digits, indicating that the computer has been adding too much fuel for normal operation. After a while the LTFT will also reflect this relative addition in fuel. Now if the vacuum leak is bad enough, the computer will not be capable of adding enough fuel to achieve proper air/fuel ratio. It will add fuel until the STFT reaches it's max calibration, usually 25%. Then a code is set in the computer indicating that the engine is running lean (P0171 or P0174) and the STFTs have maxed out. The opposite would be true if an engine was running rich due to a fuel leak (P0172, P0175). Keep in mind that the computer has no idea if the O2 sensor is reading properly in some cases. For example if an O2 sensor was sticking rich, the computer would assume it was reading correctly and begin taking away fuel to compensate. This is referred to as a "false rich" condition. The computer would be leaning the engine and setting a possible P0172, P0175. The codes would indicate the engine was running rich but it ACTUALLY is running lean. If you use only the false rich codes to diagnose and don't observe all the fuel trim and O2 sensor data, you may make a false diagnosis. Also, each bank has it's own fuel trim reading. If your engine is a 4 cylinder, then it has only one bank, Bank 1. On a V-style engine you can isolate which bank is running rich or lean by watching that bank's fuel trims. If one bank is running properly, and another isn't you can narrow down a developing problem to one side of the engine or the other. Using your new knowledge of fuel trims, you can take the guesswork out of interpreting your fuel system's condition and possibly save some hard-earned money.