What does it do and why are they so expensive ?
Cheers
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Lambda probe = oxygen sensor - see
auto.howstuffworks.com/catalytic-converter.htm
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Sept 2004 - 'Britain on the Move'
curious ? see www.mikes-walks.co.uk
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Hi
The lambda sensor basically measures oxygen in the exhaust to ensure that the Air/fuel ratio doesn't drift - in engineering terms, lambda is the greek symbol that is used to denote the air/fuel ratio (prior to combustion), where a lamba of 1 is the stoichiometric ratio (the ideal ratio, where the chemical constituents of the air and fuel tally perfectly). It's important to measure this, as incorrect lambda can result in nasty emissions that both harm the environment and kill the catalytic converter pretty quickly (this, to the best of my knowledge, is one of the main reasons that lambda sensors were originally fitted). In perfect combustion, with a lamba of 1, there should only be water and carbon dioxide as waste products (with heat as the useful product!) but in real situations there are loads of other things - the measure of oxygen in the waste gives some indication of how 'good' the combustion was, and if it was poor the engine setings and mappings can adapt to improve things. It's just a useful form of feedback.
I don't know why they're expensive, might be that they're precision made - they're actually quite basic but require the use of metals such as platinum (which is more expensive than gold).
Matt
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I believe all it does is measure the oxygen content of the exhaust gasses so that the ECU can calculate whether the combustion process is as efficient as possible. It then calculates other things such as mixture and even possibly ignition timing based on the result.
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There is a narrow window, each side of stoichometric where the catalyst can work.
If the mixture is richer, then there will not be enough oxygen in the exhaust to oxidise any unburnt HCs and CO.
If the mixture is weaker, there will be too much oxygen to allow the reduction of NOx.
Within the narrow window, the oxygen 'taken' from the NOx is used to oxidise the HCs and CO.
Because there is a time delay between the injection of a slug of fuel, and the measurement of the resulting slug of exhaust gas, and the fact that the lambda sensor is strongly non-linear with respect to excess air ratio (the definition of lambda), the feedback control loop is not smooth. The sensor/ecu/injection works in an oscilatory limit cycle, about 1-2 times per second at tickover, but speeding up as the engine revs up, and the time delay shortens. So, the output from the sensor is always changing, but in an average sense, mixture is closely controlled.
number_cruncher
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What does it do
www.honestjohn.co.uk/forum/post/index.htm?t=14677
and why are they so expensive
£30 for my mate's Vaux Carlton from a motor factors.
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It controls fuel injection. if it fails overfueling occurs. which will cause cat failure. as I think has been said motor factors are cheaper.
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Was mech1
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Some work by comparing the amount of oxygen in the air with that in the exhaust, and have some form of venting to the outside. If the sensor is mounted underneath the car it obviously gets coated in mud, which blocks the vents.
Sometimes my Lambda light has come on and the diagnostic code indicates a fault in the sensor, but I have found I can clear this by gently brushing the outside of the probe and hoovering it. I don't suppose driving through floods does it much good either.
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Also, some sensors draw their reference air via the wiring harness (i.e down the cable) - and therefore don't have visible air vents. The quantity of reference air required is very small.
Do take care when cleaning them, as the internal elements are surprisingly fragile. Also, never spray chemicals on them such as WD-40 - as this doesn't do the sensor, or the connector, any good at all.
The heaters are often the cause of failures which put the diagnostic light on. (these are required to get the sensor up to a minimum working temperature of about 300degC).
.....Why are they so expensive ? ....Because the car manufacturers put a huge mark up on them. They cost about £10 to make.
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