How long will the engine light stay off if the fault still exists?

Sorry for the randomness. VW 1.9tdi in a ’08 T5.

Failed MOT because the engine warning light is on (well, it didn’t fail but here in Spain they won’t measure emissions if there’s an engine fault, so no ticket). Retest tomorrow.
Took it to garage, fault is No.1 glowplug. Can’t fit a new one in time for the retest. They deleted the fault, the light is off – will it still be off when I get to the test station? Plan B is a bit of black ĺeccy tape over the light I guess.

Until the fault get reported again.

Don’t VW have two ECU computers? My TT has, you can delete the fault code off one and the light goes out, but it needs a full clear down to get rid of the full fault. (I have a fault on a sensor, so when I get MOT I clear codes before MOT to make sure, even if there aren’t any)

I could be talking bollox obvz, it’s just what I’ve been told.

My fault is intermittent, can go a whole year before it comes on, or every half year… so bought a fault code reader with erase option.

You might be lucky if you clear it down before you MOT it..

Turned it on and off a few times and it hasn’t reappeared… If it doesn’t come on in the morning it’ll be ok, as they do the test with the engine running.

When the massive cloud of white smoke indicator comes on 🙂

It depends?

Vw had a lot of problems with oxidistion of the plug causing faykts due to arcing I think. Caused intermittent codes.
It also depends on your exact system. A lot of VWs detected glow plugs faults by unballanced resistance but a 4 cylinder had two groups of 2 so 1 failed plug often wouldn’t trigger it.

Most faults set a stored code the first time the problem occurs and lights the MIL on the second occurrence.

I would have thought glow plugs would fail on a start that involves them (i.e. cold start) probably 2 in a row, although I dint actualky know about VW specifically, it may run a test on prior to every start, or eveb periodically while running. Stored codes will show up on the scan although may not be an instant fail, as the reason for needing 2 for a MIL is sometimes you just get an electronic hiccup

ISTR I had two replaced a couple of years back, would need to dig out the receipt to find out which ones, but stands to reason the other 2 won’t be far behind. Mechanic definitely said just no.1 though.

Internetland suggests the light can be turned off by holding the trip reset when turning the key.. not sure if I believe that tbh.

Worth giving it a go isn’t it ..

I haven’t had an engine warning light and i have a broken glowplug.

Literally broken as in the lead isn’t actually attached to the plug.

ISTR I had two replaced a couple of years back, would need to dig out the receipt to find out which ones, but stands to reason the other 2 won’t be far behind. Mechanic definitely said just no.1 though.

They’re qbout £9 each just get them all done.

It is possible. I know VW went to a one wire per plug in various engines in various years . I think they all are now.

I’m guessing if the coolant measures warm enough, the glow plugs won’t be called upon and the code won’t set. Unless VW shaft you by testing the plugs on every start.

All else fails i guess you could start the car, run it warm, go to mechanic and have fault cleared, drive straight to testing station and hope.

Don’t VW have two ECU computers?

In the old days, the ECU was the engine control unit and that was that. Nowadays there are loads of ECUs (electronic control units) all over the car. The engine ECU is just one of them.

ECUs store codes, but wether or not the light comes on depends on how it’s been programmed for that particular fault. Some faults don’t trigger the light, some do. Some faults will cause a certain behaviour for a period of time and then it’ll see if it happens again. If it doesn’t, it’ll go back to normal (e.g. limp mode for dodgy boost pressure reading). This can mean the light comes on, then goes off again of its own accord when the software decides it’s been ok for long enough.

If you have a glowplug fault it might be intermittent, in which case it’ll probably stay off til it happens again; but if it’s a flat out fault it’ll reappear as soon as you start it.

07 2.5tdi T5 here, eml is on pretty much all the time for as long as i can remember. We only worry if it goes off.

When we clear the code or by an act of god it goes off itself, it usually stays off for about a week before reappearing.

No idea what the cause is, nor the vw indi we use.

No idea what the cause is, nor the vw indi we use.

They don’t sound very clued up.

All else fails i guess you could start the car, run it warm,

Actually that’s a good call, it’s got a Webasto so I can get the coolant to 80c before starting

@joshvegas wish I’d got them all done, no idea why I didn’t. Crazy, especially as quite a bit of dismantling to get to them.

My 1.9 tdi T5 has a glow plug fault. Only manifest in cold weather as there’s a temperature above which they aren’t used for starting (according to Breeze in Portsmouth, anyway) so maybe if that’s the case you’ll be ok if it’s warm enough on the day?