I hope someone can offer some advice on this: My 20yr old Fab Mk1 (1.4l BBZ engine) has failed the MOT on emissions. High CO, Low lambda. I don't have the actual numbers so I don't know how close it was to 'good enough'. The garage ran some cat cleaner through it and retested it but it still failed.

 

They recommend a weekend of thrashing the engine at high revs and retesting on Monday with a hot engine. If that still fails, with no further testing or analysis as far as I know, they are recommending a new catalytic converter.

 

As I understand it, low Lambda means not enough air in the fuel/air mix which would be consistent with partial combustion and too much CO - which the cat is struggling to cope with because of too little air. Not enough air means too much fuel for the air, which means running rich.

 

However, on board diagnostics (OBD2 handheld and VCDS lite) has called out a P0171 'system too lean' in the recent past and VCDS shows another intermittent DTC which suggest it's running too lean.

17559 -  Long Term Fuel Trim Additive Air; Bank 1; Range 1: System too Lean
P1151 - 35-10 -   - - Intermittent

 

Here's my idea: If the (pre-cat) oxygen sensor is incorrectly reading the O2 as too high and is asking the engine management to make the mixture richer this would explain why it records a DTC of too lean. But because in reality it's running a mix which is actually too rich, it fails the MOT.

 

Does this sound reasonable and if so could a replacement O2 sensor do the trick or should I just pony up for a new cat or get rid? I understand that the cat could also be bad or may have been damaged by the fault...

 

I've got it apart and the O2 sensor is an NTK 036 906 262G with the VW/Audi symbols on it so it's possibly an original 20year old sensor. It didn't look fouled and had a light sand coloured coating of deposits which I've brass wire-brushed off before reasembling.

 

The sensor has 5 wires to its connector but the socket it goes into has 6 wire, with the cable and connector it is about 53cm long. Prices seem to vary from £60 to £145 or more... If I do go for replacing the sensor is it worth it to get the £145 NGK sensor or the £60 Ridex one?

 

Any help or comments on thinking this through will be welcome.

Might be worth checking that the fuel pressure regulator hasn't been replaced with a wrongly rated one during a fuel filter replacement. 

That can play havoc with the engine ECU's ability to get fuelling ratios right.

If you let me know VIN or reg plate I can check what should be fitted, then you can compare with what's stamped on the rear end of the filter assy just in front of rear right (driver's side) wheel.

 

Thanks for this. I hadn't thought of the fuel pressure regulator

 

VIN is TMBND46Y944139010

 

 

 

 

There is no possibility that a twenty-year old lambda probe will be providing accurate data to the ECU, replace it, I did mine recently and it was only £45 for a NOS genuine item. Make sure you clear codes after fitting to clear the dynamic map and take it for a half hour thrash to rewrite the map before retest.

1 hour ago, T****y said:

Thanks for this. I hadn't thought of the fuel pressure regulator

 

VIN is TMBND46Y944139010

 

 

 

 

 

3 Bar fuel system, check that someone hasn't fitted a 4 bar one due to duff info from ECP lookup by reg or similar.

12 hours ago, T****y said:

My 20yr old Fab Mk1 (1.4l BBZ engine) has failed the MOT on emissions. High CO, Low lambda. I don't have the actual numbers so I don't know how close it was to 'good enough'. The garage ran some cat cleaner through it and retested it but it still failed.

When was the air cleaner element last changed?

4 hours ago, Paws4Thot said:

When was the air cleaner element last changed?

Thanks for this but the air filter was changed just before the test - also the plugs and oil and filter; it was given a service.  Oil was not over-filled (I checked) and is VW compliant.

 

If it passes its MOT I'll treat it to a new timing belt too.

 

(I remind myself: Don't anthropomorphise machines - they hate that).

13 hours ago, Breezy_Pete said:

 

3 Bar fuel system, check that someone hasn't fitted a 4 bar one due to duff info from ECP lookup by reg or similar.

Thanks for this. I had a quick gander at the fuel filter/regulator. It seems to be made from a white composite (plastic) material with no obvious part number markings. Most replacements seem to be alloy cans from the images. It does have black 'direction of flow' arrows printed on so maybe other markings are hidden. I'll take it out tomorrow (battery disconnect, depressurise first) to have a proper look (and check TB etc adaptation hasn't been lost after reconnecting battery).

Phone cam photos from various angles may reveal info on it somewhere.

I certainly wouldn't take it off unless you have a correct one ready to fit on, or fuel will pee out of the tank until empty, I think.

