Can a lambda sensor be faulty with no codes showing?

[flat4fanatic]

Polo failed the mot today on carbon monoxide levels (2002 1.4 16v)
mot place said likely to be lambda sensor but the only thing that comes up with vagcom is egr insufficient flow.
not got £140 to throw away on the possibility of it being what they said.

anyone any ideas?

the eml has been on for about a year, full head recondition last year due to exhaust valves, no noticeable change to performance or mpg

thanks,
steve

[Rob69]

How do they know it is an O2 sensor- Did they do a scan on it or just what the MOT tester said it might be? If you're getting an EGR fault then check the EGR valve for operation. If it was all stripped down for the head job was it set up in adaptations when it was refitted, I'm thinking it's the electric actuated EGR on that engine?

[flat4fanatic]

Thanks for the reply, really appreciated. Thats just their idea of what to try first so Im not convinced as theres no codes to back it up.
I read the codes when I got it back.
The egr fault has been around off and on for the last 3 years (got the car off a mate last year) and has been ok at mots inc the one after the head was done. Ive got the rosstech full version of the diagnostics, but I inherited it off a mate who passed away so reading the codes and clearing them is about as far as Ive got with it. Pretty sure you are right about the electric version. Can that be cleaned out?

Something that I came across whilst browsing - some people have mentioned emissions failing because of a hole in the exhaust affecting the sensor. Can detect a bit of a blow from somewhere near the front, so will have a look in a bit. Thanks

ps i doubt it was set up after the head was done as i dont think the guy who did it has the software

[Rob69]

Have a look at this old thread and yes an exhaust leak could affect O2 sensor reading
Question Pre+Post Cat / Lambda / Oxygen sensor tests - help needed with results... [Archive] - VW Audi Forum - The #1 Volkswagen (VW) Group Forum (http://www.vwaudiforum.co.uk/forum/archive/index.php/t-140004.html)

[flat4fanatic]

Had a look, bit of a leak from the overlapped joining section just behind the cat & sensor so have sealed that.
had a look at the egr and its been blanked off where the pipe goes in towards the back of the engine, going to take the blank out and see if I can get the code to go away. Thanks for the link but I think thats a bit beyond me ( give me a nice simple mk2 gti anyday). Cheers

[zollaf]

i would elave the blank in there, its been fine like it for ages, get it through the mot first. have you got the print out of the readings from the gas test. could be a leak, that can allow frsh air into the exhaust and over the lambda, which then thinks the car is running lean so bungs in more fuel to compensate. but the best way to tell is to see the readings.

[flat4fanatic]

Thanks, will dig them out after work

[flat4fanatic]

Ah, no readings available. Fail sheet says:
exhaust emissions carbon monoxide content at idle excessive (7.3.d.4)
exhaust emissions carbon monoxide content after second fast idle excessive (ssame code)
exhaust emissions lambda reading after second fast idle outside specified limits

[zollaf]

need the the readings to diagnose, impossible otherwise. they binned it instead of giving it to you, should get them to test it again and get the printout. worth sticking a can of injector cleaner in it and driving the nuts off it, test it while its nice n hot, might pass. never know...

[flat4fanatic]

Been back readings are:
0.447 co (fail)
180 hc ok
0.999 (some symbol)

no error codes showing on vcds

need to try and get it sorted this afternoon.
any ideas? Thanks


did notice the backbox was a bit hanging, but no holes/passed mot

[zollaf]

so its just the co slightly high, hydrocarbons and lambda are both bang on. co is probably about bang on as well, but the cats not working as well as it should. my money is on the cat. in this case i might be tempted to buy a bottle of catclean then go for a proper italian test tune, i.e on the redline as much as possible, foot down, get the cat as hot as you can to get it clean, then get it tested immediately, before it has time to cool even a little bit.

[Crasher]

It is likely that running it for a year with the MIL on has caused damage to the catalyst, that is why it is a warning light, not an advisory light. Your CO, HC and lambda readings are perfect PRE cat, with that lambda reading a working cat would, err well work!

[zollaf]

forgot the bit about the mil light being on for a year. blimey, you should have just put some tape over it like most people do.

[Crasher]

Ahh yes, the old burying one's head in the sand principle.

[flat4fanatic]

Thanks guys, advice much appreciated, cat was spot on. Now with full mot. Cat was significantly less that I was expecting too.
Light's been on since the prev owner blanked the egr, but replaced it with a good one last week so no light now.
Cant complain though, the car was a freebie off a mate.