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Related Pages: Glossary | Diagnostic Trouble Codes (DTC)
Contents: Introduction | Code Reader/Scanner | Diagnostics | Books
The service industry calls the Check Engine light on your dash an "MIL" or Malfunction Indicator Light. It shows three different types of signals. Occasional flashes show momentary malfunctions. It stays on if the problem is of a more serious nature, affecting the emissions output or safety of the vehicle. A constantly flashing MIL is a sign of a major problem which can cause serious damage if the engine is not stopped immediately. In all cases a "freeze frame" of all sensor readings at the time is recorded in the central computer of the vehicle.
Most of the causes for the Check Engine light are emission control problems checks specified by the California Air Resources Board (CARB) and the Environmental Protection Agency (EPA) and will not damage your car if you wait until you get home to have them fixed. However, you should check your dashboard gauges and lights for indications of low oil pressure or overheating. These conditions mean you should pull over and shut off the engine as soon as you can find a safe place to do so.
A blinking light indicates a problem that needs immediate attention. In late-model cars, a blinking light usually indicates an engine misfire so severe that unburned fuel is being dumped into the exhaust system, where it can quickly damage the catalytic converter. If that happens, you should reduce power and have the car or truck looked at as soon as possible.
In some extreme cases, the car's computer may reduce power for you, as it tries to limit the risk of damage. This does not necessarily mean you car is going to stop running.
In a Mar. 2008 article "What to do if the "check engine" light goes on" ConsumerReports says:
"The check engine light ("service engine soon" or "check powertrain") can come on from a variety of conditions from a loose fuel cap (gas cap) too a serious problem which should be checked ASAP. It doesn't mean you have to pull the car over to the side of the road and call a tow truck. It does mean you should get the car checked out as soon as possible," says Dave Cappert of the National Institute for Automotive Service Excellence, a Virginia-based organization that tests and certifies auto technicians.
Ignore the warning, and you could end up damaging expensive components. It also can be a sign that your car is getting poor fuel economy and emitting higher levels of pollutants.
Some cars have a Generator light on the dash which will set trouble codes also.
Today's automotive computers often try to compensate when there's a problem; so you may not notice deterioration in performance, In some extreme cases, the car's computer may reduce power for you, as it tries to limit the risk of damage.
(Note: The check engine light in newer cars does not come on as a service interval reminder, but in my old Mazda it came on at 30,000 miles because that was when the oxygen sensor needed to be replaced.)
Volkswagen was one of the first carmakers to install an on-board diagnostic (OBD) system starting in June 1971. They currently have more checks (over 1,000 trouble codes) than most other manufacturers. Many of these checks have been added to 2003 and later models.
The Onboard Diagnostics (OBD) defines a communications protocol and a standard connector to acquire data (Diagnostic Trouble Codes - DTCs) from passenger cars to help monitor/inspect vehicle emissions. OBD was established by the California Air Resources Board (CARB) and the Environmental Protection Agency (EPA). Detection of a problem by the Powertrain Control Module (PCM), a computer, turns on the Malfunction Indicator Lamp (MIL) (Check Engine Light) and sets a code in the computer memory which can be read by code scanners connected to a plug usually at the bottom of the dash board.
The origins of OBDII actually date back to 1982 in California, when the California Air Resources Board (CARB) began developing regulations that would require all vehicles sold in that state starting in 1988 to have an onboard diagnostic system to detect emission failures.
OBD-I was implemented on 1982-1995 GM 12 pin ALDL vehicles, 1983-1995 Ford EEC-IV vehicles, 1983-1995 Chrysler SCI vehicles, and 1991-1995 Jeeps.
Some 1994-95 vehicles are OBD-II compliant. See table below.
Onboard Diagnostics II (OBD II or OBD2, OBD-II) is a second-generation emissions diagnostic system required by U.S. EPA on all 1996 and newer vehicles (though some 1994 and 1995 model year vehicles were equipped with early versions of the system) OBDII will light a lamp called a MIL (malfunction indicator lamp), also known as the "check engine" light on the dash. The Malfunction Indicator Lamp (MIL) will illuminate anytime a vehicle's emissions exceed 1.5 times the federal test procedure (FTP) standards for that model year of vehicle. Some codes (Type B) require the condition to occur twice before the MIL comes on. Type A diagnostic trouble codes are the most serious and will trigger the MIL lamp with only one occurrence. When a Type A code is set, the OBDII system also stores a history code,
(The light comes on briefly when the ignition is frist turned on as a bulb check)
A scan-tool is conected to the OBDII connector to probe for OBDII data as defined by the SAE J1979 standard. The OBD-II standard allows for multiple electrical interfaces, which complicates the hardware used to interface with the vehicle.
