Lambda Probe - MOT Emmissions - bmw. - kris_kros
I recently had my 1997 R reg BMW 323i Auto MOT\'d at a BMW dealership in Warwick. The car failed on emmissions and i was told the Lambda Probe would require replacing. Subsequently, having had the work completed, on examining the emmissions Gas Analysis sheet i noticed that the lambda test had in fact passed and the o2 emmissions had failed. The bill was itemised for both an oxygen sensor and a lambda probe. Was it really necessary to replace both of these? Additionally, do i have any grounds for complaint given the parameters within which MOT testers can apparently work, in terms of justifying work done? I still have the paperwork somewhere if more info needed. Many thanks.
Lambda Probe - MOT Emmissions - bmw. - John S
I'm confused. There is no specific check for O2, so it can't fail on this. The car is checked for CO (carbon monoxide) and HC (hydrocarbons) plus lambda. A lambda of 1.00 means the mixture is correct for the cat to work. It must be between 0.97 and 1.03 on test. The O2 is read by the lambda probe, otherwise known as an oxygen sensor. I don't understand how you appear to have had two replaced, esp id the lambda was correct.


John S
Lambda Probe - MOT Emmissions - bmw. - kris_kros
Apologies, i shall just give you the facts. The fast idle test results were as follows:

CO Level = 0.52% Fail (max 0.30)
HC level = 118ppm Pass (max 200)
Lambda = 0.98 Pass (min 0.97, max 1.03)

A second and natural idle test were also run with the same pass/fail results as above.

On the Service invoice:

Labour charge - replace oxygen sensor
Parts charge - Lamda Probe M52 38

So, if the lambda probe reads the o2 level then how can the Lambda test pass but the CO level test fail?

Lambda Probe - MOT Emmissions - bmw. - Andrew-T
Your car is clearly suffering from incomplete combustion, for whatever reason - unless the test instrument is misbehaving?
Lambda Probe - MOT Emmissions - bmw. - Andrew Moorey (Tune-Up)
There is an element of confusion creeping in here namely with CO (carbon monoxide) and CO2 (carbon dioxide) and the misnomer of the O2 (oxygen) sensor by calling it a Lambda probe/sensor. Your car failed on excessive CO emissions, nothing else, right? Ok, the reason for this has been diagnosed by the dealer as a failed Oxygen sensor (Lambda probe). Because the word Lambda appears on the emissions certificate and as another name for the O2 probe the two become inextricably linked in the mind of the layman. The Lambda on the emissions sheet is merely the industry name for the calculation made by the emissions tester based on the levels of CO,CO2,O2 and Hydrocarbons coming out of the tailpipe, if everything is working properly then the value of Lambda should be 1.000 -.030 If, as in your case, the O2 sensor was defective (common fault on all cars) then the emissions would be out of kilter (ie high CO) and consequently the Lambda calc showed accordingly.
Hope this helps, if you\'d like to know how a Lambda probe works there is a site called \'how stuff works\' or something which is brilliant.

{The link is . DD}


Simplicate and add lightness!!
Lambda Probe - MOT Emmissions - bmw. - Cliff Pope
Thanks for that very clear explanation Andrew. I sort of half knew it, but it is good to see it set out so well.
Is it not the case that many Oxygen sensors are needlessly replaced when a bit of simple maintenance would have sufficed? I understand that they work by comparing the oxygen concentration inside the exhaust with that outside. They have vents to allow air to permeate, which stop functioning if the sensor is, for example, covered in baked mud.
Cleaning the outside carefully, vacuuming the vents clear, and giving the car a blast, might be all that is needed?
Lambda Probe - MOT Emmissions - bmw. - DL
Was the car taken for an Italian Tune-up before the test?

I reckon it might well have passed then, without the need for expensive Dealer work........
-- - Pictures say a thousand words.....
Lambda Probe - MOT Emmissions - bmw. - Andrew Moorey (Tune-Up)
The sensor works reacting to the amount of oxygen passing across it which results in the sensor itself generating a small DC voltage which the brain uses to make tiny corrections to the fuelling. The sensor should be regarded as a consumable item in the same way as a sparkplug. The length of service depends on the use it is subject to. Driving at 70mph on the motorway every day will keep it hot and clean and it could easily pass 100k, conversely a sensor subjected to arduous use- the school run, shops- will foul up and fail very quickly. They can be recovered by the Italian tune-up method coupled with a dose of Forte Gas Treatment, or by cleaning ultrasonically.
You are correct that some sensors have a vent to atmosphere but I believe I am right in saying that this air is tracked into the sensor between the wires and the insulating sheaths. I only use Manufacturers original sensors or those by Bosch or NTK.
Now back to the pressies!!!

Simplicate and add lightness!!
Lambda Probe - MOT Emmissions - bmw. - DL
I haven't found an awful lot wrong with Fuelparts Oxy sensors....or have I been lucky?
-- - Pictures say a thousand words.....

Value my car