Hi everyone,

Hope you’re all doing well.

I’ve bought a Skoda Fabia Combi 1.2 HTP - motor code AZQ - @ 115k km one month ago.

After a day, I’ve noticed that the car has a rough idle, the inside of the car wobbles a little bit. In the first instance I thought it was may be normal after several restshowing that this 3 cyl is a little rough in idle.

But I’ve put the OBDeleven into the obs port and P0172 was the code shown. So I’ve begun to diagnose it by myself.

What I’ve found is :

• the evap purge valve was blocked in close position. When the car was cold there was no succion. After the warm up, the valve was still closed.

• Leakage of cooling liquid at the thermoswitch. But it’s not a huge leakage

• The fuel pressure at the rail measures 3,2 bars

• The MAP sensor shows a value of 45 kPa at idle. When put only the ignition, I have a value of 101 kPa. When reving the car, the value descrease at 25 kPa.

• The lambda value is close to 1

• The O2B1S1 value was at 1.52 V and fluctuate a little but not much

• The O2B1S2 value vas at 0,74 V and goes to 0,78 V but doesn’t fluctuate much.

• The STFT was at -15% up to 20%

• The LTFT was at -3 to -4 %

Before replacing anything, I’ve unplugged the B1S1 and the STFT went to 0%. The cars became a little smooth. So I guessed it was because of the sensor ! I’ve change the S1 and put the Bosch brand, changed the evap valve as well and changed the fuel filter (the filter was 22 years old …)

After all that, the value were :

• O2B1S1 : 1 V but no fluctuations at all… except when reving … it goes to 3 V !!!

• O2B1S2 : 0 V

• StFT up to -20%

• No error code coming

• Rough idling

I’m completely lost…

I don’t know where I should look at…

Also when the car is cold, sometimes it happens after that I’ve reversed or moved forward a little that the cars revs up automatically, like you put your foot on the gas pedal and release and same on some few times before it stabilizes

Could you please help me a little with this ?

Appreciate a lot ! Thanks in advance

Have you still got the removed O2 sensor?

From memory, for this wideband sensor with this ECU, 1.5V steady is correct.

Have you checked basic things like spark plug condition?

My apologies I haven’t precised. I have replaced O2B1S1 by a new one from Bosch. Every sensors are connected !

I have checked the spark plugs and they are all 3 black … showing that there is indeed a rich condition. No oil or petrol on the spark plugs. They feel in good condition though. There are no misfiring.

Did you keep the old sensor after you fitted the new Bosch one, or did you throw it away?

What Bosch part number was used?

Nope still got the old one. Will post the reference after eating

I looked back at the workshop info, and 1V could be correct if you are looking at measuring group 31, (red ringed info) or 1.5V could be correct if you are in group 33 (blue ringed numbers).

Perhaps you looked at 33 before the sensor change, and 31 after?

Note 15 is for AZQ engine code.

Please check again the numbers in groups 31 and 33, with engine fully warmed up.

There may be another cause for the over-fuelling. Is the coolant temperature sensor original?

What coolant temperature does VCDS show when engine is fully warmed up?

Edited 15 January15 Jan by Breezy_Pete

Just realised that group 31 is lambda nominal value, not voltage, in the table for AZQ engine.

Group 33 is showing voltage;( for AZQ, the 1.5V I had remembered).

Thanks for your help !

For the reference, :

• old one has 03E 906 262

• The new Bosch has : 0 258 007 35

The measurements are indeed seen on group 33. I haven’t watched the group 31

On my sons fabia m1 1.2htp there are two lamda sensors available from memory depending on the Vin/age. His is AZQ. Maybe you have got the wrong lamda?

Think there maybe a number missing off the bosch part number as they are usually 10

Alasdair

My bad

It’s 0 258 007 357

From what I’ve seen this part is compatible for AZQ motors.

I could be wrong….

According to Bosch its the equivalent to old one used on Fabia Mk1 AZQ year 02.2003 - 11.2004.

Alasdair

Ohw, so it’s the bad sensor that I’ve took ?

Not sure the Bosch one matches the one that came out as far as I can see unless it was replaced by previous owner with wrong one?. What year is yours.

Alasdair

Mine’s a 2004

First registration in December 2004

Vin number : TMBHY46**********(01)

I’ve drove the car for 300 km. 150km per trip.

