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Here is a CUSTOMER'S MIL FACT SHEET . This should help answer your questions.
WHAT IS THE CHECK ENGINE/SERVICE ENGINE SOON LIGHT?
The check engine/service engine soon light notifies the driver of a possible emissions related problem. It is also known as a Malfunction Indicator Lamp (MIL).
WHY DO WE NEED A MIL?
The MIL is part of the on-board diagnostic (OBD) system required by EPA regulations on light-duty vehicles and light-duty trucks to monitor the vehicle's emission related components. The MIL alerts the driver of a potential problem; IT IS AN EARLY WARNING SYSTEM.
WHAT IS OBD (On Board Diagnostics)?
OBD was designed to monitor on-board computers which control many of the engine's systems, such as fuel and ignition. OBD systems were first developed in the early 1980's as cars became more computerized. The system is made up of sensors and actuators and diagnostic software. OBDII is a complex computer package installed on 1996 and newer cars and light trucks that predicts emissions and notifies the driver if there is a problem by turning on the MIL. OBDII is a self diagnostic system where fault codes indicate which components of the system may be malfunctioning by storing the codes in the computer. These stored codes are read by the auto technicians' scanning tools.
WHAT DOES OBDII DO FOR ME?
One of the benefits of the OBDII system is the possible reduction or prevention of harmful emissions and therefore improved air quality. The system is designed to alert drivers when something is wrong that may affect emissions. Early diagnosis and repair may also prevent more costly repairs on both the emission control systems and other vehicle systems and may possibly add fuel economy benefits. OBDII systems provide technicians with more information for diagnosis then ever before.
WHAT SHOULD I DO IF THE MIL COMES ON?
The light informs the driver of a potential problem and a need for service. This does not indicate the need to stop the vehicle immediately. The driver should consult their vehicle
owner's manual and/or a repair shop as soon as possible. A blinking light indicates a more severe problem; you should reduce speed and/or load and seek service as soon as possible.
HOW IS THE MIL TURNED OFF?
There are several ways to turn off the MIL: (1) check to see if the gas cap is tight; if not, tighten it (the MIL may not go off immediately) - a loose cap may indicate an evaporative fuel leak; (2) have an auto technician check the system with a scan tool, perform a repair and turn off the light; or, (3) the system may turn itself off after a period of time if it no longer detects a problem.
WHY AM I CHARGED FOR DIAGNOSTIC TIME ON A SYSTEM THAT DIAGNOSES ITSELF?
OBDII only provides a "hint" or a "clue" of the problem. Diagnosis time will generally be needed to isolate the true cause of your vehicle's problem and identify the proper repair. There will be a charge for this labor.
WHAT IF THE AUTO TECHNICIAN HASN'T BEEN ABLE TO FIX THE PROBLEM AFTER SEVERAL ATTEMPTS?
There may be other problems not readily detectable. For example, the light may come on due to a software problem when there is no problem with the sensor component. This is an unusual situation, but it is still important to have the software problem fixed so OBDII can function properly and detect potential problems with the car.
DO I NEED TO GO TO A REPAIR SHOP IF THE LIGHT WAS ON AND THEN WENT OFF?
The MIL may indicate a temporary problem that may only happen once. However, if the MIL goes on again, you should go to the repair shop while the MIL is illuminated. AN ILLUMINATED MIL SHOULD NEVER BE IGNORED.
In conclusion, if you can get to a garage and simply have the vehicle's computer scanned and retrieve and record the Diagnostic Trouble Code (DTC). When you get the DTC# XXXXX back here and let me know what the code is and I will help you with the likely cause of the condition associated with the DTC. Even if the MIL is no longer illuminated, the DTC is stored in the computer's memory for some time. Here in the states many "parts stores" have OBDII scanners and will scan your vehicle and tell you the codes for free, in hopes that you will buy the needed parts from them. I do not know however if this is true in the UK.
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