Engine: B5244: S80 OBD Code P0172

[Bernard46]

I have a 2001 S80 Petrol 170 bhp auto with about 140,000 miles on the clock. About 4 weeks ago it had the cam belt replaced for the second time (nearly 15 years old) and an MoT test. It failed the emissions test because the lambda sensor was kaput - no reading from it at all. Replacing the lambda sensor put the emissions back in the middle of the range where they have always been (by the way there were no codes or lights on the dashboard to indicate there was a ever problem with the O2 sensor).

Everything was fine for a week when suddenly on start-up the "Emission System Service Required" message and the engine light came on - the car had previously been driven about 12 miles and left for 2 hours, so the engine was partially warm. The message and light stayed on through several subsequent starts. Having connected an OBD code reader I saw P0172 and following research on this code I open the bonnet and poked around the engine looking for any obvious broken or leaky vacuum pipes, but couldn't see anything. I closed the bonnet and the next time the car was started there was no message and no warning light. This lasted for a day or so and then the same indicators came on, again on start-up. This time the lights stayed on so I booked it into my local garage (the one who had done the previous work) - not a Volvo specialist but nevertheless a competent outfit who have done good work in the past.

They are puzzled. They say the new lambda sensor is working fine and giving the correct readings, they can find no source of vacuum leaks - they tell me there is not much in the way of vacuum pipes on this engine (I personally wouldn't know where to look, so cannot verify this). They say it could be a MAF problem or it could be the injectors are leaking, but given the age of the vehicle they are reluctant to throw (expensive) parts at it speculatively.

The car drives fine, idles at the right speed, does not mis-fire at any speed and fuel economy is as good as ever. The only change at all that I can notice is that prior to the new lambda sensor there was always (for a lot of the past year) a strange burning (like metal) smell when one got out of the car after a journey and this no longer appears - probably not relevant to this problem.

If the dash light and the message were not there I wouldn't notice any issue. Can anyone throw any light on what might be the problem?

[bournricha]

The Mass Airflow meter can be removed and cleaned with made for purpose spray cleaner .Google cleaning MAF and you should come up with the tutorial on you tube I think .Don't be tempted to use anything other than the spray as the wires that measure the air flow are very fragile .They are known to get dirty from time to time and send a wrong signal to the ecu .Cleaning will only cost pennies and it is quite likely to be the cause of your code .

[Bernard46]

OK thanks, I'll get some cleaner and give that a try. I assume there will be no issue in disconnecting the sensor to clean.

I noticed I could get a replacement MAF Sensor for this vehicle on ebay http://www.ebay.co.uk/itm/ACP-Air-Ma...cAAOSwnDxUflpT for about £28 - would it be worth trying this as well? If I replaced the sensor does one need to "tell" the ECU?

[cheshired5]

Originally Posted by Bernard46

OK thanks, I'll get some cleaner and give that a try. I assume there will be no issue in disconnecting the sensor to clean.

I noticed I could get a replacement MAF Sensor for this vehicle on ebay http://www.ebay.co.uk/itm/ACP-Air-Ma...cAAOSwnDxUflpT for about £28 - would it be worth trying this as well? If I replaced the sensor does one need to "tell" the ECU?

Before buying a MAF, watch this. https://m.youtube.com/watch?v=QtQnE1QW-jU

[oragex]

Lambda sensor was an original Volvo part?

[reggit]

Originally Posted by oragex

Lambda sensor was an original Volvo part?

That's a good point, none original spec lambdas can be troublesome in these cars.

[Bernard46]

No the replacement lambda sensor was not bought from Volvo because of the ridiculous price (nearly £300) but it has been checked again and is working OK. The garage rang me this morning and said they have spoken to a Volvo expert who says he believes given all the symptoms that it is a problem around the inlet manifold area so they have asked for the car back on Wednesday to investigate further. Watch this space!

[Bigneil]

I can't remember the technical explanations, but there is a specific type of lambda sensor in some Volvos that just shows a constant on or off signal based on oxygen level rather than rich/lean. Wideband type. ...

Have a quick look at my lambda sensor thread. Various clever people are suggesting exhaust leaks and intake leaks can manifest as lambda sensor issues.
http://www.volvoforums.org.uk/showthread.php?t=240298

[KBB]

Fault code should be treated as a symptom and not at this stage the cause of the fault. P0172 refers to a "system rich" condition. Under these conditions scan tool live data can give a good direction to go in. Freeze frame data will let the tech know what condition the engine is under at the time the DTC is being set i.e. idle or under load.

If it is setting at idle note the STFT values. MAF sensor that are loosing calibration through heavy contamination usually over estimate air flow at idle and command a rich condition, the O2 sensor will counteract with negative fuel trim values.

When the engine is under load the MAF then under estimates air flow the O2 sees a lean condition and counteracts with high positive STFT corrections.

The starting point has to be live data, freez frame data and then looking at the fuelling commands at idle and then held at load at 2500rpm.

[Bernard46]

After resetting the code it will remain off for a short time - up to 2 or 3 days depending on use is the best I can say. I can drive the car (start and stop the engine) on a number of occasions but it seems the code will come back eventually. When it does come back it comes up instantly at start-up i.e. I can turn the ignition key and everything is normal until the instant the starter motor engages and cranks the engine - it is at this point the code appears. Since the engine fires almost immediately the starter engages it is difficult to say whether the code has come up before it fires or immediately after. It always appears at this point. It never comes on at any other time. It is not as if the engine has had a millisecond to react to any readings.

Today I used the car twice, the first time after it had been standing idle outside for 36 hours - it seemed to struggle a bit after starting as if perhaps it was firing on 4 out of the 5 cylinders and then after 15 - 20 seconds all cylinders started to fire, it ran more smoothly and the revs climbed from 900 to 1500. The car did two short journeys of 3 miles each about 30 mins apart and was then left for 6 hours. This time when it started OK and again drove 3 miles. It was left for an hour or so and then on the next start-up the code came on.

I would have expected the mixture to be a little rich at initial start-up even on a warm engine so I'm a bit puzzled by this code showing instantly at startup, always on a warmish engine (at least the gauge reached normal temperature before the initial 3 miles was up, and the car had been left for slightly over an hour) - could it be that a temperature sensor is telling the ECU it is colder than it actually is and therefore requesting to much fuel?

One other factor that may be relevant - this engine has always (over 15 years) been inclined to absolutely drench the engine in fuel at startup - at least given the very strong smell of petrol a second or two after it has started - and the smell comes from the bonnet area, not the tail pipe. Indeed if on the very rare occasion (once or twice a year) it fails to start on first turn it is quite difficult to start because of apparent flooding. I've never looked under the bonnet at start up to see if there is fuel escaping, and I've no reason to believe it is - I have always assumed this was the way a petrol fuel injected engine worked. This is all probably completely irrelevant but it might just be worth knowing.