Audi A4 / Check Engine Light (MIL) / What it means / How To Turn it Off

A few days ago, my wife started the car and noticed the check engine lamp (also known as the malfunction indicator light or "MIL") remained illuminated. I opened the owner's manual to see if it provided any clues, and it basically said that it is part of the car's OBD II system and that I should take the car into Audi service to have them diagnose the trouble. This of course means that if I want to know what has caused that lamp to illuminate, it's going to cost me a half a shop labor hour, or approximately $55. Paying $55 dollars, plus the inconvenience of the trip to the dealer, only to be told my car's problem was something small like the gas cap not being tightened, not only seemed like a huge waste of time, but an expensive one at that.. Instead, what I did was put the money into a diagnostic tool that I can keep in the car and use to diagnose these problems myself.

OBD is an abbreviation for On Board Diagnostics and if your car was sold in the United States after 1996, the US Government has federally mandated that it have an ODB II interface (regardless of make or model). With the right diagnostic tool, the OBD II interface will allow you to retrieve a code that explains what caused the check engine light to activate, to turn off or "reset" the check engine light, and (with some of the higher end models) retrieve real-time data and potentially make other tweaks to the settings in your car's computer.

Warning

THE MALFUNCTION INDICATOR LIGHT MAY INDICATE A SERIOUS PROBLEM IN NEED OF REMEDY BY A TRAINED PROFESSIONAL. UNDERSTAND WHAT YOU ARE DOING BEFORE YOU DO IT. THE MATERIAL AND METHODS PRESENTED HEREIN ARE NO SUBSTITUTE FOR HAVING YOUR CAR CHECKED BY A TRAINED PROFESSIONAL. I AM NOT RESPONSIBLE FOR ANY DAMAGES CAUSED BY THE USE OR MISUSE OF THIS INFORMATION. PROCEED AT YOUR OWN RISK.

The first step is to locate your car's OBD II Interface, on a 2002 Audi A4 Avant Quattro this can be found on the driver's side, directly behind the hood release leaver. You will not generally be able to see it without looking under the dash.

Ok, let's connect the scanner.

So it looks like we have found the cause of the check engine light, above the scanner shows the diagnostic trouble code (DTC) "P0118 - Engine coolant temperature circuit, high input". This particular scanner has an internal dictionary that it uses to interpret the code (so no need to look it up to learn that it means "Engine coolant temperature circuit, high input", it does that part for you).

Some of the cheaper scanners don't interpret the fault code, but they are generally available on the Internet. Having said that, this won't help you if your car brakes down on the side of the road so you may want to consider getting a Audi A4 Service Manual, which would of course be useful for diagnosing and solving all future problems.