 

9 minutes ago, Breezy_Pete said:

Phone cam photos from various angles may reveal info on it somewhere.

I certainly wouldn't take it off unless you have a correct one ready to fit on, or fuel will pee out of the tank until empty, I think.

 

I use an endoscope style camera on a cable - still can't see any numbers... Yes, I expect some fuel loss if I disconnect it - I hope to drop it down far enough to view it without disconnecting it from the fuel lines - but just in case I do disconnect it I don't want the fuel pump to decide to help things along.

 

I've got a 3-bar filter with regulator for a PR 1A1 car ordered. Autodoc, so it will be a week or so 😞

Fair enough, sounds like you know what you're doing. 

Try throttle clean and readaptation usually gets it through the mot.

On 04/08/2024 at 08:44, Breezy_Pete said:

Fair enough, sounds like you know what you're doing. 

Ta.

 

I managed to shift the fuel filter enough to see the last six digits of the part number ... 201 051 and the UFI brand mark. So it looks like it's a 3 bar filter/regulator. Having cleaned the road muck off it now appears to be an alloy can - not plastic as I said above. No VW brand marks that I could see so probably not a 20 year old OE installation but it's been there for a while I think - so probably worth changing. I've ordered a Febi Bilstein replacement which will arrive from Autodoc when it arrives (with a new O2 sensor too).

 

No disconnecting fuel lines or leaks but had the battery disconnected and a catcher tray ready anyway

 

Everything else screwed into the underside of the car seems to use a Torx head. Why the hell did they use a Philips for this one?

 

I'll reset the intermittent codes with VCDS lite (and thus the fuel map too, I hope) and take it for a 45min thrash in low gear before taking it for a retest of emissions this afternoon. If it still fails I've got until the end of the month before I have to fix it or get it off the road.

An update: I'm just about to take it for a retest of emissions but a little earlier today I tried to run the O2 sensor ageing test (Engine controller, Block 36. Basic settings, press and hold the brake) It wouldn't run. Although the coolant temp was 87 deg it claimed the exhaust temp was 72 deg - this after giving it a bit of a thrash. I'm sure now that sensor has failed.

14 minutes ago, T****y said:

An update: I'm just about to take it for a retest of emissions but a little earlier today I tried to run the O2 sensor ageing test (Engine controller, Block 36. Basic settings, press and hold the brake) It wouldn't run. Although the coolant temp was 87 deg it claimed the exhaust temp was 72 deg - this after giving it a bit of a thrash. I'm sure now that sensor has failed.

Sorry. Block 34...

 

Block 36 is the post cat O2 sensor and passed its test with 'B1S2 OK'.

 

Found the info through

which may be familar to @Breezy_Pete

Edited 5 August, 20241 yr by T****y
additional info

Wow, that's a while ago.  Did it all work out in the end?

 

55 minutes ago, Breezy_Pete said:

Wow, that's a while ago.  Did it all work out in the end?

 

Well... have you still got the car?

 

The really good news is that my car lives to fight another year. Despite what I found when trying to run the O2 sensor test it just passed the emissions test and I have another year of MOT.

 

'Drive it like you stole it' said the guy when he advised me to thrash it. 5,000 RPM for 20mins at a time is scary - especially when I don't know how old the timing belt is!

 

I'll try to remember to update this thread when I get the new sensor in but meanwhile, thanks to you and all the others who advised.

I hope this nearly wraps up this thread and proves useful to somebody.

 

In addition to failing the MOT the car had been hesitating a bit on moving off - especially first thing in the morning. Me being more assertive with the gas pedal seemed to get around this problem - at the risk of damaging the clutch. Also a couple of times a few months previously I had had the check engine light on (MIL) with P0171 system too lean and P0402 EGR flow excessive codes. I changed the MAP (intake manifold pressure and temperature) sensor and the MIL stayed off. My (very cheap) V-Pow hand-held OBD2 scanner showed no DTCs but after the car failed the MOT I checked with VCDS lite and found intermittent P0171 and P0402 faults which had not got bad enough to trigger the MIL (or show in the hand-held scanner).

 

After several attempts I finally managed to run the pre-cat O2 Sensor Ageing Test. I found some sources which described how to do it. But this is how I did it:

 

In VCDS lite with engine running at idle but up to operating temperature open the Engine Controller and go to Measuring Blocks.

