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P0171 code garage no idea what to do next

allenhh

Member
Joined
Jul 7, 2019
Messages
36
Location
devon
Car
C180 2002
Story so far, W203 C180 2002 went in for MOT and the CEL came on, code P171(lean system) and so failed due to high emmisions. The bright sparks at kwikfit said the CAT had gone so needed a new one, £700..so I had one fitted at another garage for £350 back to kwikfit for retest, and low and behold failed again on emmisions, but the excuse this time was it was a cheap CAT that caused the fail. Went to a merc specialist who replaced 3 yes 3 alleged leaking vacumn pipes, cleaned MAF as well .No change in the emmisions.So they are now at a loss on what to do next, so I thought I would ask this font of knowledge their opinion.I have a feeling though I shall just scap the car as its turning into a money pit.Thanks in advance.
 
You don't say if it was CO of HC but if running lean I assume probably CO, if it was HC then it's probably electrical and weird.

Can you look at live data or are you just stuck with the code? If it's running weak then - as with the MB specialist I would expect vacuum leaks first but as these now fixed start with the sillies, is the petrol tank venting OK and are the fuel filters clean (newish and unobstructed - no sh*t in the lines)? Do you often run it near empty? Do you use supermarket fuel?

If these are fine then I would assume (possibly wrongly) that in the absence of other codes the fuel pumps are fully functional so it points to injectors. Have you tried some injector cleaner? If not make sure you put in a decent quantity of PREMIUM fuel (high octane non supermarket branded) with the cleaner and give the car a good blast to clean out the system Can you try a leak off test (may be more indicative of overfuelling which is not your case but can give an indication) can you get them out and checked?
 
Thanks for that, it failed the MOT on all the emmisions,all of them were way over, and one of the mechanics said he could smell fuel in the exhaust, which makes sense as the emu is sending more fuel as it thinks its lean.I recently bought this car so dont know about the fuel, it has been regulary serviced by mercedes so they have been chucking cleaners etc in the fuel (even though it says dont do this in the owners manual)The garage are having another look this afternoon so I shall see what happens.
 
Good luck, the smell of fuel in the exhaust could still point to too lean (weak) a mixture since if it is really too weak the combustion process may not be complete. I am assuming there is no obvious misfire or other sign of not running properly and that your's is Petrol. If there is any sign of not running properly then it could be a combination of injectors and/or the other things limiting the fuel supply hence incomplete combustion and the smell. Was it read by a decent code reader because a second read by a Star or MB specific reader may reveal more especially if you can look at live data (and know how to interpret it) which will hopefully give you a more conclusive answer.
 
There is a software update for this issue.

Also check the oxygen sensors for oil ingress.
 
The garage has a star reader so I hope they used it, they are having another look to see whats happening, and I wonder @BlackC55 if they would update the software or is it only MB garage that will do that?If oil was in the O2 sensor wouldnt star indicate this?
The car runs fine with the CEL on and tickover is steady it even comes up with 35mpg though I think that is a bit of wishful thinking on the cars part....
 
Another punter misdirected by Kwikfit... They may have meant well enough, or they may have intended to make a mug of you, but either way, with something like this your first port of call should be getting the codes read by somebody who knows what they're doing - ideally by a good independent, not a pricey main dealer.

Is the 'specialist' you then went to a real MB specialist, or just a self-proclaimed 'German Car Specialist'? (There's one of those near me; I've been there once, but I won't make that mistake again...).

I hope it's not too expensive to fix properly, and wish you better luck for the future.
 
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Sounds to me like the oxygen sensor is faulty, telling the ecu that it has a lean mixture, the ecu then dumps more fuel in which is not burnt properly, the gas tester at the mot centre is reading emissions from the exhaust which would quite understandably read rich mixture, high hc and co.
Fuel getting poured down the exhaust can also damage the cat.
Other things can cause these results including a faulty ecu and faulty wiring loom which I have seen before causing Oxygen sensors to burn out.
Fingers crossed it's the O2 sensor as by far the cheapest fix.
 
Another punter misdirected by Kwikfit... They may have meant well enough, or they may have intended to make a mug of you, but either way, with something like this your first port of call should be getting the codes read by somebody who knows what they're doing - ideally by a good independent, not a pricey main dealer.

Is the 'specialist' you then went to a real MB specialist, or just a self-proclaimed 'German Car Specialist'? (There's one of those near me; I've been there once, but I won't make that mistake again...).

I hope it's not too expensive to fix properly, and wish you better luck for the future.

I think kwikfit were trying it on as £700 is a bit steep for a cat, I had read the code myself so I knew it was lean but they said their reader would give more detail. wrong...any way I got a refund from them.The specialist is real (I hope) as they have a star reader but not sure if they are any good as they have had the car for a week now and they still cant fix it.
 
