Apologies in advance for the long post!

 

My car has had a recent full service - spark plugs, oil filter and air filter change etc and I recently had my MOT done on my 1.2 12V Skoda Fabia and it failed on emissions, I was advised that the likely cause was that both lambda sensors needed replacing, when I said I had limited funds they said well it's most likely just the upstream lambda sensor that needs replacing (the first one accessed from the top of the engine bay). They charged 93.50 Including VAT @ 20% for the part - no labor was declared on the invoice or in person as said I do my own servicing and could have quite easily replaced the sensor if I had known, anyway I've since looked on ECP and they seem to be anything from £25-£78 (lowest & highest prices).

 

My questions are:

 

1> Can a lambda sensor fail with no Emissions warning light? – Mine didn’t illuminate and I thought that was the whole point of the warning light was to alert you when the lambda sensors are not operating within their parameters etc.

 

2> How long do lambda sensors generally last and would it fail that quickly over 1 year when you compare to last years emissions results? - All test results at the bottom of this post!

 

3> What would I notice as a driver if I had a faulty lambda?  There was no excessive smoke, no rich mixture fuel smell, no fast or lumpy idle and fuel consumption was good

 

4> What will I notice as a driver with the new lambda sensor installed (if anything?) - as I haven't noticed any difference on performance/idling etc

 

5> Are there any tricks that can be performed during the emissions test by an MOT tester to get a deliberate fail if business is slow? Could the tester have used a faulty detector in the exhaust or manipulated the RPM's or test to put the emissions outside the set parameters?

 

6> Do you think I was overcharged for the lambda sensor or that labor was sneakily included - if so is not declaring labor separately in the interests of transparency against the law ?

 

7> Although the garage retained the old sensor if I put a complaint to head office could the lambda probe be tested for operation as I'm not fully convinced I actually needed one?

 

I would really appreciate to get comments from people (& MOT testers!) that know about emissions tests/lambda sensors because if it needs replacing then obviously I’m happy the tester did his job to protect the car and environment from further damage, but if I got done over then will be putting complaint to head office/VOSA and naming/shaming etc!

 

Also I can’t necessarily blame the person who fitted the part for this, but on the 1.2 engine the air filter is built into the engine cover and there are 2 pipes that connect to the air filter box, 1 on the bottom and 1 on the side. The 1 on the side was left unplugged – good job I checked - I always do whenever anyone looks under the engine cover because if you don’t know the pipes connect on this engine cover it’s easily missed when carelessly tugging it off etc. I’m not sure exactly what this hose is for – can anyone enlighten me? But as it was only off for 1 shortish journey I hope it hasn’t done any lasting damage to anything else!!

 

Below are the results from last years test, this years refusal, and this years pass after the replacement lambda, the engine oil temp was warm on all 3 tests. I notice the C0 % vol limits changed between the fast idle tests this year, so was the tester running different settings to throw it off? Also the fast idle speed was 2350/2650 on the first failed test and 2500/3000 on the second passed test - don't know if that makes any difference or if the testing equipment accounts for the variation in RPM.

 

Last years performed at different MOT centre:

Fast Idle Test: Pass (Manual Check)
C0 % VOL: Limits: <0.20 Actual: 0.02 Result: Pass
HC ppm vol: Limits: <200 Actual: 0 Result: Pass
Lambda: Limits: 0.970-1.030 Actual: 1.002 Result: Pass

Natural Idle Test: Pass (Manual Check)
C0 % vol: Limits: <0.30 Actual: 0.03 Result: Pass

 

This years refusal:

Fast Idle Test: Pass (Manual Check)
C0 % VOL: Limits: 0.30 Actual: 2.51 Result: Fail
HC ppm vol: Limits: 200 Actual: 114 Result: Pass
Lambda: Limits: 0.97-1.03 Actual: 0.93 Result: Fail

Natural Idle Test: Pass (Manual Check)
C0 % vol: Limits: 0.50 Actual: 0.50 Result: Pass

 

This years pass after lambda replacement:

Fast Idle Test: Pass (Manual Check)
C0 % VOL: Limits: 0.20 Actual: 0.01 Result: Pass
HC ppm vol: Limits: 200 Actual: 6 Result: Pass
Lambda: Limits: 0.97-1.03 Actual: 1.00 Result: Pass

Natural Idle Test: Pass (Manual Check)
C0 % vol: Limits: 0.30 Actual: 0.04 Result: Pass

 

 

1) yes it is possible for the engine management light to not be illuminated with a failed sensor, especially if it's the bank sensor which has failed.

2) theres no hard and fast rule as to how long a sensor will last, but they are a consumable part, they generally last around 100k sometimes less.

3) sometimes you will not notice a thing, judging by your results it was running a little rich hence why you have a low lambda reading and a high co reading, you may get slight misfires under heavy acceleration with a rich fuel mixture like that but to be fair it's not running massively rich.

4) what you need to understand is that there is 2 probes, and failure of one is not catastrophic, so the engine can run perfectly happy with only one working, the second probe is only there to check the efficiency of the catalytic convertors, but it can run o. This sensor alone in a kind of limp mode in such a way that you would never even know the difference.

5) yes ,there are tricks, but I call red herring on that theory, its difficult to manipulate the results on a fuel injected engine, for example you could do something like unplug a fuel injector but the results would be wildly out of kilter, your fail results are only a tiny bit out

6) they vary in price, the wideband sensor which is the one before the cat is around £120 from skoda if i recall, a good quality pattern part from Bosch or ntk is £80 to £100

Going by your results from your failure I think they made a good call on changing the lambda probe, the car was obviously running rich, I don't think you were ripped off personally but maybe they should have consulted you before fitting a new probe to the car and spending your money

re; the different test limits, this could be because the emissions were only out a by a small amount so they decided to carry out an extended test which uses vehicle specific data from a list rather than generic limits based on year/cc etc.

Agree; sounds like they fixed your car for you, at reasonable cost.  You may well find your fuel consumption figures improve slightly too.

 

Regarding the two air pipes that go from the engine cover/airbox: one supplies fresh filtered air into the engine breather system, the other to the EGR valve.  Having either/both disconnected for a short time will cause no problem whatever, relax.

Thanks for your replies guys and for putting my mind at rest - much appreciated.