Vauxhall Astra 1.8 Auto, year 2000, 78,000 miles
How can I tell if my lambda sensor is faulty?
Checking my fuel consumption on a rolling 12 months basis, I've noticed that it's dropped from 31 to 29 mpg. The annual consumption has never varied more than 1 or 2 tenths. It's been well serviced and has no known mechanical / electronic problems and no fault codes are stored in the ECU.
At it's recent Mot it passed with CO 0.00 %vol and HC 0 ppm at lambda 1.003. The engine management light has never come on, apart from the ignition-on test.
I've been told that lambda sensors are only good for 60k. Is this true?
When it starts to fails, can it simply give incorrect readings thus causing high fuel consumption?
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Don't think there's anything wrong with your lambda sensor, not with a reading of 1.003!
I've know of a Nissan Primera 2.0i used as a taxi that has over 500k miles and still all the original engine sensors - so 60k is nothing. If H/G goes or you fill it with leaded petrol, or a coil pack dies then, yes, it may fail.
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>>I've been told that lambda sensors are only good for 60k. Is this true?
NO...although I have read on some web sites that sensors do age, as detailed below my omega one failed at around 130K.
>>When it starts to fails, can it simply give incorrect readings thus causing high fuel consumption?
Yes, when mine went the Omega was giving anywhere between low 40s & mid 20s. At some point I think the ECU decides it doesn't believe the readings any more and uses a default setting, and generates a fault code. But i think that alarm threshold is quite a long way from 'normal' lambda.
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Thanks, guys.
Is there any way of testing a lambda sensor, other than in a laboratory?
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Hook a scope up to it and you'll see it switching. Typically 02.-0.8V or thereabouts. Frequency will increase with rpm.
MoT tester will give you an emissions printout which will confirm its working....
Most cars the MIL comes off if its really dead.
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Given that the OP's fuel consumption has only dropped a few % then I suspect an oscilloscope would be needed to see if the sensor has become slightly 'lazy'; I wouldn't expect so at 60K.
There is a good summary of lambda sensor resources on ye olde interweb here:-
www.cavweb-forums.co.uk/showthread.php?t=39815
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A classic symptom of a sluggish EGO sensor is a feeling of a slightly 'surging' engine when cruising on a light throttle at moderate speeds.
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Sounds fair enough Aprilia, I don't doubt you'll forget more than I'll ever know about car fault diagnosis ;-)- I think thats' what I mean anyway.
However my day job does often involve debugging fairly complex closed loop electronic control systems, and cars seem to include ever more complex electronic control systems......
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The data originally posted - CO 0.00 %vol, HC 0ppm and lambda 1.003 - were taken directly from the MOT emissions test printout. I don't have "scoping" facilities, a DVM is my limit.
Two thoughts
- I don't regard 31 > 29 as a small drop when taken over a full 12 months average
- the consumption was particularly bad over the last winter (but then it was colder) could the heater circuit have failed causing a very slow warm up of the sensor?
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- I don't regard 31 > 29 as a small drop
How are you calculating the mpg? By brimming it and calculator, or using the car's fuel computer?
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>>the consumption was particularly bad over the last winter (but then it was colder) could the heater circuit have failed causing a very slow warm up of the sensor
On the Omega that would generate an EML on and hence a fault code, so I would think it's the same on an Astra.
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DD - I always brim my tank to the same level, first shut off, and record the odometer total mileage. My calculation is based on the total distance covered in 12 months divided by the fuel used over the same distance. I repeat this calculation every month to eliminate seasonal variations.
My trip computer reads about 4% optimistic by comparison - despite using the same sensor for distance. I believe the T/C uses the injector timing signal to calculate the amount of fuel used.
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All the evidence so far points to the lambda sensor being OK although the MOT emissions test results do seem to be a bit too good to be true. Was there a NOx level on the readout?
Unfortunately, the list of other things that can affect your fuel consumption is almost endless.
Weather.
Driving conditions.
Tyre brand/model wear and pressures.
Engine oil spec. and level.
Engine wear.
Gearbox oil and level.
Auto gearbox wear.
Another speedhump here or there.
An increase in congestion at junction X.
Different fuel types.
Sticky thermostat.
etc.
In the end, if you are absolutely convinced it is not due to other factors, you need to do a cost-benefit analysis. How much are you willing to spend replacing parts in the hope that you'll get your 2mpg back?
Kevin...
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There is nothing wrong with your lambda sensor, a reading of 1.003 means it's doing its job. You need 10x that error for a failure and a reading of 1.125 for an economy drop of 2 mpg.
Check the wheels for binding brakes, then check the plugs for type/gap/replacement on the last service, then go through the above list, starting with fuel type....
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I have, of course, been through the list of usual suspects eliminating the items on Kevin's and Mark's lists. I use annual consumption is it smooths out seasonal variations although I can't yet rule out last winter which was colder than the previous 5 winters, with the same car.
The MOT emission test results don't include Nox. The previous years test (Mar 05) gave CO 0.01%vol, HC 0ppm with lambda at 1.001. For both 05 and 06, the natural idle test gave the same CO as the fast idle test.
From your informative postings I'm reassured that my lambda sensor is operating normally - I'd been worried by comments elsewhere that they're only good for 60k!
I'll continue my search for the "fault" and re-post if I find anything significant.
Thanks.
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I think you lambda is fine.
Main suspects would be:
Thermostat
ATX lock-up clutch or program fault (not stuck in Sport mode is it?)
Engine oil viscosity different to last year.
Different brand tyres or tyre pressurs.
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Thermostat - engine runs at same indicated temperature as it always has and warms up in same distance from cold.
Auto transmission - lock-up and gear selection sound as usual with no warning light displayed.
Engine oil - Mobil 1 0W-40 used throughout it's life.
Tyre brand/pressure - no change in consumption noted when original Michelin XH1 changed for Goodyear NCT5 four years ago, pressure consistent and regularly checked.
What symptons would a failing EGR valve give as the X18XE1 in my Astra is prone to this, apparently?
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