Generic (Universal, SAE) DTCs typically begin with 'P0' or 'P2' ('P' referring to Powertrain), followed by a 3-digit number (i.e., P0420 or P2420)., manufacturer specific codes format is "P1xxx".
Some manufactures also have their own set of 2 digit codes in addition to the SAE codes.
I saw one posting on the web which stated: "Some codes that are kept secret so that certain diagnosis can not be made without the equipment which is reserved for authorized dealers only."
See the DTC page
Emergency Brake may have to be set to read.
The Powertrain Control Module (PCM), monitors Emissions, engine and automatic transmission and produces Diagnostic Trouble Codes (DTC) according to the OBD standard. The check engine light comes on if a problem is detected.
Chrysler - Some 95 and later OBDII vehicles, you may be able to retrieve OBDII codes with this procedure, if equipped with digital dash. (Works in '97 Jeep wrangler but not '98 and newer.)
The "Single board engine controller (SBEC)" was an earlier version of a PCM.
Computer Code Readers & Scanners:
Diagnostic Link Connector (DLC)
Note: There are SEE: obdii.com/connector.html .
Throttle Actuator Malfunction (P1580), Mass Airflow Circuit (P0103): These codes may be set due to the driver attempting to start a VW with a low or dead battery.
A drive cycle or trip, by the way, is not just an ignition cycle, but a warm-up cycle. It is defined as starting the engine and driving the vehicle long enough to raise the coolant temperature at least 40 degrees F (if the startup temperature is less than 160 degrees F). See Drive Cycle below.
There are sometimes bugs in the computer causing the MIL light to come on unnecessairly. You may want to check with your dealer to see if therer are any Service Bulletin's regarding this. They can upload a new program to the computer (reflash the EEPROM).
For example, if the fuel cap (gas cap) isn't tight or the tank is filled while the key is on or the engine is idling, it can trigger a variety of codes (e.g. P0440 - Evaporative Emission Control System Malfunction, P0457 - Evaporative Emission Control System Leak Detected, P1486 - LDP has detected a pinched hose in the
evaporative hose system) causing the MIL light to come on. (GM has not issued a technical service bulletin on the problem with filling the tank with the key on, but is advising its dealers and fleet customers to reflash the EEPROM with revised OBDII programming that waits to check the evaporative emissions system until the vehicle is in motion.)
A freeze frame stores the code along with a snapshot of vehicle operating parameters to aid in identifying the problem. This block of values is referred to as Freeze Frame Data and may include engine rpm, vehicle speed, air flow, engine load, fuel pressure, fuel trim value, engine coolant temperature, intake manifold pressure and open or closed loop status.- The MIL may be turned off and the code erased after some number (could be up to 40) of successful driving cycles.
An OBD Drive Cycle:
The OBDII drive cycle begins with a cold start (coolant temperature below 122 degrees F and the coolant and air temperature sensors within 11 degrees of one another).
NOTE: The ignition key must not be on prior to the cold start otherwise the heated oxygen sensor diagnostic may not run.
To find out if a 1994 or 1995 vehicle is OBD II compliant, check the following: The Vehicle Emissions Control Information (VECI) Label. This label is located under the hood or by the radiator of most vehicles. If the vehicle is OBD II compliant, the label will state OBD II Certified on the upper right-hand side of this label. Government Regulations require that all OBD II compliant vehicles must have a common sixteen-pin Data Link Connector (DLC). NOTE: Some 1994 and 1995 vehicles have 16-pin connectors but are not OBD II compliant, only the ones where the Vehicle Emissions Control Label states that, they are OBD II Certified.
As of 2008 ome states were considering an advanced OBD system that would allow them to do away with emissions testing. If the "check engine" light comes on, the system automatically would send a remote signal to state officials, who would contact motorists who don't have the problem corrected within a reasonable amount of time. Privacy advocates are criticizing the idea as being too intrusive.
www.obd-codes.com has tips for fixes
Diagnostic Code Readers under products.
Diagnostic Trouble Codes (DTC)
National OBD Clearinghouse at Web er State Univ.
parameter identification data (PID) - Glossary
Technical Library, "OBD II Past, Present and Future",
Sensor Guide CD ($40 at Automotive Diagnostic Software Products)
Chrysler Code Retrieval at BAT Auto Technical (BAT= Bruce, Alan and Tony)
OBD II Resources and Information at OBDIIcsu.com
The ABC's of OBD II at TomorrowsTechnician.com
Erase the Adaptive Memory in the TJ PCM
On Board Diagnostic Systems at innovationhouse.com
OBD2.pdf at VolvoSpeed.com
FAQs at actron
Diagnostic Trouble Codes (DTCs) for OBD II (OBD-2) equipped GM vehicles
OBDII Research Center at the National Center for Vehicle Emissions Control and Safety at Colorado State Univ.Z