MIL ~ Engine Check Light

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MIL ~ Engine Check Light

Postby FioranoCars » Mon Mar 07, 2016 12:34 pm

Hi all,
some feedback please, as while updating some notes for a trainee, I could not find a reference we had to a nugget of information!

Namely, that the MIL can only be lit by an emissions related fault, not some other arbitrary fault or condition. I had a reference, but it's gone, and I'm sure that there was some talk about this, but can't find anything now! There are some other blanks, if anyone fancies adding their voice...

The note was about the background of the light and it's purpose, how it came about etc, it was just to dispel a few myths and concentrate the focus on the MIL true purpose ... that it's not a all round diagnostic light or some other reporting, it's purely and if my previous research was correct, legally solely for the purpose of emissions (fuel, exhaust, etc) and air quality.

----- Start
The Engine Check Light / OBD / EML are all often used incorrectly as references to the Vehicle Malfunction Indicator Lamp (MIL).

The MIL was required by the 1990 Clean Air Act (CAA) of the USA and was required as standard on all MY1996 and newer light-duty cars and trucks, the EU {reference needed} mandated it's implementation {date}, elsewhere {reference needed}. The light symbol has become largely standardised, it is {not ?} a requirement to use this symbol.


The MIL is controlled by the On-Board-Diagnostics (OBD) which is accessed via the OBD access port, currently a standardised 16pin socket. It has been Enhanced and is now called the EOBD, and future needs will without doubt bring a different port, and more complex reporting, probably wireless access via Bluetooth or Wifi, or even remote telemetrics via 4G!

The light must self test at ignition ON, lighting briefly to show it is carrying out it's tests, and turning off shortly after startup if all the tests complete without fault.
The light indicates 2 severity of errors:
- permanently lit - Standard Fault - you should have the car checked, but no legal time frame currently.
- Flashing / Blinking - Severe Fault - avoid heavy load, drive slowly, minimise driving time.
In the UK it is an MOT failure, so if stopped you could be given a MOT rectification notice and fined for driving an unfit vehicle. The same could be applied the the AirBag or TMPS warning lights in theory!

The MIL fault reporting covers, amongst other items
- Engine misfires
- Exhaust emissions
- Fuel Anti-Evaporation
- Catalyst performance
- Failure of sensors monitoring or controlling the combustion process or the fuel storage

It cannot be used for issues outside the scope of the specifications, so cannot be used for Service Intervals, or other non-emission related faults. {reference missing}.

The vehicle will apply different strategy for lighting and turning off the MIL based on several factors, these include
- The number of running cycles an error is present before lighting the MIL
- lower level faults requiring more cycles, maybe up to 5 before the MIL is lit
- medium level issues may require 3 sequential cycles of the error
- High level faults may trigger the light without delay
The delayed illumination of the MIL normally also includes a "Pending" Trouble Code record being held in the system.
Once the MIL is illuminated a "Freeze-Frame" record of data is normally kept, there is normally a limited amount of data with this record and often only 3 {?} records of Freeze Frame are held, with newer records overwriting the oldest.

- The number of error free cycles encountered before turning the light off is used differently for some errors {category?}, other more serious errors {category} will not be switched off automatically regardless of error free cycles, and will require intervention using a OBD scantool to clear the error once the repairs have been completed.

Tampering with the MIL and it's emissions related monitoring is not permitted {what legislation?}. This includes removing the bulb or covering the symbol with opaque tape or paint!

----- End

Thanks in advance for any input, corrections, references etc!

Richard Lukins
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