2005 Volkswagen Polo 1.2 E (55 PS) - Help with Diagnostic Codes

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Sun 14 Sep 2014 19:00
2005 Volkswagen Polo 1.2 E (55 PS) - Help with Diagnostic Codes - Braunschweig

A few months ago, I was regularly getting an engine light along with code P0420 (catalyst system efficiency below threshold - bank 1), but no noticeable effects on driving or performance.

I chucked some Cataclean in the tank a few weeks back, cleared the code and the engine light has only just come on again today, but now it's code P0172 (fuel trim too rich - bank 1) and acceleration is slightly hesitant sometimes. The same code has come up everytime I've driven the car today.

How can it have suddenly gone from code P0420 to P0172? Is it likely to be the catalytic convertor or the pre-cat lambda sensor, or both?

Thanks in advance for any advice.

Edited by Braunschweig on 14/09/2014 at 19:02

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Sun 14 Sep 2014 20:09
2005 Volkswagen Polo 1.2 E (55 PS) - Help with Diagnostic Codes - Railroad.
P0420 is generated when the pre and post lambda sensor readings are too similar, indicating that the catalyst efficiency is low. The usual cure is to replace it. Beware of cheaper non genuine parts though. They're cheap for a reason.
The fuel system is closed loop once the engine is warm. The ECU receives information from the lambda sensors regarding how rich or lean the mixture is. The ECU is continuously switching from rich to lean and back again. This can be seen on a graph if you have a suitable scan tool, or with a voltmeter. Fuel trim is the ECU's correction functionality. Ideally both the short term and long term should be around the zero point. A positive percentage means the engine is running lean and the ECU is correcting by richening by the percentage shown so that the correct emissions can be maintained. A negative percentage means the opposite. Around 10% either way is perfectly normal, but a fault code of fuel trim too rich or too lean will be generated at around 25% correction.
Don't jump to blame the sensors because they may not be where the fault lies. Check for air leaks, exhaust security and for leaks, fuel pressure and pump operation, spark plug condition and even valve timing.
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Sun 14 Sep 2014 21:53
2005 Volkswagen Polo 1.2 E (55 PS) - Help with Diagnostic Codes - Braunschweig
At my last MOT in October 2013, the car literally scraped through the emissions test. The tester reckoned the cat is bad. It's just strange that the error code has now changed...the cat can't have fixed itself, unless like you say there's a leak somewhere which is causing the problem.

Why does the fuel trim error make acceleration hesitant sometimes?
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Mon 15 Sep 2014 09:09
2005 Volkswagen Polo 1.2 E (55 PS) - Help with Diagnostic Codes - elekie&a/c doctor

Excellent description from mr RR.But you need to remember that the fault codes logged are the result of a problem not the cause.Unfortunately these 3 cylinder engines are notorious for engine running and performance issues with loads of timing chain and burnt valves giving low compressions.

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Mon 15 Sep 2014 11:14
2005 Volkswagen Polo 1.2 E (55 PS) - Help with Diagnostic Codes - MrEckerslikefromRamsbottom

Strikes me that the invention of the transistor is responsible for all the troubles in this world...

What was wrong with twin SU carburettors? You could get hours of pleasure fettling them with a little screwdriver! Having a computer do it whilst your driving is no fun at all!

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Mon 15 Sep 2014 11:49
2005 Volkswagen Polo 1.2 E (55 PS) - Help with Diagnostic Codes - Braunschweig

Yes the three cylinders aren't the best engines...I've had a few problems with it in the 150,000 miles I've had it. When it runs, it runs fine and fuel economy is good.

So from what I've described is it likely to be a problem with the cat / lambda sensors? The garage said at the MOT last year that it scraped through the emissions and suggested the cat was on its way out.

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