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ricointheuk

Cat light flashing / misfire

35 posts in this topic

Hi everyone :)

I have an 1996 A8 4.2Q. While travelling back from Leeds to Worcester the CAT light began to flash and the car ran roughly just above Birmingham. I continued home as it did not get worse.

The next day I went to investigate the problem and started the car and all was good. I then went on an errand and when the car reached temperature the CAT light began flashing again and the misfire was back.

Looking at the other threads along this theme I haven't seen any with the exactly the same symptoms. Vag-com gave me codes that related to: random misfires, cylinder 3 misfire and CAT overheating. Do you think it is the CAT sensor that is faulty and is messing with the engine management as described in another thread?

Thanks

Richard

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Welcome to TSN Richard.+++

It might be worth checking the lambda sensor on that bank (possibly the pre cat one- Bank1 Series 1). Also check the coil pack/plugs on no. 3 by swapping them over and seeing if you get the code on another cylinder. The fact that it runs rough once the engine has warmed up could point to the Lambda sensor as they start working once they have reached around 300 degrees C; before that its the ECU dictating what goes on when the engine is cold. Also heat could caused arcing of the spark in the plug/coil pack assembly.

Do you have the full version of VAGcom or the share ware?

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Agree - Coilpack. A coilpack fails, which runs slightly but barely noticeably rough. Then for reasons known only to Germans in white coats, the ECU goes mad and starts shutting off other cylinders to compensate, which really makes it run rough. All the non-firing cylinders results in unburnt fuel in the exhaust which is what causes the cat light to come on. It can start with a mildly rough running engine, and appear to get quickly worse. Ignition off for a few minutes and start again and things seem ok breifly, because the ECU is attempting again to fire on all 7 good cylinders.

Vagcom is usually pretty good for this, so the cylinder that is reported faulty get another coil pack in and more than likely it will be all good.

We've got a 1996 ABZ we're breaking up for spares at the moment so we can probably sort you out with a coilpack....

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Agree - Coilpack. A coilpack fails, which runs slightly but barely noticeably rough. Then for reasons known only to Germans in white coats, the ECU goes mad and starts shutting off other cylinders to compensate, which really makes it run rough. All the non-firing cylinders results in unburnt fuel in the exhaust which is what causes the cat light to come on. It can start with a mildly rough running engine, and appear to get quickly worse. Ignition off for a few minutes and start again and things seem ok breifly, because the ECU is attempting again to fire on all 7 good cylinders.

....

The unburnt fuel is the reason all the other cylinders start to misfire too. The Lambda sensor 'sees' the unburnt fuel from no 3, adjusts the mixture leaner to compensate which results in a mixture too lean to fire, hence the misfiring gets worse. Simplest way to resolve the issue temporarily is to disconnect no.3 injector and the lambda sensor from that side and it should all run nicely on 7. The A8 will run very happily with no lambda sensors at all for a wee while (don't boot it too hard though as there is a possibility the engine will run excessively rich which could kill the cats).

My vote goes to a coilpack too.....

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Exactly same symptons with my 98 4.2Q yesterday - just after putting some crappy jet petrol in. Sounds like coil pack for me as well then, another thing to add to the list for the IN 1 service next week.(gearbox oil already planned)

Car's going to CT Cars in East Horsley - they seemed ok with my previous S8 and a couple fo guys at work use them for A8 A6, so fingers crossed that overall cost wont be too high

Iain

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I've just experienced a similar problem which currently appears to be the plug "caps" being old & current tracking down the outside. They are not cheap items, either, about £23 each. I was given to believe there was no difference between those used on a V6 & as they're only £13 each it was worth a shot. Trouble is, they're about 1" shorter. Never mind.

The coils have tested okay so hopefull this will sort it out, I'll let you know.

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Thanks for the welcome :) and the information. I will do the coil pack and let you know how I get on. Probably won't be to next month.

Ska, I have VAG COM shareware and Bentley Manual software.