Sounds to me like the oxygen sensor is faulty, telling the ecu that it has a lean mixture, the ecu then dumps more fuel in which is not burnt properly, the gas tester at the mot centre is reading emissions from the exhaust which would quite understandably read rich mixture, high hc and co.
Fuel getting poured down the exhaust can also damage the cat.
Other things can cause these results including a faulty ecu and faulty wiring loom which I have seen before causing Oxygen sensors to burn out.
Fingers crossed it's the O2 sensor as by far the cheapest fix.

It had 2 new sensors last year, though that doesnt mean they are working, but shouldnt the star come up with an error code if they are knackered?
 
Personally I'd be taking it for a run and looking at / saving all the relevant data. It is most often a sensor problem but if you save all the real time data relating to air, sparks, timing and fuel over a few miles of varied driving you can often figure out what's happening. A breaking down coil, knackered spark plug or bad timing on the simple electrical side can cause problems just as can wrong mixture, pumps, filters, injectors and faulty sensors, lambda or otherwise. If the wiring is faulty the reader will almost always show it, as it will if a sensor had failed or is inconsistent but not if it's just plain wrong. You can often do some basic tests on sensors with a multimeter and a scope which can sometimes show if they are giving a wrong signal The net result of your issue is incomplete combustion if HCs are high so it's a case of working back from there. Good Luck
 
If they have an official Star they can do software updates like we can.

Look for oil in the engine loom at the o2 sensors. If they are a MB specialist they will know all about it.
 
If they have an official Star they can do software updates like we can.

Look for oil in the engine loom at the o2 sensors. If they are a MB specialist they will know all about it.

I did mention the update to the garage but didnt get a reply, however I have got a list of what has been done, smoke test done and 3 vacumn/breather pipes replaced,found a little oil in wiring loom at ecu,maf voltage was out so changed maf but no change to emmisions so put the old one back,remove lambas clean and refitted, fuel pressure checked,emmisions still out ,so they have run out of things to do. Anyone have any thoughts or shall I just scrap it..
 
The plot thickens, I purchased a bluetooth OBD reader and looked at the fuel trims, the short term hangs around 0 and-5, the long term is at 16.41 from what I can gather thats way to high and the ECU has memorised this value, I have disconnected the battery for a couple of hours, but the value comes back to 16.41. I found somewhere here someone had the same problem and had the ECU flashed at a mercedes dealer,and this fixed the problem. Has anyone heard of this?
Thanks
 
The plot thickens, I purchased a bluetooth OBD reader and looked at the fuel trims, the short term hangs around 0 and-5, the long term is at 16.41 from what I can gather thats way to high and the ECU has memorised this value, I have disconnected the battery for a couple of hours, but the value comes back to 16.41. I found somewhere here someone had the same problem and had the ECU flashed at a mercedes dealer,and this fixed the problem. Has anyone heard of this?
Thanks
UPDATE
just been stung by stealership for 4 hrs "investigation" and they cant find anything wrong.The only suggestion they have come up with is get a new O2 sensor for a mear £400 and that might cure it.yes I have heared ducks fart before.So it looks like the car is off to the scrapyard. Thanks to everyone for their help..
 
Story so far, W203 C180 2002 went in for MOT and the CEL came on, code P171(lean system) and so failed due to high emmisions. The bright sparks at kwikfit said the CAT had gone so needed a new one, £700..so I had one fitted at another garage for £350 back to kwikfit for retest, and low and behold failed again on emmisions, but the excuse this time was it was a cheap CAT that caused the fail. Went to a merc specialist who replaced 3 yes 3 alleged leaking vacumn pipes, cleaned MAF as well .No change in the emmisions.So they are now at a loss on what to do next, so I thought I would ask this font of knowledge their opinion.I
I did mention the update to the garage but didnt get a reply, however I have got a list of what has been done, smoke test done and 3 vacumn/breather pipes replaced,found a little oil in wiring loom at ecu,maf voltage was out so changed maf but no change to emmisions so put the old one back,remove lambas clean and refitted, fuel pressure checked,emmisions still out ,so they have run out of things to do. Anyone have any thoughts or shall I just scrap it..

have a feeling though I shall just scap the car as its turning into a money pit.Thanks in advance.
I had a similar problem on my C200 kompresser failed mot on both high co and hc, first found engine breather pipe split behind supercharger to one way valve ,no better, changed maff, no better, found oil in engine loom back to ecu, fit another loom, no better, took it to main steelers £150 for a scan - upstream sensor fault, no better, fit another ecu and had it cloned, no better bought myself a good scanner because i was pissing in the wind, found downstream sensor faulty fit new and he presto passed- i think it was the loom knackering up everything, maybe not the ecu but peace of mind, if this helps, many long unpayed hours of scratching my head .
 

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