Would someone mind pointing out which vagcom tests to run and how to tell which lambda needs replacing?

I've read not to take fault codes as gospel as the precat can throw the postcat off or something along those lines......and how a lambda fault can cause an evap canister fault.

Kinda hoping sorting the lambda will improve mpg in case it's causing too rich a mixture.....

Endless gratitude in advance!

The lambdas will be shown as B1, S1 or B1,S2. B stands for bank as in which bank of cylinders, in an inline engine there is only one bank. S stands for sensor, sensor 1 is the first sensor that the combustion gasses will encounter as they leave the cylinder, so it will be the one before the cat and sensor 2 will be the one after the cat.

Lambda readings at normal operating temperature when the system is in closed loop mode have a direct impact on the amount of fuel injected. When the carbon cannister is purged, the air entering the cylinders will have fuel vapour in it, so as the purge occurs the fuel injectors will inject slightly less fuel so as to keep the fuelling correct. It could be that the cannister solenoid is not opening correctly and not allowing enough or too much vapour through. As the ECU will be expecting a normal mixture, it will be getting a leaner/richer mixture which the lambdas are picking up and throwing a fault.

It could be more like an evap cannister fault causing a lambda code to be logged.

There are lambda ageing tests that you can run, if you look at the label file for your ECU, you'll see what you can measure.

Thank you so much!

Given that I've changed the EVAP canister, it's either the solenoid or the lambda. What would be normal readings that I should look out for?

Or will it be easier for me to just do a bit of trial and error....change one lambda, see how it goes?

Trial and error replacing things is hit and miss, as 'fault' codes do not define a faulty sensor, they give symptoms and effects of the cause of the problem. The best thing to do is to get the codes read, post them up on here and get the benefit of the experienced people here.

With VCDS you can do output tests, where it will actuate various items and you should be able to hear/feel them working.

You mean get the codes read with VCDS as opposed to the handheld fault reader? If it's the latter, I'm getting these two:

http://wiki.ross-tec...24/P0140/000320

http://wiki.ross-tec...25/P0441/001089

Someone once mentioned getting a vacuum pump to test the viability of the pipes to and from the charcoal canister, but since I don't have one at my disposal, I was hoping it might be easier to ask someone to run the relevant VCDS output tests on both lambdas?

Am I barking up the wrong tree?

Just to be sure....a faulty lambda can send too rich a mixture? If so, more incentive to get this sorted and improve mpgs...

Edited 28 February, 201214 yr by fsa

What you want is...

http://www.vagcom.co.uk/petrol_engine_demo.htm

As for lambda reading values...

I think I supplied you a little while back with the nominal readings - but I can dig em out again if you need?

Chris! Oh yes please....I think you gave the ones for the pre-cat but the post-cat is currently throwing codes....or maybe it was the other way around... You are a lifesaver, sir!