If your "Check Engine Light" is on, it means your vehicle has one or more OBD II Trouble Codes. The "Malfunction Indicator Lamp" (or MIL ) comes on to warn you when the Onboard Diagnostic System (OBD II) detects a problem that cold cause your vehicle's emissions to exceed 1.5 times the legal limit.
Types of Trouble Codes
A trouble code is an alphanumeric value that corresponds to a particular type of fault. The list was originally created by the Society of Automotive Engineers (SAE) for use by all vehicle manufacturers who have to comply with OBD II emissions regulations in the U.S. The same list of basic codes have also been adopted by European and Asian auto makers. The list of trouble codes is subdivided into four basic categories:
These codes are further divided into two major groups:
Click Here for info on the latest SAE J2012 Diagnostic Trouble Code Definitions originally published 3-07-2013, updated 12-13-2016
How Trouble Codes Are Set
The OBD II system monitors all of the emissions-related functions in your vehicle, including the ignition and fuel systems, engine misfires, catalytic converter, evaporative emissions (EVAP) control system and various other emission control systems (EGR, PCV, etc.). Many of these functions are monitored anytime the vehicle is driven (continuous monitors) while others are only monitored when certain operating or driving conditions have been met. The OBD II system may run certain self-tests (such as checking for fuel vapor leaks) periodically.
If a component or system is not operating normally within acceptable limits or fails a self-test, the OBD II system records one or more "Diagnostic Trouble Codes" (DTCs) that correspond to the fault, and turns on your Check Engine Light to let you know there's a problem.
In most cases, the Check Engine Light will come on and remain on until the code has been cleared or the problem has been fixed. In some cases, such as engine misfire, the light may flash on and off while the fault is occurring. Or, the light may go out after a period of time if the fault is no longer present.NOTE:If your Check Engine Light is on and there are trouble codes in your computer, your vehicle will NOT pass an OBD II emissions test. The light must be OFF and there must be no codes in memory to pass an OBD II emissions test. In addition, all of the OBD II self-monitors that check for faults must have run and completed without finding any further problems to pass the test.
Check Engine Light Repair
The Check Engine Light only tells you some kind of emissions-related fault has been detected. The light does NOT tell you anything about the problem that triggered the code, whether or not the code is serious or minor, what you should do next, or what might happen if you choose to ignore the light (which many people do!).
Also, the Check Engine Light is NOT a general purpose warning light. Serious problems such as engine overheating or loss of oil pressure will generally NOT turn on the Check Engine Light, but they should turn on other warning lights.
WARNING:If any other warning lights are on such as the coolant temperature, oil pressure or charging warning lights, or your engine is running abnormally (sputtering, misfiring, stalling, losing power) or is making any unusual noises, odors or is smoking, STOP! The problem may be serious and may result in additional problems or expensive engine damage if you continue driving. See Common Car Problems for additional help.
How to Read Trouble Codes
The only way to know why your Check Engine Light is on is to plug a scan tool into your vehicle's OBD II diagnostic connector. The scan tool will then display the trouble code or codes that have been set. Most scan tools will display the trouble code and a one line description or definition of the code. Some basic code readers may only give you the number but no definition.
NOTE: You cannot diagnose a Check Engine Light without a scan tool! (although you can access trouble codes on some vehicles via the Driver Information Display if you know the procedure for doing so).
If you don't have a scan tool, ,many auto parts stores and even some repair shop will do a FREE plug in diagnosis to read out the codes. Most repair shops and dealerships, however, usually charge a flat diagnostic fee of $75 to $100 or more to perform a scan tool diagnostic check to read out trouble codes.
If You Have a Trouble Code...
Write down the code number(s). DO NOT erase the codes until you have written this information down because you will need it later to diagnose the problem.
The code(s) will tell you which sensors or systems are experiencing a problem. If the scan tool does not provide a definition, you will have to look up the code definition where ever you can find it (online, in a shop manual.
How to Clear Trouble Codes
The best (safest)way to erase trouble codes is to use a scan tool. The tool communicates with the vehicle computer and tells it to erase the codes. It does not alter any other learned settings that should be retained for normal vehicle operation.
The worst way to clear codes is to disconnect the battery. In most older vehicles (those made before 1996), disconnecting the battery even for a few seconds, or removing the power fuse for the engine computer (PCM) will erase the codes, along with all of the other learned settings the computer has stored over time.
NOTE: Clearing the codes will not keep the Check Engine light off. Sooner or later, the codes will reset and the Check Engine light will be back on UNLESS the problem(s) that caused the code(s) to set in the first place has been diagnosed and repaired.
Caution: Disconnecting the battery to clear codes on many 2006 and newer vehicles is NOT a good idea because it may cause the PCM to forget critical settings. This may have an adverse effect on the operation of the transmission, climate control system and other functions. This, in turn, may require special "relearn" procedures using a factory scan tool or professional grade scan tool to restore proper operation of the affected systems.
Note: Clearing codes with a scan tool, or disconnecting the battery will also reset all of the OBD system monitors back to zero, meaning your vehicle won't be accepted for an OBD plug-in emissions inspection until all of the monitors have run and completed (system ready).
Trouble Code Help:
Windows software Training & Reference Guide for your PC or laptop.
Larry's Guide to Check Engine Light Diagnostics (PDF ebook)
Click Here for Most Common Trouble Codes (and what causes them)
Click Here for Oxygen Sensor Code Diagnosis
Click Here for Mass Airflow (MAF) Sensor Code Diagnosis
Click Here for Coolant Sensor Code Diagnosis
Click Here for Engine Misfire Code Diagnosis
Click Here for Crankshaft Position (CKP) or Camshaft Position (CMP) Sensor Diagnosis
Click Here for Exhaust Gas Recirculation (EGR) Code Diagnosis
Click here for Evaporative Emission Control (EVAP)Code Diagnosis
Click here for Loose Gas Cap Code diagnosis
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