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Will any error which causes the check engine light to go on show up as a DTC during a regular OBDII scan? This is obviously only in relation to OBDII compliant vehicles.

EDIT July 26th, 2016

So I just ran across this page for VAG-COM, which is software for talking to VW-Audi Group cars. On their page, DTC Comparison (Why a generic OBD-II scan tool can be useless) it says:

Here's a real-world example from my car from September 2002. My car (a 2000 GTI 1.8T) had stalled out several times over a period of a few weeks. One evening leaving work, the car stalled, but I was able to restart it. The Check Engine Light was not on, but I scanned for codes.

It then goes on to say that no DTC's showed up during a generic OBDII scan, but that ten proprietary VM-Audi codes showed up when scanned with their software.

This would seem to indicate to me that manufacturer specific codes don't set the CEL, although I guess this could vary from manufacturer to manufacturer.

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The CEL only comes on when a specific trouble code has been stored, that's it's purpose.

  • But could a cel be set by a manufacturer specific dtc which doesn't show up on a generic obdii scan? – Robert S. Barnes Jul 24 '16 at 20:56
  • @RobertS.Barnes - I believe so. If your scanner does not pick up the vehicle specific DTC's, I don't believe you're going to see it when you pull the codes. – Pᴀᴜʟsᴛᴇʀ2 Jul 24 '16 at 23:50
  • You should still see the numerical code itself, just not the english-language translation of it. You'll need to look it up online. – tlhIngan Jul 25 '16 at 2:10
  • @Pᴀᴜʟsᴛᴇʀ2 So are you saying that a vehicle specific code can set the CEL, and there's nothing in the standard requiring that code number to show up during a generic obdii scan? – Robert S. Barnes Jul 25 '16 at 4:57
  • @RobertS.Barnes just from personal experience any DTC that requests the MIL will show up on a generic obd2 scan. If you can't find any codes the lamp maybe shorted to power. – Ben Jul 25 '16 at 11:15
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I scanned an Audi TT some years ago and one of the manufacturer specific codes to be returned informed me that there was an intermittent wiring fault on the passenger side wing mirror. This was traced to a lose connector in the door.

It's quite obvious that this wouldn't illuminate the CEL but was obviously logged in the ECU. That said, I would expect the inverse (i.e. CEL illumination) to almost always write an event into the fault log on the ECU.

  • While that seems sensible, is there anything that actually requires it? – Robert S. Barnes Jul 26 '16 at 10:29
  • Local legislation usually. In the UK, a CEL is not a requirement whilst in the US, it has been for many years. UK spec Mk3 Golfs, for example, do not have a check engine light. I don't know if there is any guidance about the type of events that should cause the light to illuminate however. – Steve Matthews Jul 26 '16 at 11:16

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