Thanks

Richard

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Hi :)

I have changed the coil pack on cylinder 3 and no difference. I'm getting codes for:

cylinder 3 misfire

multi cylinder misfires

o2 sensor circuit no activity detected bank 2 sensor 1. (SAE P0154 VAG 16538)

and the CAT light is still flashing.

Next?

Thanks

Richard

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You might have to move the lead to the coilpack around and see which one then goes.

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You might have to move the lead to the coilpack around and see which one then goes.

Just to confirm I have understood - I need to move my new coil pack from cylinder to cylinder to see if anything changes? (The lead is to short to move anywhere else)

Thanks

Richard

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No, just the lead. But seeing as it is too short, that does not help.

Did you say you've checked/changed the plugs at all?

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No, just the lead. But seeing as it is too short, that does not help.

Did you say you've checked/changed the plugs at all?

I have recently changed all the plugs, so in theory they are ok. I should pull it out and check just to cross it off the list.

Thanks

Richard

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Hi :)

Ok plugs look good and tried the old ones again. Same problem, what now :)

Thanks

Richard

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Have you changed any O2 (Lambda) sensors yet? That sounds possible given those codes.

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Have you changed any O2 (Lambda) sensors yet? That sounds possible given those codes.

Thanks :)

I will try that next :)

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Have you changed any O2 (Lambda) sensors yet? That sounds possible given those codes.

1. Should I change both o2 sensors while I'm at it?

2. Is it easy access or does the exhaust need to be removed?

3. Any other info on procedure you think I should know?

Thanks

Richard

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1. Should I change both o2 sensors while I'm at it?

2. Is it easy access or does the exhaust need to be removed?

3. Any other info on procedure you think I should know?

Thanks

Richard

Change them both.

Before you do that, see if you can swap the leads where they plug into the ECU. See if that makes a difference.

If not, change them. They will be well in there! You'll need a big spanner and lots of Plusgas to loosen them. You'll need good hand skills to change the 02 leads too! If I get time, I'll try to do an EKTA picture for you.

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They are possible to do in place but you will need a modified ring spanner. I went to Helfrauds and bought a 22mm ring spanner (I think it was 22mm anyway), then cut it in half, then cut a slot in the ring about 5mm wide on the opposite side to the handle. You can use an angle grinder or a dremel to cut it. Then if you have small hands, a high threshold to pain and a good set of muscles you can just reach them to undo them, but they aren't easy....Bizarrely I found the drivers side the easiest once I'd removed the intake trunking. I replaced both mine with NTK ones from here:

Just Lambda NTK, Suppliers of NTK lambda sensors by post..

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As an alternate to Kiwi's suggestion get a box spanner of 100m length. Cut the slot as Kiwi suggests and once it is fitted over the sensor use a jubilee clip around the bottom to stop it expanding on the nut. Then just use a 1/2 drive ratchet and a Allen socket. When you do it up it 50 Nms and you should use a sealer according to Elsawin.

Mike

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Thanks you for the information. I have ordered the parts. I will let you know what happens next week :) May be I will take pics and vid if that would be helpful to the forum?

Thanks

Richard

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My misfire and CAT light were caused by leaking breather pipes and a faulty ignition module.

Now running just fine.

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My misfire and CAT light were caused by leaking breather pipes and a faulty ignition module.

Now running just fine.

Thanks :)

I will look in to that as well

Richard

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Hi :)

I have changed the lamda probes and an air intake boot that had a split. Unfortunately the misfire is still there :( and the CAT light is still flashing.

Any ideas what I should try next? Am I correct in thinking that there are oxygen sensors by the CAT's? can they be changed? is that difficult?

Thanks in advance

Richard

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My misfire and CAT light were caused by leaking breather pipes and a faulty ignition module.

Now running just fine.

The breather pipes are all ok. They need changing but are still sealed. Is there a way to test the ignition module. What cost is it?

Thanks

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