since i bought the car at the beginning of last month ive had the MIL flick on 3 times each time the only 'common' fault has been P0420

i dont think the exh is blowing ive looked but not very hard and it had a cat back mild steel system fitted for its MOT (just before i bought the car)

just thought it was weird that i could clear the code and it wouldnt come back for a week then *bing* MIL on and when i get FES out its the same P0420 back again... FES always reports "Invalid Signal > Fatal > Light ON" yet i can clear... what does Fatal actually mean in FES anyhow?

I would study the two lambda readouts and see what your car is spotting. It compares the two readouts and triggers the P0420 if the results are different than expected

What other fault codes is it displaying when you get the P0420?

last 2 times None... just P0420

1st time i cleared i had these

There's a P0136 there on your post cat O2 sensor
That'll trigger a P0420 given time as it returns out of limits performance

so it could be the post CAT lambda?

could make sense as it would have had to have been disturbed to put the replacement exhaust in... how susceptible are the lambda wires to failure when replacing exhaust pipework?

Depends how clumsy you are

Study your lambda outputs and see what your car is trying to tell you. If you're lucky you may have some freeze frame data which will tell you exactly what was happening at the time and why the MIL occured

Very often the fault needs to occur a number of times before the car OBD is convinced there is a problem there which is why you can go a week perhaps before it reappears

ill be de-modding my ELM cable then... didnt think about FF data, can FES read FF?

MIL back as of last night... barely gone thru £30 worth of fuel... still need to hook up to car to grab fault codes and monitor lambdas. what exactly should i be looking for with lambda monitoring? i know the parameter tab has pre and post cat lambda 'monitoring' they flick between open and closed loop and running lean and rich but im guessing thats 'normal' if i can get the graphs hooked up for looking at lambdas what should i be seeing??

If your MIL is P0420 then that is a cat efficiency warning. It doesn't mean there's anything wrong with your cat or lambdas but it does mean something isn't quite right with what is coming down the exhaust

The front lambda does most of the work in telling the ECU how to trim the fuel for best economy so you should see that oscillating when at steady rpm and everything warmed up and in closed loop

What would trigger a P0420?

Pre cat lambda should be oscillating and post cat lambda signal should be lower than front lambda and fairly steady in this state. If the front and rear lambda signals were very similar then it would mean the cat isn't doing much and get the OBD suspicious. It won't fire the code straightaway, it'll make a note, put it on pending, and only if it keeps looking wrong will it put up the MIL

A misfire, an exhaust leak, wrong mixture will also make the lambda traces "not look right" and can trigger P0420


Here's a good set of readings for comparison. With engine at warm steady rpm, front lambda (V1) is switching hi/lo around every sec and 2nd lambda (V2) is much lower and fairly steady. So this would get thumbs up from the OBD as it thinks "Yep, cat is doing it's job"

The signal from o2 sensors should be like this: http://merkur_sound.sweb.cz/lambdy_eng.png , this is from my 1.6 stilo.

@deckchair:- what program is that your using? looks useful as a ....

@Hanz:- Cheers bud nice one

That's a Gendan program I'm using. I find it difficult to see what's going on with FES graphs

The lambdas signals are largely ignored if you are playing with the gas. Lambdas are used as fuel trimming when the rpm is steady either at idle or steady speed



They should look like this at idle, also Gendan program. Lambdas are switching every sec and lambda 2 is much lower than lambda 1 so cat seems to be working well

Deckchair5: Do you think, that o2 sensor after cat is not ok on my graph? I think, that values between 0,6-0,7V on idle are ok, because there is not oxygen after cat - all residual oxygen is burned in cat.

You need to look at the graph traces when the rpm is steady and not "playing with the gas".

When in steady state then readouts should be similar to the graph I posted above. Your OBD will more often spot a problem before you do as it's constantly monitoring them and knows what to expect. For instance, it will look very different if you have aircon switched on as you will have a high idle = more fuel going in


See the effect here on the post cat O2 sensor at idle when I switch aircon ON half way through the trace. The figures are not so important (as long as they are within the max/ min and changing rapidly enough), it's what's happening v what you expect should be happening that makes OBD suspicious of a problem

I don't know... I found this graph from some Lexus: http://www.mypicx.com/uploadimg/930206311_09092010_1.gif
It looks very similar to my graph.
When I look to your last picture, after aircon switched on, 2nd o2 sensor moved to 0,7-0,8V values.
But I have not aircon.
Maybe our catalytic converters are on different phase of lifetime...
But I was half a year ago on emission check with perfect result.

The link doesn't work for me but beware of comparing your car with Toyotas as they often use different types of lambda sensors

Your O2 sensors are working fine, they are within range and responsive so your car will warn you if there are any problems


Here's a car with 3x O2 sensors, 2 x pre cat and one post cat. The post cat sensor is very unresponsive and needs changing

It is clear, that o2 sensor with values about 0-0,1V is bad one.
My opinion is, that after cat o2 sensor should have about 0.6V, like it is explained here in message #10 : http://www.fullsizechevy.com/forum/...al-maintenance/391069-p0430-wont-go-away.html

Rather than explain what it should have then it's easier to explain what it shouldn't be doing because that is what triggers the OBD

The post cat sensor shouldn't be copying the pre cat sensor when at idle as that would indicate a bad cat efficiency, it should be a fairly steady waveform whilst the precat sensor goes hi/lo

http://www.obd-codes.com/p0420

just been out there for a while in a nice warm car!

heres some screen dumps from FES:-


Import CSV data for FES:-
View attachment importCSV.zip

and 'fuel trim' screen dump from Pro Scan 5.7


sooooo.... still seriously expecting Post-CAT lambda?!

See if you can post up pre cat and post cat lambda waveforms on warm steady idle for a longer period

Your .csv shows your post cat lambda working but copying the pre cat lambda signal too much and that's what your OBD is concerned about