2005 VW Polo 1.2 - P0172 - Fuel Trim too Rich

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Wed 17 Dec 2014 23:49
2005 VW Polo 1.2 - P0172 - Fuel Trim too Rich - PX-230A
I replaced the catalytic converter two weeks ago, along with the pre and post lambda sensors - the converter had a hole in it so needed doing. Brand new OEM parts were used.

The car has been running perfectly since (it was very sluggish and used tons of fuel). I now have error code P0172 coming up. The car is again noticeably sluggish when accelerating between 1500 and 2000 RPM. Fuel economy seems fine. When I clear the code, it returns within a few miles - the engine light never comes on though.

Any ideas what the cause could be?
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Thu 18 Dec 2014 00:51
2005 VW Polo 1.2 - P0172 - Fuel Trim too Rich - elekie&a/c doctor

This will be the 3 cylinder engine that gives continuous problems.I would get the cylinder compressions tested.

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Thu 18 Dec 2014 09:01
2005 VW Polo 1.2 - P0172 - Fuel Trim too Rich - PX-230A
Thanks - I can certainly give that a try.

Is the problem not likely to be electrical or sensor related?
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Thu 18 Dec 2014 09:01
2005 VW Polo 1.2 - P0172 - Fuel Trim too Rich - Railroad.
If you have a scantool check manifold vacuum by reading the MAP sensor live data. You could have a valve timing problem, and this would show that in low but steady manifold vacuum.
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Thu 18 Dec 2014 13:32
2005 VW Polo 1.2 - P0172 - Fuel Trim too Rich - PX-230A
Compression is spot on...always a good sign!

Somebody suggested either the coolant temperature sensor (is this the same as the engine temperature sensor?) or an injector fault?
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Thu 18 Dec 2014 13:45
2005 VW Polo 1.2 - P0172 - Fuel Trim too Rich - Gibbo_Wirral

It could always be a faulty part. New ones can come faulty. I was on another forum recently and there was someone with a similar problem - EML coming on after a new cat was fitted. The cat was swapped and all has been fine.

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Thu 18 Dec 2014 13:59
2005 VW Polo 1.2 - P0172 - Fuel Trim too Rich - Railroad.
If the compression pressures on all cylinders are much the same and around 175psi then you can pretty much eliminate a valve timing problem. A manifold vacuum check is always worth doing. You can tell a lot about an engine by measuring manifold vacuum, including a blocked exhaust which could be the cause of a problem generating P0172 - System too rich. The ECM is taking fuel away to compensate for the rich mixture, but you need to ask yourself why is the engine running rich, and sufficiently rich for the ECM to compensate to the point where this code is set. A blocked exhaust is one reason why this could happen, and it would certainly explain you car's poor performance.
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Thu 18 Dec 2014 16:40
2005 VW Polo 1.2 - P0172 - Fuel Trim too Rich - PX-230A

Thank you for all the replies!

The only section of the exhaust that is original is the rear section (everything from the first silencer backwards) - the centre section (flexi-pipe and downpipe from catalytic converter) was changed recently due to a hole. Can the rear section actually get blocked? I thought the silencers were just hollow spaces?

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Fri 19 Dec 2014 09:15
2005 VW Polo 1.2 - P0172 - Fuel Trim too Rich - Railroad.
Once again, use a vacuum gauge to test manifold vacuum. It will tell you if the exhaust is blocked. Do a Google search on how to test manifold vacuum. There's loads of information on the Internet to help you.
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Sat 20 Dec 2014 13:42
2005 VW Polo 1.2 - P0172 - Fuel Trim too Rich - Railroad.
Another cause of an engine running rich would be a restricted fuel return line. Make sure you haven't accidentally kinked or trapped a return fuel pipe when you fitted your new exhaust.
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Sun 31 Jan 2021 13:16
2005 VW Polo 1.2 - P0172 - Fuel Trim too Rich - Nikolahlad

I have the same problem now, have you ever found a solution and if so, what is the solution?

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Mon 1 Feb 2021 11:20
2005 VW Polo 1.2 - P0172 - Fuel Trim too Rich - edlithgow

4WIW I cleared some debris from my rear exhaust section by detaching it and back- flushing it out with a garden hose.

I'd tried measuring manifold vacuum and couldn't interpret the results (at least from any description I found on the internet) so I made an adaptor to measure exhaust back pressure directly.

On a cat-equipped car you can do this by removing the oxygen sensor, This allows direct measurement of exhaust back pressure if you can get an adaptor that fits the sensor port.

More simply, it gives an alternative exit for the exhaust gases, so if the car then runs better, that indicates a blockage aft of the sensor port.

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