When I’ve done the first 150Km, I’ve done and obd scan. No code error and stft of -9,5 to -11,5%.

Then I’ve done the trip back and done the scan … (drum roll) surprise code P0171 - system too lean … the opposite now with a STFT of -18 to -21%.

I’m becoming mad… xD

VIN obscured.

Do not post your full VIN. This is to prevent cloning. Thank you.

Briskoda Admin

Sorry havent a clue either. Maybe purge valve is sticking closed then stuck open or the PCV valve is faulty? Sons had poor idle. low then high etc plus poor mpg. I replaced purge valve and also his brake servo vaccum pipes were split so taped up and idle returned to normal. Have read that some folks needed to replace charcoal canister as well as purge valve.

He had problems last MOT with emissions due to small leak in centre of exhaust. Sealed it and emissions were ok. It was a tiny leak that I and the MOT guy thought wouldnt make that much difference.

Its odd it jumps from rich to lean.

Alasdair

On 15/01/2026 at 21:14, Breezy_Pete said:

I looked back at the workshop info, and 1V could be correct if you are looking at measuring group 31, (red ringed info) or 1.5V could be correct if you are in group 33 (blue ringed numbers).

Perhaps you looked at 33 before the sensor change, and 31 after?

Note 15 is for AZQ engine code.

Please check again the numbers in groups 31 and 33, with engine fully warmed up.

There may be another cause for the over-fuelling. Is the coolant temperature sensor original?

What coolant temperature does VCDS show when engine is fully warmed up?

I think so that the coolant temp sensor is the original one.

The ECT is at 84-88 when warmed up. At idle it oscillates a little because of the leakage, I guess it influences a little

14 hours ago, Alasdair1 said:

Sorry havent a clue either. Maybe purge valve is sticking closed then stuck open or the PCV valve is faulty? Sons had poor idle. low then high etc plus poor mpg. I replaced purge valve and also his brake servo vaccum pipes were split so taped up and idle returned to normal. Have read that some folks needed to replace charcoal canister as well as purge valve.

He had problems last MOT with emissions due to small leak in centre of exhaust. Sealed it and emissions were ok. It was a tiny leak that I and the MOT guy thought wouldnt make that much difference.

Its odd it jumps from rich to lean.

Alasdair

When you say the purge valve, is it the the evap purge valve ? If so I just put a new one.

Also I didn’t have at any moment the engine light on the dashboard.

Also when driving the STFT is normal… it’s only at idle that it’s negative as hell

What the general mileage of this car ? The average that I should be having ?

The fuel tank is 45 l right ? Because once I’ve filled that tank and it took 50,5 L… normal ?

Edited 18 January18 Jan by King848

There's a little nipple-like feature that can be seen if you look into the fuel filler neck with the cap unscrewed.

This is supposed to only be pushed in by the fuel cap as it's screwed back on; but it is possible to accidentally or deliberately push it with the petrol pump nozzle while filling.

It's particularly easy to do this accidentally if filling with the pump on the wrong side of the car, where the nozzle may go in off-straight due to sideways pull on the pump hose.

The vent valve actuated by pushing the nipple is supposed to only connect the air space above the fuel level to the carbon canister when the cap is on, tank sealed. However, if pressed and held down while filling, you can inadvertently fill this planned air space and the carbon canister with liquid fuel instead of vapour. The canister is not designed to cope with this.

This may be what has happened and is causing your problem. Removing and emptying the canister may help, I guess. I seem to remember reading about at least one person successfully doing that.

Thanks a lot all for your help.

You mean I should remove the charcoal canister, empy it from the petrol that it contains and fit it again ? It's for the refueling problem right ? the excess of amount. Not for the fluctuating Lean-Rich condition or may be I am not understanding correctly.

If the car is in total rich condition, when driving the STFT should be close to 0 or remains in negative ?

I was thinking that maybe a charcoal canister full of liquid fuel could be causing the 'system too rich' fault.

Each time the purge valve opens to clear fuel vapour, much more fuel than expected by the ECU may be coming through.

It just seemed possible when you mentioned the over-filling of the tank recently.

Thanks !

I’ll do it and let you know if that worked

Very interesting. On my 1.2 tsi (cbzb) is exactly the opposite, system to lean. Especially on idle lots of misfires, i change the coil pack, leads and spark plugs. So nothing to do with the ignition.