 

Select block 034. The four fields shown are:

 

1) Engine RPM
2) Exhaust/cat.temperature bank 1 or 3 [°C]
3) Dynamic value bank 1 or 3 [ ]
4) Result [Test ON / Test OFF / B1-S1 OK / B1-S1 n.OK]

 

The O2 sensor is also the temperature sensor for this field. Field 2 ought to be somewhere around 200-300 degrees or more.

 

[When I first tried this a few days ago Field 2 showed 72 degrees - which is an impossibly low temperature for a running engine.]

 

Select *Basic Settings*

 

Press and hold the foot brake and bring the revs up to around 3,000 and hold. One source suggests it should be OK to just floor the gas pedal and the engine management will self limit the revs - but I didn't have the courage to try this. Field 4 changes to 'Test ON' and after a few seconds changes to 'B1-S1 OK' (we hope).

 

I found a thread on VWVortex (https://www.vwvortex.com/threads/vag-com-procedures-for-testing-the-maf-o2-sensors-and-catalytic-converter.1265064/) which suggested that Field 3 should be above 0.80 and that a new sensor would read 1.99. However, the thread referred to something other than the BBZ engine.

 

So Field 3 should change to show a number greater than 0.80 or the sensor is considered shot. In my case Field 3 changed to 0.60 so I guess the sensor is beyond its useful life - especially as it suggested the exhaust gasses were at 72 degrees at one point.

 

Directly after running the O2 sensor ageing test you can select block 046 and test the Cat efficiency. You trigger the test in a similar way. Select block 046, press and hold the foot brake and bring the revs up to about 3,000 RPM and hold. It takes maybe 20-30 seconds and I got 99.6% - I don't know what's 'good enough' for this but this feels OK.

 

Immediately after the new sensor was delivered I ran into a completely different problem: the front passenger door window regulator broke, so I took the car off the road and had the door in pieces until I fixed that.

 

When I finally fitted the new sensor and after clearing all DTC fault codes in VCDS lite (which should also reset the learned fuel map), I took the car for a long test run. Trying to get the onboard monitors to get into the 'Ready' state.

 

On re-running the pre-cat O2 Sensor Ageing Test Field 3 changed to 0.84. It's a cheap Ridex sensor but I guess from the number it's better than the old one. Directly after running the O2 sensor ageing test I also re-ran the Cat efficiency test. I got 99.6% again.

 

I ran it for a few days and checked the DTCs again - no intermittents, but then I don't know how often they were being recorded before. It may be my imagination (or the weather) but the car seemed less hesitant.

 

I keep detailed records of the fuel I put in and the mileage I get - I've been averaging just over 40mpg (around 7l/100km) over the last few tankfuls. I hope I'll get a slight improvement. I'll maybe add another brief update in a few weeks after it's used a couple of tankfuls.

 

I'll wait until then before I change the fuel filter.

Don't waste your time and money changing the fuel filter, unless you are having specific symptoms of fuel starvation then leave well alone, old filters filter BETTER than new ones!

Not if he puts genuine or quality filter, easy job about 5 minutes too.

15 hours ago, sepulchrave said:

Don't waste your time and money changing the fuel filter, unless you are having specific symptoms of fuel starvation then leave well alone, old filters filter BETTER than new ones!

Yeah, I've already bought the replacement filter (Febi Bilstein aftermarket job), I ordered it at the same time as the O2 sensor as I was not sure what is currently fitted. I'm holding off replacing it to see whether replacing the O2 sensor has had a beneficial effect. On the principle of change one thing at a time I'm going to wait until I've used a couple of tankfuls and see whether the intermittent 'too lean' faults have returned - and whether fuel economy has improved or not.

21 minutes ago, Blue8793841 said:

Not if he puts genuine or quality filter, easy job about 5 minutes too.

Hmm, cheapo Febi Bilstein unit is what I've got. Maybe it'll stay on the shelf - we'll see. Glad to hear it's an easy job - the Youtube vids seem OK.

In more than 10 years on this forum, I can't remember a single instance of someone solving a fault on a petrol Mk1 Fabia by changing the fuel filter.

 

There are a few cases where a problem has been created instead, by filter with wrong pressure rating  on the regulator part being installed.

Edited 28 August, 20241 yr by Breezy_Pete

10 minutes ago, Breezy_Pete said:

There are a few cases where a problem has been created instead, by filter with wrong pressure rating  on the regulator part being installed.

As the part number that I was finally able to read says it's the correct pressure/part for the car, the one I had already ordered will either stay on the shelf (or might be sent back).