My dad's 1999 Octavia 2.0 GLX has been showing a check engine light every couple of weeks or so. It clears with vagcom, but it just keeps coming back. I've read up what i can, and now i'm stuck. Here's what i've done so far...

The fault that's being logged is -

16556 Fuel Trim; Bank 1: System Too Rich P0172-35-00--

First off, i decided to check out the MAF sensor, as i've read plenty of stories about them failing. We did a run in the car and monitored block 002 (airflow). From the info i've read, it should peak at around 0.8 times the car's power output. The 2.0 is 115bhp, so i was looking for about 92g/s of airflow. The car peaked at 98g/s, so i assume that's OK?

Then i read about block 032, which displays the current additive and multiplacitive fuel trim. The additive fuel trim is at -18.0%, and the multiplacitive is +2.3%. As i understand it, this suggests that the engine is running too rich at idle (as it's trimming the fuel back by 18%), but is fine higher up in the revs. This is where i'm stuck. What could be causing the car to run rich at idle? Or have i missed something else?

Any help would be greatly appreciated.

Had this on my RS, unfortunately I can't remember what cured it :(

One for people like Lummox and Big K, might be worth a PM

If you do a search I think you may pick up a post on this by Gwilo.

He changed the MAF and this cleared the intermittant DTC.

I think he bought the replacement MAF from Euro car parts and cost about

Think that bank 1 is refering to the lambda probe.

This can alter the fuel mixture with the ecu.

So think it may want one of those.

Sarah

Bank 1 is the the first probe before the catalyst, it is detecting unburnt fuel in the exhaust. Either the air mass reading is wrong, it is over reading the air going in and therefore putting too much fuel in, or the probe itself is faulty.

However, maybe both of these are working quite correctly and a dribble of fuel is getting in after the injectors have shut off. The cheapest and easiest first thing to do is try some fuel injector cleaner.

will be the lambda or the maf or both i'd put monies on the maf.

I have the same error.

What fixed this?

One of the things that can cause this is a non-functioning fuel pressure regulator. The fuel pressure is too high at idle/cruise and hence the amount of fuel injected is too much, the ECU will try and reduce injection time to compensate but the reduction required is outside the limits and a code is set.

I'm not familiar with this particular car but would suggest a look at the vacuum pipes in this area.

It the engine using oil?

Quite often the 2.0 will burn a little oil and as that enters the inlet system it can effectively alter the mixture thus making it richer. Pop the inlet pipe off and check the throttle body for oil.

Many people change MAF's and Lambda sensors, but more often than not they arent the source of the fault.

IIRC it is block 32 (will check) that also works out altitude adaption, this should be 0 +or- 10%. Watch that value and then clamp off the engine breather hose and see if it alters. If it does its breathing heavily and will probably be down the piston rings.

I had the same problem... it gradually got worse and after several visits to the garage had a new lambda probe fitted - i decided to clean the MAF and cured it overnight.

The most likely problemis the lambda probe (bank1 probe1) if the problem continues you will have to replace the lambda probe, try cleaning the throtle body and reset the basic settings first.

The lambda probes can and do go wrong, But if the engine is using oil this leads to lambda probe failure. So replacing the probe sorts it, but only until the new probe gets clogged up again.

I had the same fault and recently posted a thread about it... I had a dirt MAF ... the problem continued for ages and eventually led to replacing the Lambda... Clean the MAF and see what happens.

I reseted the error with vag-com (suggester by the service),

and it didnt come back. Seemed that it might have been some

error since it showed only once when temperature rose from

near -20 to -5C in about 30minutes.